#66 JUNE 2018

QUANTUM REVOLUTIONS CONTEXT − TECHNOLOGIES − SCOPE OF APPLICATIONS − OUTLOOK VIEWPOINT CONTENTS

IN THIS ISSUE Quantum Adventures “When I read Heisenberg, I understood the extent to By Jérôme Ferrari, which Heidegger’s famous QUANTUM THE VIEWPOINT OF JÉRÔME FERRARI 2 writer and Goncourt prize-winner 2012, teacher. statement “science does not CONTENTS 3 think” was not only wrong, regardless of how one REVOLUTIONS THE QUANTUM WORLD AND US 4 interpreted it, but also © Siddtarth Siva ridiculously condescending.” THE NEXT QUANTUM REVOLUTION No theory has ever been as fruitful in terms of understanding the Universe that surrounds us and applications. Our society, that of information and communication, 8 is the child of quantum mechanics.

a model of educational writing, to gain a slightly clearer OUR PERCEPTION OF QUANTUM MECHANICS HAS CHANGED 9 understanding of what was going on with quantum physics and at what level. I still find that it is inconceivable to talk THE SECOND QUANTUM REVOLUTION 11 about the ultimate nature of reality, as one naturally does in metaphysics, without listening to what science has to say since – at the beginning of the 20th century – by penetrating the world of the atom, it completely changed the fundamentals of hroughout my secondary education, I was deeply in the problem. This clearly demands an effort, especially when, QUANTUM ENGINEERING 2.0 love with mathematics, albeit a love that was not like myself, one is obliged to stop at the point where The second quantum engineering revolution concerns research which aims to design and build innovative, even disruptive returned. Physics however repelled me from the devices. This is a field enjoying rapid expansion, with considerable scientific and technological implications. outset. It was therefore with resignation that I mathematical formalism begins, in other words the heart of Tfound myself once again taking epistemology classes when I the matter. But a grasp – albeit imperfect and solely 15 enrolled for a philosophy degree after two years of preparatory qualitative – of the fundamental concepts of the theory is INTRODUCTION 15 classes. The reading list we received included Physics and enough for us to become aware of the fact that we are plunged A Round-Up of Quantum Technologies 16 philosophy by Werner Heisenberg. When I read it, I into an existential inferno from which, paradoxically, one has understood the extent to which Heidegger’s famous statement no desire to escape. COMMUNICATION 19 “science does not think” was not only wrong, regardless of Every year, when I gave my class on la raison et le réel, I tried Future Challenges and Prospects 19 how one interpreted it, but also ridiculously condescending. to give my students a glimpse of this awe-inspiring landscape Generating and Detecting a Single Photon 22 And that was about all I understood. in the hope of helping them to understand its importance and possibly its beauty. It must be said that my attempts were METROLOGY AND SENSORS 25 Because, for the first time, I found myself faced with a writing rarely crowned with success. When I presented the double-slit style whose perilous elegance and lyrical outbursts could do Future Challenges and Prospects 25 nothing to conceal the monstrous difficulties that lay behind experiment, including to final year high-school science Detecting Spins 28 it. I was convinced at the time that contemporary atomic students, I received nothing but unbelieving stares in return. science had on the whole confirmed – at least conceptually – The students suspected me of spouting nonsense and it was COMPUTING 32 the materialist conjecturing of Lucretius and Epicurus. What did only with the support of my physics colleagues that I was not definitively categorised as deranged. Yet this experiment is Future Challenges and Prospects 32 Heisenberg mean when he spoke of elementary particles as The Search for the Integrable Qubit 36 not hard to understand: the problem is that when carried out being situated somewhere “between the possible and the real”? The Benefits of Quantum Computing 40 with electrons, its results are simply improbable and Quantum Computing for Tomorrow? 43 I for one understood nothing. But I was absolutely fascinated. outrageous. This is doubtless because it unceremoniously I therefore decided to ease my frustration by obtaining the dispels our most spontaneous intuitions that quantum physics texts that the founding fathers had written for non-specialists, has such difficulty in effectively popularising – apart, of along with a number of popularisation works. After reading course, from the many bizarre fantasies that use it as a pretext. them, I no longer understood nothing, but had a completely I still hope that this situation will change and that those who, erroneous grasp of everything, not what one might really call like myself, are not physicists, will be able to familiarise OUTLOOK progress. I had to wait until 1993 and the publication of themselves with a theory that is so rich and stimulating and THE UNIVERSE IS ALL QUANTUM 45 Regards sur la matière, co-authored by Étienne Klein and one that has been determined as a result of an intellectual Bernard d’Espagnat, a book which I still today consider to be journey as fascinating as any great novel. 45

2 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 3 THE QUANTUM WORLD AND US THE QUANTUM WORLD AND US

• The First BY ÉTIENNE KLEIN (Fundamental Research Division) Quantum Revolution During the 1920s, quantum physics appeared in order to explain the bizarre behaviour (given classical physical laws) of atoms and particles. Radically new concepts were then invented and led physicists to think differently about matter and its interactions. In the end, a decade of creative effervescence and intense Étienne Klein is a physicist and labour were enough for a small number of them (Schrödinger, Heisenberg, head of the Physical Sciences Pauli, Dirac…), scattered to the four corners of Europe, to lay the foundations of Research Laboratory (Institute “This separation for Research into the Fundamental one of the finest intellectual constructs of all time. Laws of the Universe at CEA). from ordinary physical space thus creates a distance between the representation of

of this system. Therefore the superposition principle example, if a chemical bond exists, it is because the phenomena and applies not only to waves, but also to any physical (roughly speaking) an electron can be located on the phenomena system. The addition rule thus becomes universal, in several atoms at the same time, as if it were that we consider everything to be like a wave, even if “suspended” between several positions, as required themselves, a distance experimentally this is not true: when an electron is by the principle of superposition, in the same way as directed to a plate covered with a chemical product in a diatomic molecule, certain electrons, said to be within which all kind which whitens on contact with it, this is manifested valence, are both on the left hand side and right-hand by a point impact, not as a wave which would occupy side of the molecule, even if this notion appears at of unprecedented the entire space… first glance impossible. Many instruments which are questions today commonplace, owe their existence to an In mathematics, the entities which satisfy the understanding of this quantum law: transistors, can be heard.” property of being added to each other are “vectors” lasers, LEDs, etc. and the assembly they form constitutes a “vectorial space”. This is why entities a, b, c representing the Just as strange is the fact that it is the philosophical uantum physics is based on a rigorous and Any particle is defined by a certain number of various possible states of the physical systems are consequences of the superposition principle which complex mathematical arsenal (referred to characteristics that are identical for all the particles From left to right called “state vectors”. They are of course functions of raised problems with interpreting quantum physics. as a “formalism”), the fundamental of the same type. Therefore all electrons, wherever Erwin Schrödinger space and time. It is easy to see why. The formalism of quantum principle of which is however simple to Q they are and regardless of their environment, have physics works within abstract vectorial spaces which Paul Dirac Wolfgang Pauli state. We know from experience that there are several strictly the same mass and the same electrical charge. This description of physical states by state vectors (or, are remote from the physical space in which the However, in addition to these universal characteristics, Werner Heisenberg types of waves (acoustic, electromagnetic, etc.) and if one prefers, the equivalent affirmation that the events described by this formalism take place. This that they all share a common property, independently electrons have quantities which can vary from one to principle of superposition is applicable to them) is separation from ordinary physical space thus creates the other, for example, position or speed. In classical of the matter of their medium: they are capable of thus the fundamental concept of quantum physics. It a distance between the representation of the physics, all of these quantities form the “state” of the is remarkable that a hypothesis that is so simple, being added together, in that the sum of two waves of phenomena and the phenomena themselves, a particle. How is this represented in quantum physics? lying in the depths of a labyrinthine formalism, has distance within which all kind of unprecedented a certain type is still a wave of the same type. This is Answer: by entities, noted a, b, c…, which are simply led to such considerable progress in the field of questions can be heard: how does the quantum the “superposition” principle characteristic of the required to obey the superposition principle. In other physics. Thanks to this hypothesis, many formalism concretely connect to the experiments? wave concept, that has been taken up and generalised words, if a and b are two possible states of a certain incomprehensible phenomena of classical physics What rules are to be used? How does this new physics by quantum physics. Let’s see how. system, then state (a + b) must also be a possible state have been described elegantly and concisely. For describe reality?

4 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 5 THE QUANTUM WORLD AND US THE QUANTUM WORLD AND US

of a state in which decay has taken place and one in he dramatizes the suspense of the explosion, which it has not taken place, thus a state in which the something is lacking; the image does not involve us. cat is dead and another in which it is alive. Contrary to what semi-informed culture would have us By placing an animal’s life at stake, Schrödinger believe, Schrödinger did not intend to suggest the accentuates the affective involvement. However the possibility of such a state of macroscopic animal had to be one whose death does not leave us superposition, considering this to be impossible, indifferent, such as a mosquito, or that some of us eat and relies on the fact that we have never observed (chicken, pig, etc.). What makes the feline metaphor this to call into question the notion of a particle in a so special is that it has particular resonance in our superposed state. imaginations: The cat is both domestic yet independent, indoors and outdoors, it is an animal In his commentary on this exchange of letters, with nine lives and whose eyes are a source of Étienne Klein quite rightly points out that from an profound mystery for poets. For Lewis Carroll, it epistemological point of view, the transition from already raised the enigmatic question of presence “Any animal would Einstein’s barrel to Schrödinger’s cat adds nothing to superposed on absence (the Cheshire cat was also to the thought experiment. He finds the feline metaphor become the medium for a quantum thought do: From the strict both superficial and arbitrary: “Any animal would do: experiment). heuristic viewpoint, From the strict heuristic viewpoint, the paradox of Schrödinger’s chicken, goose or sow would have had Maybe the metaphor was too perfect! Although the paradox of exactly the same importance as that of the cat”. [2] Schrödinger was seeking to make it problematical to Yet it is clearly the image of the dead/alive cat which apply his equation to the concept of the particle, the Schrödinger’s ensures its popularity and, even if Schrödinger gives metaphor escaped him and became the emblem of no information on this subject, one can wonder about numerous increasingly realistic re-interpretations of chicken, goose the reasons for the choice of this animal. superposition. “Schrödinger’s kittens” are today being or sow would have produced in the very real experiments by Serge The Cat of A thought experiment is always a combination of Haroche. Consequently, the metaphor is an obstacle had exactly the reasoning based on unachievable conditions to understanding. Schrödinger’s cat is the perfect (hypotheses that are apparently counterfactual even example of a thought experiment in a superposed same importance as if one sometimes subsequently discovers that they are state of exemplary success and pitiful failure. • Opportunity real) and a story implying a suspension of incredulity that of the cat.” BY VINCENT BONTEMS with regard to another “possible world”. In general If researchers used animal metaphors in the quantum Einstein experimented on himself, riding a photon, context, it was perhaps to tame a conceptual difficulty (Fundamental Research Division) From Schrödinger’s cat to Tollaksen’s pigeons, not forgetting Dirac’s fish or or falling into a vacuum. Unlike Galileo, he would rather than to resolve it, in other words to provide a Gamov’s tiger, animal metaphors abound in quantum mechanics. Unlike in never have thrown an animal into the void, even in fictitious representation of equations where there is other fields, they were not invented by those attempting to popularise science his imagination! In his thought experiments leading no conventional realistic representation. We cannot to a positive result, he strove to represent his own test this hypothesis here on other examples, but but by the scientists themselves. What is the purpose of these metaphors? awareness, to employ reflexivity. With the barrel, we quantum zoology is clearly intended more to surprise What does “quantum zoology” teach us? can feel him maintaining a certain distance: even if than to teach.

n 8th August 1935, Albert Einstein wrote metastable powder (ready to explode at the slightest Vincent Bontems Bontems is a philosopher a letter to Erwin Schrödinger in which he spark) and, in this barrel a radioactive atom with a of science at the Physical Sciences Research drew his attention to the counter- half-life of one year: “Initially, the ψ function of the Laboratory (Institute for Research into intuitive nature of some consequences of system characterises a well-defined macroscopic the Fundamentals of the Universe at CEA). Othe equation to which the latter had given his name. state. But your equation means that by the end of a The structure of the wave function means that a year this is not the case. The ψ function then tends to particle can be in a “superposed” state, in other words describe a blend containing the system which has not the sum of an A state and a non-A state. This yet exploded and that which has already exploded.” [1] [1] Letter quoted by Françoise Balibar, paradoxical state led to Einstein’s scepticism with “Correspondence between Einstein and th regard to the completeness of the quantum It is in his answer of 19 August that Schrödinger Schrödinger” in Michel Bitbol and Olivier formalism. For Schrödinger, this tended more to imagined a similar thought experiment in which a cat Darrigol (dir.), Erwin Schrödinger. Philosophy show that the “particle” concept was no longer valid is locked inside a box with a radioactive atom. In the and the Birth of Quantum Mechanics, Paris, at the quantum scale. event of decay, an ingenious detection system triggers éditions Frontières, 1992, p. 312. breakage of an ampoule containing a lethal gas: As In his letter, in order to exaggerate the strangeness of long as the box has not been opened, the system it [2] Étienne Klein, Il était sept fois la superposition, Einstein invented a stratagem which creates with the atom, mechanism and cat is révolution. Paris, Flammarion, 2005, p. 231. transposed it to our scale. He imagined a barrel of described by a state vector which is the superposition

6 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 7 THE NEXT QUANTUM REVOLUTION THE NEXT QUANTUM REVOLUTION

• BY ALEXEI GRINBAUM (Fundamental Research Division)

Alexei Grinbaum is a physicist and philosopher of science. He works at the Physical Sciences Research Laboratory (Institute for Research into the Fundamental THE NEXT Our Perception of Laws of the Universe) at CEA. QUANTUM Quantum Mechanics REVOLUTION has Changed Quantum mechanics manuals written in the 20th century are not wrong but not “We today know that No theory has ever been as fruitful in terms of understanding quite up to date. a large part of the the Universe that surrounds us and applications. Our society, specificity of quantum that of information and communication, is the child of quantum n 1958, for example, Freeman Dyson declared “resources”. For instance, quantum physics yields theory lies in its mechanics. that he was convinced that quantum mechanics quantum non-locality, or quantum contextuality, or would become a natural form of thinking quantum discord. These notions correspond to description, not of an because scientists would become used to it. He resources that can be used to achieve particular non- isolated physical system, Iwas apparently convinced that conceptual problems classical results. with quantum theory would resolve themselves with but of the rule of the Our Perception of Quantum Mechanics has Changed 9 time. Nothing of the sort happened. On the contrary, Let us look at non-locality. We know today that a large composition of systems.” research into various ways of understanding quantum part of the specificity of quantum theory lies in its mechanics has profoundly modified its language. The description, not of an isolated physical system, but of new conceptual and mathematical vocabulary, in the rule of the composition of systems. The Second Quantum Revolution 11 terms of quantum information, began to emerge at the end of the 1980s. Both in the technological and The non-locality resource exists by virtue of quantum theoretical research worlds, an intuitive conception of entanglement, measured by inequalities first quantum mechanics today involves the use of formulated by John Bell. Their violation was

8 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 9 THE NEXT QUANTUM REVOLUTION THE NEXT QUANTUM REVOLUTION

The Second • BY ALAIN ASPECT Quantum Revolution (Paris Saclay University)

Physicists needed nearly half a century to open the door to the second quantum revolution, with its many applications in a variety of fields such as computing, simulation and metrology. Alain Aspect is a physicist at he development of quantum physics in described within the context of quantum physics, the Graduate School Optics Institute. the first half of the 20th century is one of but which are so new and unexpected that there is He was a CNRS gold medal winner the most extraordinary intellectual continuously renewed interest, not to mention the in 2005 and is also the only French adventures in the history of mankind. awarding of numerous Nobel Prizes. TThis physical theory, which completely changed our winner of the Niels Bohr medal (2013). view of the world, explains the properties of matter, Even more unexpectedly, given the widely accepted of light and of their interactions. It also radically feeling that the founding fathers (Niels Bohr and his altered how we live, with its countless applications: students) had completely clarified the subject Lasers, transistors, integrated circuits, which are the (essentially by responding to Einstein’s objections), basic components of the information and it is at the conceptual heart of quantum theory that communication technologies... a new phase of major progress began in 1960 with Such a string of successes could have led to fears the work by John Bell, followed by increasingly that the field would have nothing left to offer: detailed experiments. The violation of Bell Nothing could be further from the truth. Whether inequalities provided irrefutable evidence of the demonstrated experimentally by Alain Aspect (see p. scale but a resource that we can harness for real-world the behaviour of electrons in solids, light-matter reality of quantum entanglement, an extraordinary 11). At the experimental level, anyone having a pair of applications. Furthermore, “non-classical” does not interactions, or ultra-cold quantum fluids, to property of a pair of objects which behave as a single quantum particles in a lab can use them to accomplish necessarily imply “quantum”: there exist mention but a few examples, we are seeing the quantum system, even when remote from each “With these new a classically impossible task . At the theoretical level, “postquantum” models, which are used to study the appearance of phenomena, which can certainly be other. however, it is possible to develop a general theory of informational properties of our world. tools, we can ask such resources, similarly to the use of electrical circuits in a general approach to the design of a With these new tools, we can ask fundamental fundamental questions computing machine. questions previously firmly associated with metaphysics: What is a system? What is an observer? previously firmly The days of the Schrödinger’s cat enigma are over (see Other types of theoretical enquiry are even more p. 6). This metaphor corresponds to the property of counter-intuitive, e.g. what is a “quantum causality”? associated with quantum superposition; yet superposition, which If each sub-system of a compound system is measured changes with a change of basis, is not usually in an isolated laboratory, it is conceivable that the metaphysics: considered as a resource. Entanglement, another Fig. 0: Atomic corral. This tunnel causal order of the observations within the enigmatic property of quantum mechanics, is more effect microscope image shows an laboratories would not be described as “L1 is before What is a system? robust and also directly useful in areas from quantum “atomic corral” produced with iron L2“ or “L2 is before L1“: it can itself be indefinite, as cryptography to quantum simulation. atoms deposited on a copper What is an observer?” in a quantum superposition. This indefinite character surface. The atoms are shown by This usefulness is also theoretical. Quantum of causality is fundamental: it is not due to the electronic density peaks. information enables a precise description of the observer’s ignorance. If we learn to use it as a The waves show the wave function boundary between the classical and quantum worlds. resource, it could offer us a completely new approach of the copper electrons which are In the case of non-locality, determining a limit to understanding physical space-time. reflected by the iron atoms. The involves a bound on the correlations between the subsystems of a compound system. Whatever the The hope is therefore that the tools of quantum possibility of observing and material substrate of such a system, correlations information will allow us to reach technological and controlling individual quantum between its parts are stronger than anything predicted industrial applications, but also much more: a new way objects is an essential ingredient of From top to bottom

by classical physics. This difference is not a matter of of defining the foundations of physical theories. IBM © D. Eigler, the second quantum revolution. Niels Bohr and John Bell

10 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 11 THE NEXT QUANTUM REVOLUTION THE NEXT QUANTUM REVOLUTION

© A. Aspect et G. Roger, Institut d’Optique « Quantum It was at the same time that we learned to manipulate In this article, Einstein and his co-authors show that the electrons, ions, atoms or photons one by one (Fig. 0). We From the Bohr-Einstein debate quantum formalism allows the existence of particular states mechanics today know that quantum mechanics can be applied to testing the Bell inequalities: of two quantum objects, for which certain observables are PM a’ b’ could not have successfully to single objects and not only statistically to a heavily correlated, even when the objects are remote from PM entanglement revealed each other (Fig. 1). Einstein therefore concludes that for each large number of identical objects such as the atoms of a gas. v v come about And we have seen new calculation methods appear, known object of a pair, these observables have a precise value prior 1 2 Quantum mechanics could not have come about without PM S PM without major as “quantum Monte-Carlo”, which are well-suited to to measurement, even though this is not the case in the major revisions of concepts accepted in classical physics. The C C describing the behaviour of these individual elementary quantum description. Einstein therefore concludes that a 1 2 b revisions quantum objects. sacrifices were so radical that several physicists, including quantum mechanics does not reflect the total physical of concepts Einstein and de Broglie, and to a certain extent Schrödinger, reality. N(a,b) , N(a,b’) It is doubtless no exaggeration to say that these two did not accept that they were final, unlike Bohr who made N(a’,b) , N(a’,b’) accepted th conceptual advances at the end of the 20 century – the this the keystone of the interpretation of the new theory Niels Bohr was apparently deeply disturbed by this argument which uses the quantum formalism itself to demonstrate its in classical awareness of the importance of entangled states, the known as the “Copenhagen interpretation”. Einstein did not understanding of the dynamics of individual quantum own incomplete, provisional nature. His writings show his Fig. 2: Experiment at the Optics institute of Orsay (1982). After the pioneering experiments of the 1970s in physics. » question the mathematical formalism of quantum objects – marked the beginning of a new quantum deep conviction that if the EPR logic was correct, the whole mechanics, nor its predictions, but he appeared to believe Berkeley, Harvard and then Texas A&M, the use of new lasers allowed the development of an effective source of revolution. There is no reason why we cannot imagine that of quantum physics would be called into question. He that the sacrifices recommended by Bohr reflected the fact pairs of entangled photons, within the vacuum chamber located in the centre of the photo. Polarisation beyond the progress made in fundamental physics, for which therefore immediately disputed the EPR logic, stating that that quantum theory was incomplete. This position was to measurements established the existence of correlations violating the Bell inequalities, thus proving the we have doubtless only observed the first manifestations, in a quantum state of this type, it is impossible to speak of inseparability of the photons of each pair, even when remote from each other. In this experiment, the two photons lead to epic debates between them, in particular that which the individual properties of each object, even if they are and the conceptual questions which remain more open than were 12 m apart at the time of measurement and the orientations of the polarisers were modified during began with the 1935 publication of the Einstein, Podolsky remote from each other. With Schrödinger, who discovered ever, this new quantum revolution could in turn profoundly propagation of the photons between source and detectors, thus guaranteeing relativistic separation of the change our society by leading to a new technological and Rosen (EPR) article, the title of which poses the the extraordinary nature of these states at the same time, we measurements, which could not be connected by a signal travelling at a speed less than or equal to that of light. revolution, the revolution of quantum information and question: “Can quantum-mechanical description of physical would now talk of “entangled” states, to indicate that the two metrology… reality be considered complete?”. objects are inseparably interconnected, that they form a single object regardless of the distance that separates them.

One might have imagined that this debate between two giants of the physics of the 20th century would have split the This polarizer, made with 10 optical quality plane-parallel world’s physicists into two camps. In fact when the EPR plates inclined at Brewster's angle, has been built for testing article appeared in 1935, quantum mechanics was enjoying Bell's inequalities. success after success and, apart from Bohr, most physicists ignored this debate, which they felt to be outside the scope of laboratory science: It appeared that preference for one or In 1964, there were no experimental results enabling this Fig. 1: The Einstein, Podolsky and Rosen thought experiment (1935). other of the positions was a matter of personal taste (or conclusion to be reached. The experimenters therefore set In an EPR quantum state, the two particles 1 and 2 are totally correlated in epistemological position), with no practical consequences to work to build a “sensitive” experiment, in one of the rare terms of position and speed. Measuring the position of the first particle can for the use of the quantum formalism, something which situations in which quantum mechanics predicted a violation © Institut d’optique Graduate School give an indeterminate result (M1, M1’ …); but if we have found M1, we find the Einstein himself seemed not to dispute. Nearly thirty years of the Bell inequalities. The most convincing experiments second particle at M2 with certainty, the position of which can be deduced were to pass before a scathing refutation of this consensus were performed with pairs of entangled photons on which from M1 by a fixed translation r0, and similarly, if we have found M1’ for the was to be seen. polarisation measurements were carried out. After a first generation of pioneering experiments carried out in the first, we find M2’ deduced from M1’ by the same translation r0. The speed It was in 1964 that John Bell, an Irish theoretical physicist United States in the early 1970s, a new series of experiments measurements are also totally correlated. The speeds are always found to be working at CERN in Geneva published a short article which carried out at the Optics Institute in Orsay in the early 1980s opposite: V2 = - V1 or V2’ = - V1’. was to radically alter the situation. In just a few lines, he We can thus know with certainty the value of the position of particle 2, or its showed that if we take the EPR argument seriously and (Fig. 2), following schemes that were increasingly close to the “ideal thought experiment” gave indisputable results, speed, by taking a measurement on particle 1. As a measurement on particle complete the quantum formalism in the spirit of Einstein’s providing an excellent fit with quantum mechanics and 1 could not instantaneously affect remote particle 2, EPR conclude that, ideas, then we reach a contradiction with certain quantum clearly violating the Bell inequalities. A third generation of before the measurements, particle 2 had a perfectly determined position and predictions. Einstein’s ideas are the most natural imaginable: experiments published as of the end of the 1990s and They are the equivalent of explaining the similarities [1] At the same period, speed value, even though the quantum state cannot simultaneously specify recognised by two experiments in 2015, carried out in these values with zero uncertainty. EPR conclude that the quantum between twins by their identical genetic heritage. That an a different scheme, carried explanation of this type had to be rejected in the field of Vienna and Boulder, confirmed these results [1]. It was time description of this situation is incomplete and one must seek to find how to out at Delft, was to intricate quantum objects never ceases to surprise us but, thanks to to face the facts: Entangled twin photons are not two complete the quantum formalism. Thirty years later, John Bell was to artificial atoms called Bell’s theorem, experimentation can be used to find out separate systems carrying two identical copies of a same set "NV centers", and a first sign demonstrate that it is impossible to complete the quantum formalism within whether, in nature, there exist correlations as strong as those of parameters. An entangled pair of photons must be the spirit of the EPR ideas, that is by conserving the notion of the physical predicted by the quantum formalism and which violate the considered as a single, inseparable system, described by an of the violation of Bell's reality of a system (set of parameters determining the results of any famous “Bell inequalities”. These inequalities apply to all the overall quantum state which cannot be broken down into inequalities was obtained measurement on this system) and the notion of relativistic locality, which correlations which could be explained in the Einstein two states relative to each of the two photons: The properties with this new system, opening rules out any influence propagating faster than light. viewpoint, which will therefore have to be abandoned if we of the pair are not just the combination of the properties of great opportunities for observe their violation. the two photons. quantum computing.

12 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 13 THE NEXT QUANTUM REVOLUTION QUANTUM ENGINEERING 2.0

BELLEVUE

4,5 km FM BELLEVUE Quantum channel Z FM 8,1 km APD1+ GENEVA APD1- Laser

R++ CORNAVIN P F L R-+ classical channels R+- & KNbO, R-- 10,9 km

APD2- BERNEX

APD2+ 9,3 km Z FS Quantum channel

7,3 km δ2 FS QUANTUM BERNEX Fig. 3: Geneva experiment (1998). This experiment used the optical fibres of the commercial telecommunications network. The source (in Cornavin) ENGINEERING 2.0 is more than ten kilometres away from the detectors (Bellevue and Bernex). Entanglement of individual be broken in the event of progress in mathematics This experiment demonstrated that (hypothetical) or in the power of computers The second quantum engineering revolution concerns research which entanglement continues to exist for two quantum objects is the basis of (probable). aims to design and build innovative, even disruptive devices. This is a field photons more than ten kilometres apart. quantum technologies The application potential for entanglement in enjoying rapid expansion, with considerable scientific and technological In an comparable experiment performed quantum technologies goes well beyond quantum implications. in Innsbruck, the propagation time in the Quantum inseparability manifests itself even if no communication. By coding information on quantum fibres is used to randomly choose the interaction has the time to propagate between the bits carried by individual quantum objects – atoms, orientation of the measuring instruments two remote photons (unless at a speed faster than photons, ions, superconducting microcircuits – and light, which is ruled out by relativity). In the 1982 (polarisers) during the few microseconds by entangling these qubits to produce a quantum experiments (Fig. 2), the entangled photons were that elapse between emission of the computer, one could develop exponentially more about ten metres apart at the moment of Introduction 16 photons and their observation. We have powerful computing methods than those used on measurement, which was already sufficient to classical computers. Nobody today knows if it will today shown that entanglement can establish the relativistic separation. In the more one day be possible to entangle a large enough survive hundreds of kilometres, notably recent experiments, sources of a new type enable number of qubits to take advantage of this Communication 19 owing to an entangled photon source entangled photons to be injected into two optical exponential advantage. No fundamental physical installed on a Chinese satellite. fibres leaving in opposite directions (Fig. 3) or even law prohibits it but the technological leap required to transmit them from a man-made satellite to would seem to be immense, because one would need remote stations. Inseparability was verified at to go from the twenty or so entangled qubits Metrology & Sensors 25 distances of several hundred kilometres. Even at currently available to hundreds of thousands of such distances, everything happened as if the two physical qubits corresponding to a hundred of ideal photons were still in contact and the result of the logical qubits. The challenge is impressive but to Computing 32 measurement taken on one instantaneously affected keep our spirits up, we should remember that the the other. This is what is sometimes called “quantum situation was similar thirty years ago with regard to non-locality”. One must not however conclude that the detection of gravitational waves. Will we have an quantum inseparability allows “faster than light authentic quantum computer in thirty years? Even telegraphy” dear to writers of science-fiction, owing if not, we can be sure that there will be many “niche” to the fundamentally random nature of the applications of quantum technologies, whether in NOTE individual measurement results. This does not quantum computing, simulation, or metrology. And however prevent the creation of radically new at the fundamental level, even in the event of failure, This text uses a certain number of elements applications in the field of quantum communication, we could maybe dream that we had identified the previously published by the same author notably for new secure quantum cryptography elusive boundary between the quantum world and in “Demain la Physique”, Odile Jacob, 2005. methods, unlike classical cryptography which could the classical world?

14 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 15 QUANTUM ENGINEERING 2.0 INTRODUCTION INTRODUCTION QUANTUM ENGINEERING 2.0

Quantum manifesto The second pillar is about quantum computing As early as 1982, Richard Feynman had observed that the evolution For the past thirty years, the interest shown in quantum technologies A Round-Up of a quantum system is a problem made extremely difficult by the has constantly grown. This led the entire scientific community in exponential growth with its size of the Hilbert space and its quantum 2016 to propose a “Quantum Manifesto” to the European states. He concluded that only another controllable quantum system Commission so that a large-scale programme could be launched. A would be able to determine it. The maximum number of the most • of Quantum Technologies FET (Future and Emerging Technologies) flagship, endowed with a simple coupled quantum systems, that is two-level systems, that can billion euros over 10 years, was then decided on by Brussels. The BY be simulated with the most powerful computer today available is first calls for projects, as of 2017, are currently being evaluated. Here only 46, which proves that Richard Feynman was absolutely right! PHILIPPE CHOMAZ For thirty years, the interest shown in quantum technologies has constantly we give a first look at the quantum technologies, using the standard However, a very pleasant surprise was the fact that manipulating breakdown into four fields of activity adopted by the flagship. AND DANIEL ESTÈVE grown. Europe is investing heavily in developing them and the CEA teams are the quantum state of a quantum bits register instead of a classical (Fundamental Research Division) playing a full role in this. The first pillar concerns quantum communications bits register is a powerful resource resolving difficult problems. Not all computers are Turing machines functioning at varying speeds. The theoretical and then experimental demonstration in the 1980s The complexity classes of the numerical problems established for of cryptographic protocols with inviolability guaranteed by the laws classical computers do not apply to quantum computers and, more of quantum physics opened up a new field of cryptographic research interestingly, certain problems that cannot be handled with classical (see p. 19). In one of the protocols relatively directly inspired by the computers would be within the grasp of a quantum computer. Peter experiments which characterised the properties of quantum Shor’s 1994 invention of a quantum algorithm for factoring integers, the difficulty of which underpins universally used encryption entanglement, pairs of photons with entangled polarisations are methods, made a deep impression. A register of N quantum bits transmitted to two remote locations. Any attempt to measure these Philippe Chomaz is executive scientific The four pillars of the European program (qubits) with 2N basis states corresponding to the 2N values of a photons by an adverse party destroys the quantum entanglement director of CEA’s Fundamental Research Division. on quantum technologies. classical register and its quantum state can be an arbitrary present, provides no useful information and can even be detected. superposition of these states. © Alain Porcher CEA / IRFU Commercial quantum cryptography systems, such as those of the Swiss company ID-Quantique, are already enabling messages to be A quantum algorithm consists in applying a series of quantum logic transmitted, but do not allow large-scale encoding of gates which implements the evolution resulting from the algorithm. communications because the transmission rate of the secure bits At this stage, the intrinsic linearity of quantum mechanics enables Engineering/Control still remains too low. The development of high throughput protocols these evolution phases to be applied to an arbitrary superposition is thus an important challenge. Furthermore, photon losses in the of the basis states, or even to their complete sum. As reading the fibres limit the transmission range to a distance that is dependent Software/Theory register gives a value of only 0 or 1 for each bit, or one of the basis on the protocol, but does not exceed a few tens of kilometres on a states of the register, the art of the quantum algorithm consists in SIMULATION

COMPUTATION telecom fibre. The route to high-rate networks thus entails the concentrating the evolution towards states giving a/the solution to COMMUNICATION Education/Training

SENSING/METROLOGY development of optimised protocols in association with high fidelity a particular problem. This art is a difficult one and a quantum Daniel Estève is director of research and quantum repeaters capable of transmitting the quantum computer is not a universal machine capable of resolving all head of the Quantronics group at CEA’s Basic science information despite losses on the fibres. The research performed at problems more quickly than a classical computer, but rather a Condensed State Physics Department (Saclay CEA in quantum photonics (see p. 22) is linked to, but not only to, machine capable of efficiently resolving certain problems beyond

Institute of Matter and Radiation). © Quantum Manifesto this pillar. the reach of classical machines (see p. 40). But what interesting

ince it was created, CEA has been concerned In many areas, CEA was thus a pioneer and became a with many cutting-edge fields of fundamental reference institution. This is notably the case for those DATA or technological research which are or can areas concerned by this issue: microelectronics, prove to be important in the performance of quantum electronics, nanosciences and high- SFrance’s sovereign missions and indeed its sovereignty. performance computing. Quantum Technologies Timeline To find out more Alain Aspect also likes to recall the story of the teams from the Ecole Normale Supérieure coming to CEA/ In the 1980s, new and revolutionary ideas were Atomic Quantum Intercity Quantum Quantum Universal The FET programmes: Saclay in Claude Cohen-Tannoudji’s 2CV Citroën car launched by physicists such as Alain Aspect and his Quantum clock sensor Quantum link simulator internet Quantum computer www.horizon2020.gouv.fr/ to learn about quantum mechanics in the classes given historical experiment on entanglement and non-lo- cid73297/les-fet- by Albert Messiah, a young theoretician recently cality of photons, or Richard Feynman and his semi- dans-horizon-2020.html returned from the United States, where he had been nal article “Simulating Physics with Computers”, in- the disciple of two pillars of the Manhattan Project: troducing the notion of the quantum computer. This The FET flagship Niels Bohr and Robert Oppenheimer. Quantum was the beginning of the second quantum revolution on quantum technologies: mechanics, a science which had been born in the (see p. 11). This emergence did not go unnoticed by www.horizon2020.gouv.fr/ 1920s, was then really taking off and shaking up several CEA teams, who were already looking at cid123504/1er-appel-du-fet- science and technology. It was the beginning of the quantum electrical transport in nanostructures and flagship-sur-les-technologies first quantum revolution which was to lead to the more generally at electrical circuits in quantum -quantiques.html invention of the laser, the transistor, computers, conditions. They thus became pioneers in this new 2015 2035

atomic clocks and satnav devices or mobile phones. field which was both scientific and technological. © Quantum Manifesto

16 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 17 QUANTUM ENGINEERING 2.0 INTRODUCTION COMMUNICATION QUANTUM ENGINEERING 2.0

Fig. 1: Artwork of a “global” quantum Internet with hybrid optical and satellite links. The nodes (bright spots) correspond to ground or orbital stations for generating, storing or problems can be tackled with quantum computing? Certain The fourth pillar is called “Quantum metrology and sensors” measuring qubits coded on photons or in atoms. The links between the nodes show the photon problems of linear algebra, such as the inversion of sparse matrices, qubits distribution channels consisting of optical fibres (terrestrial) or free space (space-based). the problems of quantum learning or optimisation are more It aims to exploit a range of quantum phenomena which are known particularly explored. Determining the energies of molecules is also but which have as yet been little exploited for performing more arousing great interest, because the presence or absence of an precise measurements, in the same way as quantum entanglement electron in a molecular orbital can be naturally described by the before its benefits for quantum information had been realised. Let state of a quantum bit. Optimisation of the electronic configuration us consider the example of atomic clocks. In an atomic clock, atoms of molecules with two or three atoms was thus recently carried out following trajectories in an interferometer are probed by optical but about a hundred perfect quantum bits would be needed to tackle pulses creating transitions between atomic levels and the an interesting problem, which leads us to the problem of scaling up. interference signal obtained can be used to align the excitation frequency with that of an extremely fine atomic line, even very However, increasing the number of qubits is only of any use if the precisely at its apex. The use of collective quantum states of groups quantum coherence is maintained despite the inevitable process of of atoms in place of independent atom states is already opening up decoherence. The main scaling up challenge for quantum computing prospects for improving the sensitivity of atomic clocks (see p. 25). • based on the evolution of a register is thus to carry out this quantum In addition, in gravitational wave detectors such as LIGO and error correction which is made difficult by the impossibility of VIRGO, the use of beams of light in which the noise on the optical BY making copies of the quantum bits to be protected, an operation that interference measurement phase is compressed at the expense of ELENI DIAMANTI is precluded in quantum physics! The development of quantum bits the noise on the other phase now improves the sensitivity of the that are inherently more robust and easier to correct than those of detectors and should quadruple the effective detection volume of AND SÉBASTIEN TANZILLI today and/or an integrable architecture compatible with quantum gravitational signals in the universe. Noise compression in the signal (CNRS) error correction, are the main challenges in the field. The research measurement phase is thus a quantum technology which improves in progress at CEA is following these two strategies (see p. 32). At the signal to noise ratio of an electromagnetic measurement. The CHALLENGES & PROSPECTS Saclay, they are looking at new types of quantum bits with better research carried out at CEA in the field of quantum sensors notably quantum coherence than that of the superconducting quantum bits concerns magnetic resonance, for which spectacular progress has previously used. In Grenoble, CEA and the “Quantum Silicon” already been made (see p. 28). This could eventually revolutionise consortium are developing quantum bits manufactured with measurements in chemistry and biology, with the ultimate goal of microelectronics on silicon techniques which, once the basic detecting a single molecule or increased resolution. Progress in Quantum building blocks have been finalised, should allow large-scale integration compatible with the enormous resources required for Eleni Diamanti is CNRS researcher at the quantum error correction (see p. 36). Quantum Inforamtion team of the LIP6 laboratory at Sorbonne University and a The third pillar aims to resolve the evolution of a quantum system Cryptography steering commitee member of the Quantum Conclusion Engineering, Fundamental Aspects It means solving it through “Feynman style” quantum simulation † with another quantum system. To do this, one must find the right In association with an increased effort in fundamental Quantum communication links, both terrestrial and satellite, are beginning to & Applications Research Group (IQFA). match between interesting quantum problems and physical systems physics, these four pillars form the new quantum engineering, be deployed. The goal is ultra-secure transmission of information in response to that are easier to produce, adjust, control and measure. The an interdisciplinary mixture of research and engineering. This the threat from quantum computers and from algorithms capable of compromising simulation of important Hamiltonians of condensed matter, such second quantum revolution opens up immense possibilities, as that of Hubbard describing the dynamics of interacting electrons but we must be wary of promising too much too fast, because classical encryption techniques. Quantum communication will truly take off in a network, can thus be carried out with ultra-cold atoms on an the conceptual and technological hurdles are many and once we can assemble the building blocks for generating, transferring, storing optical array. There are not as yet any quantum problems for which difficult. But meeting this type of challenge is exactly what we and synchronising quantum information between remote sites. simulation has provided hitherto unknown information, but this expect of a research organisation such as CEA, with its could happen soon. One of the main potential benefits of simulation research engineers (unique status) and its culture, ranging Richard is to give access to the smallest scale of a quantum phenomenon, from fundamental research to the industry of the future. public life and represents a strategic challenge for Sébastien Tanzilli is Research Director. Feynman which is generally impossible on the original system. From classical communication companies, major industrial groups, banks and He heads the Quantum Information to quantum cryptography Governments. The protocols used today for with Light & Matter (QILM) team encrypting and decrypting messages use at the Côte d’Azur University and heads The means of communication and processing of increasingly complex mathematical codes with the IQFA Research Group. classical information have revolutionised society in increasingly long public keys, to keep pace with the recent decades: The five continents are connected increasing power of the (classical) computers by submarine optical cables, the world is a mesh of capable of breaking them. However, with classical terrestrial and satellite links, capable of carrying and algorithms, the confidentiality of the messages routing information with no losses and at high speed cannot be guaranteed for ever. A more effective and over virtually unlimited distances. long-lasting strategy is therefore required: Quantum However, there is a strict limitation when one physics then comes into play, to guarantee the wishes to ensure ultra-secure communication of long-distance and long-term inviolability of information. Data security is in fact present at all communications, offering solutions for private and times in a very large number of areas of private or totally confidential encryption keys [1].

18 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 19 QUANTUM ENGINEERING 2.0 COMMUNICATION COMMUNICATION QUANTUM ENGINEERING 2.0

The genesis of quantum [1] V. Scarani et al., Rev. Mod. Phys. 9, communication 1301-1350 (2009). [2] B. Korzh et al., Nat. Phot. 9, 163 (2015). The photon entanglement experiments of the 80s (see p. 11) profoundly transformed the field of [3] J. Yin et al., Science 3456, 1140-1144 Fig. 2: Coherent superposition of states, qubits and entanglement. quantum optics and opened the door to the realm (2017). The coding of qubits offers an infinity of possibilities thanks to the range of combinations given by the coherent of quantum information. From the conceptual [4] A. Martin et al., New J. Phys. 14, 025002 superposition of two basis states |0> and |1>. Let us consider a photon with horizontal polarisation after passing viewpoint, the physicists realised that the intrinsic (2012). through a polariser (where SPU represents a single photon source). If we add a half-wave plate which we rotate through properties of quantum theory could be used to [5] E. Diamanti et al., NPJ Quant. Inf. 2, an angle Ө (shown in red below), we obtain a superposition sinӨ |H> + cos Ө |V> of the horizontal |H> and vertical encode quantum information bits of use for several 16025 (2016). |V> polarisation states. This leads to a qubit of the form α |0>+β |1>, with the relative weights α and β varying with information exchange protocols initially referred to the angle Ө while satisfying the normalisation rule |α|2 + |β|2 = 1. by the generic term of quantum communication. We [6] D. Aktas et al., Laser & Photon. Rev. 10, readily talk of “qubit” (contraction of the words 451-457 (2016). Photon qubits coded on the polarisation observable are commonly used for quantum cryptography, similarly to “quantum binary digit”) and entangled qubit pairs, the time and frequency observables. The photons which carry them are emitted from a single photon source (SPU), by analogy with the usual bits (Fig. 2). From the here situated with Alice who codes the qubits on the polarisation observable via a polariser (Pol) and a half-wave plate. technological viewpoint, the encounter at the end of aims eventually to build a quantum connection on A new era, that of intercontinental quantum The blue arrow indicates the direction of propagation. the 1980s between quantum optics and non-linear optical fibre between Beijing and Shanghai (about communication, is thus opening up to researchers optics allowed the development of new single and 1,200 km). However, at present, this technology can and to “quantum engineers” (Fig. 1). entangled photon sources that were more compact, only function over distances limited to a few efficient and easy to use. Other techniques also hundred kilometres owing to the absence of secure emerged, allowing the production and operation of Outlook Alice α H + β V relays and repeaters. λ artificial emitters, such as semiconducting quantum pol 2 In order to increase the rate, range and security of “Thanks to recent Θ dots or colour centres in diamond crystals. A whole field of research is therefore being quantum communication links, current research is developed to make quantum networks more efficient experimental progress SPU Towards Bob The time has now come for the construction of true readily turning to the latest technological and increase the range of quantum communication. quantum networks, allowing the generation, innovations in photonics and micro-electronics in manipulating Such relays and repeaters have to be designed, to distribution, manipulation, storage and processing [4, 5]. This should allow the creation of true allow bipartite entangled photon states to be of quantum information, in the same way as is quantum crypto-systems, ranging from prototype to entanglement at teleported and/or stored in two remote locations, carried out in our classical networks on a daily certified devices. Thanks to recent experimental followed by the synchronised reemission of the telecommunication basis. The age of the quantum internet promises to progress in manipulating entanglement at Entanglement represents the generalisation to two or more quantum systems of the coherent superposition of states photons. The principle is intuitive: the higher the revolutionise communication, through “absolute” telecommunication wavelengths, researchers are wavelengths, defined for creation of a qubit. Let us consider a source which emits pairs of entangled photons (EPPS, see below). The number of resources involved in a network, the data security. today envisaging large-scale quantum communication most usual way of creating such a source is to use a non-linear crystal which transforms a single photon into a pair of greater the number of nodes on which it is possible protocols, whether in terms of the number of users researchers are today photons with half the energy (not shown on the figure). The pair of entangled photons must be considered as a quantum to test the presence of the quantum information. connected, or the distances separating them (Fig. 1). envisaging large-scale system, from its moment of creation up to the moments when the photons are detected, even at great distance from The situation today These nodes are either generation stations (single each other. When a measurement is made on one of the two photons, the result of the measurement on the other is photon or entangled photon sources), or information Entanglement should thus allow the development of Quantum cryptography is used to establish remote quantum communication measurement or storage stations. More specifically, certified crypto-systems or, in other words, which immediately determined. private keys, which are then used in classical quantum memories enable the qubits to be stored are independent of the hardware used (sources and protocols.” Here, a source emits an entangled photon pair (EPPS), on which the quantum information is coded on the polarisation encryption protocols. It is based on the exchange of without revealing or degrading the information they detectors). Moreover, new hybridisation ideas are randomly generated bits, except that the 0 or 1 bits observable. The pair of photons is then prepared in a well defined state |Ψa,b>, while the states of the individual carry and ideally with very good write and read constantly emerging: Some aim to introduce are now state superposition (qubits). To send qubits photons are not: we talk of entangled qubits. Experimentally, the photons are sent to two remote users, Alice and Bob, efficiency. These devices are generally based on a quantum cryptography into existing telecom over long distances, the preferred medium is the who each possess a polarisation separator cube followed by two detectors at 90° to each other. This enables them physical medium, for example crystals doped with systems [6], others envisage post-quantum solutions photon, which allows information encoding on to project the state of the received photon into an analysis basis, here the horizontal and vertical polarisations base. rare earth ions, atomic vapours at ambient based on classical cryptography which cannot at observables such as the polarisation of light (Fig. 2). By turning the half-wave plate (red arrow), they can change the analysis basis. The crucial point is that until Bob has temperature, or cold atom clouds. present be attacked by a quantum computer. Finally, made the measurement, Alice’s photon has no defined polarisation, because only the state of the pair counts from The photons are emitted one by one (colour centres from the systems development viewpoint, a drastic in diamond, quantum dots) or in pairs (parametric the viewpoint of the information. By using this strategy, the two communicating parties can reveal non-local correlations In parallel with this research, quantum communication increase in the range of quantum networks will sources in non-linear optics). Some of the various or establish secret keys of use for the cryptography operations. has just taken its first steps in space: A source carried probably involve interconnection between protocols for the quantum establishment of on a Chinese satellite has enabled entangled photons technologies, employing satellite links for large scale encryption keys thus use individual qubits, while to be distributed between two ground stations point-to-point bridges and optical fibres for others used entangled qubit pairs [1]. separated by a record distance of 1,200 km [3]. multipoint local bridges.

An already relatively mature quantum technology is Polarizing cube Polarizing cube giving rise to systems developed and commercialized Θ Θ by a few small firms. The Swiss company ID- To find out more V a EPPS b V Quantique has notably used it on several occasions in real situations, for example to collect on-line votes † Conclusion Site of the CNRS “Quantum in the Canton of Geneva. True networks are thus Alice Bob Even if years of R&D will still be needed before the general public has access to a universal quantum Engineering, Fundamental Aspects 1 currently operational in several major cities around Internet, the establishment of ultra-secure private keys between remote sites is today being seen as a & Applications” research group: Ψ = ( H H + V V ) the world, as has been the case in Tokyo since 2011 H a,b a b a b H way of bypassing the quantum computer threat which looms over classical encryption systems. 2 (about 50 km). A secure link of more than 300 km http://gdriqfa.cnrs.fr was recently demonstrated in Geneva [2]. China

20 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 21 QUANTUM ENGINEERING 2.0 COMMUNICATION COMMUNICATION QUANTUM ENGINEERING 2.0

second, temporal resolution of about ten Conceptually, the simplest and most elegant Electrical connector Fibre connector picoseconds). For the simplest detectors, the approach for generating a single photon consists in superconducting film is too thin to reliably absorb exploiting the spontaneous emission of an isolated the photons and the detection efficiency is only atom. If an atom is excited from its fundamental about 20 to 30%. In collaboration with the Dutch state to its first excited state, for example using a start-up Single Quantum, CEA designed and built laser pulse, it spontaneously relaxes to its • fibre detectors using an optical microcavity to fundamental state by emitting a single photon. A CONTRIBUTORS reinforce absorption, with record detection variant of this protocol was for example utilised by Alain Aspect to conduct his famous experiments on (Fundamental Research Division) efficiency of better than 90%. CEA is also exploring the integration of superconducting nanowire the violation of the Bell inequalities (see p. 11). It is Detector detectors on photon circuits, to take advantage of of course extremely delicate to manipulate their remarkable performance in quantum individual atoms. With a view to practical computing and simulation. applications, it is easier to use solid “artificial atoms” which imitate their properties. Quantum dots of Fig. 1: General view of a single semi-conductors (see box) here play a leading role. photon detector, with optical fibre access Photon gun and electrical contacts. Down: Electron microscope view Jean-Michel Gérard is head of the In 2000, developing a single photon source for Quantum dots of the superconducting nanowire Photonics, Electronics and Quantum quantum communications still seemed to be Engineering Department (PHELIQS) impossible, given the level of the constraints. This Selecting the right artificial atom is not however constituting the active zone of the device sufficient to manufacture an efficient source. at the Institute for Nanoscience source, a sort of “photon gun” had to emit a light (H. Machhadani, INAC 2017). Usually, spontaneous emission tends to radiate in and Cryogenics (INAC) in Grenoble. pulse at the chosen moment, which was pure (no all directions in space. For practical applications, the multi-photon pulses) and efficient (few pulses radiation diagram must be made as directional as without a photon). In most cases, it must be possible or, better yet, the entire emission must be monomode, in other words the single photons channelled in a well-defined mode of the emitted successively must be prepared in the same electromagnetic field. This point is particularly quantum state (spatial mode, polarisation, temporal crucial for quantum dots which, owing to how they wave packet, etc.). Finally, it is important for them are built, are integrated into a material with a high 2 μm to have a certain spectral agility, so that distinct APPLICATIONS refraction index. A major part of the emission from Julien Claudon is a researcher in the sources can function at exactly the same wavelength the quantum dot therefore remains trapped in the Photonics, Electronics and Quantum in quantum networks or simulators. The laser diode, material, through total internal reflection. To Engineering Department (PHELIQS) at the a key component in high-rate optical produce an efficient monomode source, it is Institute for Nanoscience and Cryogenics communications, cannot perform this new function, essential to “model” the spontaneous emission. (INAC) in Grenoble. as it generates coherent states for which the number Generating and Detecting of photons varies randomly from one pulse to the With this in mind, an approach today widely used is a Single Photon next, following the statistical Poisson distribution. to place the quantum dot in the centre of an optical 200 nm Single photon sources and superconducting nanowire detectors: Revolutionary The quantum dot, components to meet the needs of quantum communications. an artificial atom “Quantum communications A quantum dot consists of a nanometric size inclusion of a uantum communications rely on the this fault limits the range of secure quantum semiconducting material within another semiconductor with a rely on the transmission transmission of light pulses consisting of cryptographic links. A new detector, introduced in band gap. We know how to manufacture such nanostructures of light pulses consisting a single photon. This is the fundamental 2001, is based on absorption of the light by a relatively easily using semiconductor thin film deposition Q difference with optical fibre superconducting nanowire. A conductor such as this techniques. Quantum mechanics teaches us that a quantum dot of a single photon. telecommunication protocols, for which a bit is usually conducts the current with no resistance, at has a discrete spectrum of electronic states, like an atom, owing This is the fundamental carried by several thousand photons! The highly a temperature lower than its critical temperature to the confinement of the electrons in a very small volume. difference with optical specific needs of quantum communications thus (Tc ~10 K for NbN). However, in a nanowire, the These artificial atoms offer a unique range of advantages which stimulated the development of new optoelectronic energy contributed by the absorption of a single has today been clearly established: Time stability, ability to emit fibre telecommunication components, capable of emitting or detecting a photon is sufficient to locally destroy the at telecom wavelengths for dots of the right size, very pure 1 μm 1 μm protocols, for which single photon almost perfectly. superconduction. The wire becomes partially single photon emission, very fine emission line, well-suited to resistive and a potential difference leap is detected quantum optics experiments. It should however be noted that a bit is carried by several Since the 80s, we have been able to detect a single at its ends. A detector of this type can therefore only these last two properties are obtained at the cost of operating thousand photons!” photon using avalanche photodiodes but, at the function at a cryogenic temperature, but this at cryogenic temperatures (1-50K). Fig. 2: Electron microscope views of two single photon sources, using an optical microcavity with wavelengths suitable for transmission on optical drawback is easily offset by a number of remarkable Bragg mirrors (left) or a photonic trumpet (right)). The quantum dot is placed in the centre of the cavity fibre (1.3 µm-1.6 µm), these components generate a advantages (broad spectral range incorporating or inside the base of the trumpet. The red lines schematically represent the propagation of the lot of noise or spurious detection events. In practice, visible and near infrared, noise of a few counts per radiation emitted by the quantum dot.

22 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 23 QUANTUM ENGINEERING 2.0 COMMUNICATION METROLOGY AND SENSORS QUANTUM ENGINEERING 2.0

Fig. 1: Strontium optical network clock. In the centre the sample of trapped and laser-cooled atoms can be seen.

micro-cavity (Fig. 2), a sort of “photon cage”, source in 2013, the photonic trumpet (Fig. 2). The capable of confining the light in all directions, at a quantum dot is integrated into a gallium arsenide spatial scale about the same as that of its emission (GaAs) nanowire, etched in the clean room, which Up: illustration of the stress field induced wavelength. A cavity such as this confines a few in a bending photonic trumpet. forms a cone-shaped waveguide. GaAs, which has a modes of the electromagnetic field, well separated refraction index far higher than that of the vacuum, The compression zones are shown in red and from each other in the frequency space. If we tends to capture the entire emission from the the tension zones in blue. coincide the quantum dot emission frequency with quantum dot. More precisely, theoretical studies that of an optical microcavity mode, its properties Down: evolution versus time of the photon show that 95% of the photons emitted are injected energy emitted by the quantum dot in a trumpet change radically: The spontaneous emission of a photon become ten to a hundred times faster and into the guided mode of the photonic wire. A undergoing forced mechanical oscillation. suitable mirror is placed at the foot of the trumpet We observe periodic oscillation in phase with nearly all the photons are injected in the mode. This effect is known as the “Purcell effect”. This concept to redirect the entire emission upwards. The flared the mechanical vibration. enabled one of the authors to produce the world’s form of the photonic wire enables the lateral size of • first single photon source in 2001, in the Photonics the guided mode to be gradually increased, in order BY and Nanostructures Laboratory (LPN) at the CNRS. to obtain a slightly diverging Gaussian radiation ARNAUD LANDRAGIN After a long maturing phase in the LPN, this source diagram which, for example, would be well-suited (CNRS) today offers highly attractive performance and has to direct coupling to an optical fibre for quantum been marketed since 2017 by the Quandela start-up communication applications. Photonic trumpets (Orsay). have already led to the production of highly pure, highly efficient sources (0.75 photons/pulse). In Photonic trumpet addition, their emission wavelength can be modified

Owing to the spectral tuning constraint between the without degrading their efficiency (see box). The (SYRTE) Le Targat © Rodolphe next goal is to produce ultra-bright sources of emitter and the mode, microcavity sources have a Arnaud Landragin Landragin is Director entangled photon pairs, an essential component for relatively narrow spectral tunability range. To of the Time-Space Reference Systems overcome this limitation, CEA introduced a new building quantum repeaters! Laboratory (SYRTE), a joint Observatoire de Paris/CNRS/PSL/Sorbonne Université/LNE CHALLENGES & PROSPECTS research unit. Stress effects and quantum measurements in photonic trumpets Upcoming Owing to their particular shape, photonic trumpets (Fig. 3) can be easily deformed and this degree of freedom opens up numerous opportunities. By bending a trumpet, it is for example possible to induce a localised mechanical stress near its base and modify the emission wavelength of the quantum dot. In collaboration with the Néel Institute in Grenoble, PHELIQS thus obtained a single photon source Quantum Sensors that can be tuned over a remarkably broad spectral range (25 nm). This property is crucial for applications using several single photon sources, because they are required to emit at strictly identical A quantum sensor is used to measure one or more physical values by means of “Atomic clocks today wavelengths. quantum systems or phenomena. The existing range of sensors and corresponding have significant Conversely, studying the displacement of the – very fine – emission line of the quantum dot enables applications is developing rapidly. us to identify the position of the trumpet with remarkable sensitivity. In collaboration with colleagues societal impacts by from Basel University, we were able to detect the Brownian motion of the trumpet at a temperature of enabling all time 4K, the amplitude of which is only about 100 femtometres! This recent experiment, which makes full wing to their very nature, quantum atom transition. Atomic clocks today have use of the fine nature of the artificial atom’s emission line, will perhaps pave the way for the systems are highly reproducible and thus significant societal impacts by enabling all time scales worldwide development of original quantum position, force or temperature sensors. make it possible to build sensors that are scales worldwide to be kept synchronised. both accurate and extremely sensitive. Numerous applications are directly dependent on to be kept OThis is the case of sensors based on atomic transition this: Telecommunications, transports, certification REFERENCES spectroscopy, with the energy levels defined by synchronised.” of transactions and of course satellite-based nature. A known example is that of atomic clocks positioning systems (GNSS). A second iconic which, since the 1950s, have demonstrated the A highly efficient single-photon source based on a quantum dot in a photonic wire - J. Claudon, J. Bleuse, M. Bazin, N.S. Malik, P. Jaffrennou, N. Gregersen, C. Sauvan, P. Lalanne and J.M. benefits of using a quantified and perfectly example is that of nuclear magnetic resonance Gérard - Nature Photon. 4, 174 (2010) Dielectric GaAs antenna ensuring an efficient broadband coupling between an InAs quantum-dot light emitter and a gaussian optical beam reproducible system for building frequency (NMR) with more particularly its application to - M. Munsch, N.S. Malik, E. Dupuy, A. Delga, J. Bleuse, J.M. Gérard, J. Claudon, N. Gregersen and J. Moerk - Phys. Rev. Lett. 110, 177402 (2013)vv Design of broadband high-efficiency standards. This property was the basis for the imaging (MRI) offering unparalleled medical superconducting-nanowire single photon detectors - L. Redaelli, G. Bulgarini, S. Dobrovolskiy, S. N. Dorenbos, V. Zwiller, E. Monroy, J.M. Gérard - Supercond. Sci Tech 29, 065016 (2016) redefinition of the second by means of a caesium diagnosis methods.

24 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 25 QUANTUM ENGINEERING 2.0 METROLOGY AND SENSORS METROLOGY AND SENSORS QUANTUM ENGINEERING 2.0

specifically chosen to preserve quantum coherence. fundamental limit (Heisenberg limit) improving at Considerable sensitivity For example, point defects in diamond (N-V centre) Fundamental aspects the same time as the number of particles. To reach which produce “artificial atoms” can be used to The considerable sensitivity of quantum sensors The development of these quantum sensors involves these non-standard conditions, it is necessary to sound magnetic or electrical fields by combining goes hand in hand with extreme vulnerability to fundamental production and control aspects, with develop related technologies specific to each sensor, very great sensitivity with spatial resolution. sources of decoherence. Progress in understanding the aim of achieving their ultimate limit through the such as for example reading the quantum state at Similarly, progress in micro-electronics in single atom level. The final complex step lies in the “Since the 1950s, quantum mechanics but also the related development of quantum engineering protocols. For (SYRTE) © Arnaud Landragin association with cryogenic methods, reducing functionalisation of these sensors in their technologies enable decoherence to be limited and example, we can consider the case of atomic clocks, thermal decoherence sources, allow the application environment, in order to go beyond the atomic clocks have allow the development of new types of quantum the intrinsic limit of which is linked to the finite development of more complex quantum sensors: the experimental stage and make actual instruments. sensors that are constantly more efficient and number of atoms. In this case, as in that of all the demonstrated the quantum Josephson effect and Hall effect have This can be particularly demanding, especially when sophisticated. Without aiming to be exhaustive, we sensors using a set of spins in quantum superposition become the reference methods for calibration of the applications are outside physics laboratories. It benefits of using a could mention methods for cooling atoms or ions between two states, the classical standard limit is electrical value; sensors with superconducting includes technological aspects, but also systems which, by freezing their movement, have allowed the linked to the quantum projection noise when quantified and circuits (SQUIDs), by NMR or by electronic aspects for the signal to be usable by a non- development of two successive generations of far measuring the spin state and improves at the same paramagnetic resonance (EPR) demonstrate record specialist. perfectly reproducible improved atomic clocks (see box) and the emergence levels of sensitivity to electromagnetic fields across time as the square root of the number of particles system for building of the field of atomic interferometry (see box) for a very broad frequency range (see p. 28); quantum measured. The use of quantum engineering measuring inertial forces. Another solution is to optomechanical sensors allow the measurement of methods, referred to as spin compression, enables frequency standards.” isolate an ion or a defect in a crystalline matrix extremely weak and localised stresses or forces. this limit to be overcome in order to achieve the

Fig. 1 : Cold atom gyrometer

Fig. 3: Schematic diagram of an atomic interferometer Learn more using atoms in free fall manipulated by laser separators.

Fig. 2: Schematic diagram of an optical network clock. Atomic interferometry Learn more inertial sensors Atomic interferometers exploit the wave-particle duality stated by De Broglie. The atomic wave packet is diffracted by the application of a short pulse of an Atomic clocks optical standing wave and finds itself in a coherent superposition of two partial Non-destructive detection wave packets, which differ in their momentum state and which will be spatially The principle of the atomic clock is to slave the frequency separated (Fig. 3). In its simplest version, an interferometer can be produced of a macroscopic oscillator (quartz, laser, etc.) to the using three pulses, with recombination of the two partial waves at the third frequency of an atomic or molecular transition. The pulse. The phase difference accumulated along the two arms is proportional oscillator delivers the reference frequency which benefits to the acceleration in the direction of separation and the rotation around the from the intrinsic stability and accuracy properties of the normal to the oriented area. The use of laser-cooled atoms gives access to long atomic reference. In an optical lattice clock (Fig. 2), the cold interrogation times (about 100 ms) with record sensitivity levels for gravimeters atoms (blue dots) are transferred to a trap formed by an (vertical accelerometer) and for rate gyros. Numerous applications in ground intense laser standing-wave (brown). or space geosciences and in inertial navigation are currently being developed, Conventional Optical whether with atoms in free fall, as is the case here, or with trapped atoms. The reference atomic transition is probed by an ultra-stable detection array laser (red). With a very long probe time (trapping) and a high probe frequency (optical domain), this new generation of clocks is able to achieve record performance levels: 10-17 or 10-18 relative frequency. Ultra-stable cavity A new method for detecting atoms, which is non-destructive To find out more in quantum terms, will make it possible to produce and use Int Conclusion samples of 10,000 entangled atoms. The stability of the † Optical clocks: clock could thus be improved beyond the standard quantum Sensors are the often hidden building blocks in the development of new application fields. Quantum https://syrte.obspm.fr/spip/science/fop/ limit, more particularly opening the door to chronometric sensors are opening up a broad spectrum of applications ranging from high-performance equipment geodesy by direct measurement of gravity potential. for micro-electronics or life sciences (measurement of electrical and magnetic fields emitted by a cell), Atomic interferometers: https://syrte.obspm.fr/spip/science/iaci/ Laser clock inertial navigation, Earth sciences (measurement of the magnetic field, the gravity field or even gravity potential by chronometric geodesy). Laser © Jérôme Lodewyck© Jérôme (SYRTE)

26 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 27 QUANTUM ENGINEERING 2.0 METROLOGY AND SENSORS METROLOGY AND SENSORS QUANTUM ENGINEERING 2.0

Manipulation experiment of a single-spin in diamond, under confocal microscope.

© P.Stroppa / CEA Many opportunities Detection at

In addition, quantum superconducting circuits offer ambient temperature many opportunities for detecting the weak • Once collected by the resonator, the spin signal must microwave signals emitted by the spins, with great be amplified before being sent to the detection CONTRIBUTORS efficiency. An essential circuit is the superconducting electronics at ambient temperature. In general, the (Fundamental Research Division) micro-resonator schematically represented in figure 1. amplification process adds extra noise, which reduces It comprises a capacitor C in parallel with an the spectrometer’s sensitivity accordingly. Here again, induction coil L consisting of a single wire, it has a we use a tool provided by superconducting circuits; resonance frequency of ω_0=1/√LC. It functions as the Josephson parametric amplifier (JPA). This is a an antenna capturing the signals emitted by the microwave amplifier based on superconducting spins when their frequency ω_s is tuned to that of material and Josephson junctions, functioning at very the resonator by a magnetic field B_0, in accordance low temperature (10mK) and adding as little noise to with the equation ω_s (B_0 )=ω_0. The the input signal as is allowed by quantum mechanics. Patrice Bertet is a researcher particularity of the resonators used is that the wire acting as the induction coil has a very small in the Quantronics group of CEA’s transverse dimension, of about one micron or even Condensed State Physics Department less; thanks to this, these resonators are highly

(Saclay Institute of Matter and Radiation). / CEA © P.Stroppa sensitive to the spins in the cylindrical “active zone” with a radius of about 1μm around the wire (Fig. 1). Moreover, as they are entirely made from a APPLICATIONS superconducting metal (aluminium in our experiments) with virtually no loss at microwave Prototype of a dilution refrigerator cooling frequencies, these resonators have a very high figure to 30 mK experiments of quantum electronics of merit (105 or more). and of quantum superconducting circuits.

Denis Vion is a researcher in the Quantronics group of CEA’s Condensed Detecting Spins State Physics Department (Saclay Institute of Matter and Radiation). The resonance spectroscopy of electron and nuclear spins has numerous Fig. 1: On the left, the diagram of a quantum magnetic resonance spectrometer. The core is an LC microresonator of frequency ω_0 coupled to the spins with a coupling constant g. applications in a variety of fields but suffers from limited sensitivity. The use of Sequences of two microwave pulses at ω_0 separated by a time τ are sent to the resonator in order to control the spins and generate the emission of an echo at time 2τ. After amplification by superconducting quantum circuits at very low temperature is a means of a superconducting parametric amplifier, the microwave signal is detected. A typical spin echo signal is shown on the figure. significantly improving this sensitivity, up to the detection of a very small To the right, the resonator diagram. The inductive wire which defines the “active zone” of the resonator is 500nm wide and 100μm long. It is placed just above the spins. For the measurements, number of spins. the sample is placed in a copper sample-holder connected to measurement cables (see photo on left).

agnetic resonance has an important Aarhus University in Denmark, are applying the Input Daniel Estève is Research Director impact on our everyday lives, from quantum technologies of superconducting circuits and Head of the Quantronics group. medical imaging to quality control in cooled to ultra-low temperature (10 millikelvins) to beer production. This technique uses magnetic resonance. Our main aim is to improve the 1 mm 500 nm Mpowerful magnets and RF or microwave pulses to sensitivity of detection of electron spins by several Output reveal small concentrations of molecules in a 0.3 orders of magnitude. How is this possible? x100 L Micro-resonator Spins 100 μm

substance. Improving its sensitivity – by increasing (V)

Q τ τ A the signal or reducing the noise – is a means of First of all, at the very low temperatures T at which 0 g C/2 C/2 -0,8 π /2 π Ae measuring smaller quantities of material and is thus -1 our experiments are performed, the energy k_B T, 0 50 100 150 200 250 a constant quest in this discipline. 500 nm characteristic of thermal fluctuations, is well below t(µs) the energy of a single photon, at the spin resonance One general area for work is to use the fundamental C L Active zone frequency (4−8 GHz). This implies that at thermal principles of quantum mechanics to produce Parametric ultimate “quantum sensors”. Since 2015, the equilibrium, the spins are completely polarised in amplifier Quantronics group of the Condensed State Physics their lowest energy state, which maximises the resonance signal. Moreover, the thermal noise is Spins Department at the Saclay Institute of Matter and Magnetic field 12 mK B0 Radiation (IRAMIS), in collaboration with also eliminated, leaving only quantum fluctuations 100 nm researchers from University College in London and of the microwave field (empty of photons).

28 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 29 QUANTUM ENGINEERING 2.0 METROLOGY AND SENSORS METROLOGY AND SENSORS QUANTUM ENGINEERING 2.0

production of states in which the uncertainty of a 65 spins in 1 second variable is reduced at the expense of the other. These 103 Thanks to the complete polarisation of the spins, to the compressed states provide a method for taking superconducting micro-resonator used to detect them measurements which go beyond the limit of the vacuum and to the very low level of noise obtained, record quantum fluctuations. In our experiment, a Josephson parametric amplifier generates a compressed state of sensitivity is expected. In the experiment, the spins are 102 situated under the surface of a silicon substance, on the microwave field which is then sent to the (s)

spectrometer input. In this state, the fluctuations – 1 which the resonator was manufactured. These are T Fig. 2 : The relaxation time T_1 thus measured is represented Bismuth atoms situated at a depth of 100 nm (Fig. 1), which are in phase with the signal emitted by the 10 as a function of detuning _s- _0 (blue dots). The red curve which trap an electron for which one wishes to detect electron spins we are looking to detect – are reduced ω ω the spin. The spins are excited to the frequency ω_0 by while the phase quadrature noise is for its part is the theoretical curve of the spin Purcell effect, with no adjustable a series of two microwave pulses separated by a amplified. The signal-to-noise ratio of the spin parameter. The yellow curve indicates a non-radiative contribution duration τ, which causes them to emit a signal (called measurement is thus improved by 1.2 dB [3]. 1 to relaxation, which takes over when detuning is very large. -4 -2 0 2 4 spin echo) after an additional time τ. By quantifying the ω –ω /2π (MHz) signal-to-noise ratio of the spin echo measured, we Qualitatively new phenomena s s demonstrate that the spectrometer is capable of detecting about 65 spins in 1 second, which is a five By coupling the spins and the superconducting order of magnitude improvement in sensitivity by circuits, we do not only improve detection comparison with the state of the art [1 and 2]. sensitivity, we also access qualitatively new physical phenomena in which the spin dynamics are modified We have also improved sensitivity by using a purely by the interaction with the resonator. Spin “Thanks to the complete polarisation of the spins, to the superconducting quantum effect – quantum compression – which, relaxation is a good example of this. After a sequence remarkably, is also highly promising for detecting of microwave pulses, the spins return to thermal micro-resonator used to detect them gravitational waves! Even if the Heisenberg uncertainty equilibrium in a time called the “relaxation time” and to the very low level of noise obtained, record sensitivity is expected.” principle limits the combined uncertainty of two T_1, by exchanging energy with their environment. variables said to be conjugate, it does not rule out the In the usual magnetic resonance experiments, this

exchange is via vibrations in the crystalline lattice effect relaxation is the dominant mechanism in spin Cavity-enhanced radiation (phonons) or by interaction with other spins in the relaxation (Fig. 2). Relaxation of the spin echo signal is measured as a function of the waiting time, for Fig. 2: The principle of the spin Purcell effect. Free-space radiation environment. The emission of microwave photons [1] Reaching the quantum limit of sensitivity different values of frequency detuning ω_s-ω_0 A spin’s return to thermal equilibrium is by an takes place with an extremely low rate corresponding in electron spin resonance A. Bienfait, J. J. Pla, to a relaxation time of about ten thousand years. between spins and resonator. Through exponential exchange of energy with the environment. The adjustment of each curve, we obtain a measurement Y. Kubo, M. Stern, X. Zhou, C. C. Lo, C. D. Weis, dominant process is usually relaxation by emission of the relaxation time T_1 as a function of ω_s-ω_0. T. Schenkel, M.L.W. Thewalt, D. Vion, D. Esteve, of a phonon in the crystal. Although negligible in Purcell effect relaxation We note that T_1 depends significantly on the B. Julsgaard, K. Moelmer, J.J.L. Morton, P. free space, the spontaneous emission of a detuning, increasing by 3 orders of magnitude when Bertet, Nature Nanotechnology 11, 253 (2016). However, this radiative emission is significantly microwave photon may on the contrary become Phonon relaxation the spins are detuned by a few MHz. Figure 2 also exalted when the spin is coupled at resonance with [2] Inductive-detection electron-spin resonance important if the spin is closely coupled with a shows the theoretical prediction of the Purcell effect a microwave resonator (Fig. 2). This effect, predicted spectroscopy with 65 spins/√Hz sensitivity S. resonant cavity: this is the spin Purcell effect. The Saturation Magnetic field pulse δB Readout (red curve) with no adjustable parameter. Tuning is by Edwin Purcell in 1946, had – until our Probst, A. Bienfait, P. Campagne-Ibarcq, manifestation of this effect is that the spin remarkable up to detuning of a few MHz, after experiments – only been observed with atoms or J.J. Pla, B. Albanese, J.F. Da Silva Barbosa, relaxation time T_1 must be as short as possible which the relaxation time saturates owing to another semiconducting nanostructures. We have been able T. Schenkel, D. Vion, D. Esteve, K. Moelmer, when the spins are tuned to resonance with the non-radiative process which takes over (yellow Π/2 Π echo to demonstrate that, in our experiments, the Purcell curve). J.J.L. Morton, R. Heeres, P. Bertet, Appl. Phys. cavity. We measured T_1 with the sequence T 1 Lett. 111, 202604 (2017). described in the left-hand panel: the spins are first [3] Magnetic Resonance with Squeezed excited by a “saturating” microwave pulse, after Microwaves A. Bienfait, P. Campagne-Ibarcq, A.H. which they are detuned from the cavity by a quantity 3.8 MHz Kiilerich, X. Zhou, S. Probst, J. J. Pla, T. Schenkel, _s- _0 controlled by the amplitude of a pulse ω ω Conclusion D. Vion, D. Esteve, J.J.L. Morton, K. Moelmer, P. applied to the magnetic field. After having again being -1 10 † Bertet, Phys. Rev. X 7, 041011 (2017). tuned to resonance, the amplitude of the spin echo is Magnetic resonance can thus be taken to quantum conditions using superconducting circuit tech- [4] Controlling Spin Relaxation with a Cavity, A. measured as a function of the time T during which 1 MHz niques. These techniques enabled us on the one hand to significantly improve the detection sensitivity they were detuned. At resonance, relaxation takes of the spins and, on the other, to demonstrate that spin relaxation can be rapidly induced by the Bienfait, J.J. Pla, Y. Kubo, X. Zhou, M. Stern, C.C place in a few seconds, whereas it took about 1000 0 MHz 0.38 MHz Purcell effect [4]. One fascinating prospect is to achieve enough sensitivity to detect individual spins Lo, C.D. Weis, T. Schenkel, D. Vion, D. Esteve, -2 with microwave photons, which will notably open the door to a new quantum computer architecture J.J.L. Morton, and P. Bertet, Nature 531, 74 seconds when the spins are detuned by a few MHz. Spin signal (a.u.) 10 0 0.5 1 1.5 2 based on individual spins interfaced by superconducting circuits. (2016). See arxiv 1508.06148. See also Article by Time (103 s) Johanna Miller in Physics Today.

30 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 31 QUANTUM ENGINEERING 2.0 COMPUTING COMPUTING QUANTUM ENGINEERING 2.0

per logical qubit, estimated at no less than ten “It is absolutely The problem thousand for today’s platforms. Let us now look at the THE QUBIT of quantum coherence status of the solutions being tried to resolve or work essential that the

/ CEA © P.Stroppa around the difficulty. decoherence processes Maintaining quantum coherence throughout the Abbreviation of quantum bit, it is the quantum state representing performance of an algorithm is the hardest criterion Qubits research first of all naturally focused on resulting from the the smallest unit of quantum information. It is the quantum equivalent to meet when the number of entangled qubits systems with a proven quantum behaviour, such as coupling of the qubits of the classical bit. Like the classical bit, the qubit has two increases. As each quantum bit is characterised by a atoms and ions, even though these microscopic fundamental states, labelled |0> and |1>. Whereas the classical bit can coherence time, that of an entangled state with N bits systems are hard to control individually and to with their environment only exist in one of its basis states 0 or 1, a qubit can exist in a is about N times shorter. It is therefore absolutely couple. It also then looked at more controllable must be minimised. superposition of its fundamental |0> and |1> states simultaneously. essential that the decoherence processes resulting systems, such as electrical circuits, which do not for from the coupling of the qubits with their But this environment This is quantum superposition. their part generally function in quantum conditions. environment must be minimised. But this CEA is currently active in these two areas. is essential to be able environment is essential to be able to apply the logic gates to them and read them! The residual errors The platform based on trapped ions is that which is to apply the logic gates must therefore in practice be actively corrected. currently the most advanced. It uses up to about to them and read them! twenty ions held in a one or two-dimensional trap. As a functional quantum processor requires at least The residual errors must The qubits are formed with two ion electronic states a hundred logical qubits (in other words the ideal) to and are manipulated with resonant optical pulses. therefore in practice resolve a problem of interest, quantum error • The logic gates with two qubits use the coupling of the correction would appear to be essential in order to be actively corrected.” optical transitions with the collective mode of ion CONTRIBUTORS obtain them. Classical error correction is based on displacement in the trap. The characteristics of this (Fundamental Research Division) Analysis of experimental results obtained redundancy. We make a partial copy of the platform are very good quantum coherence and very with Andreev’s first quantum bit. information, for example by adding parity bits to detect an error on a small number of bits and correct high-fidelity, albeit relatively slow, logic gates. it. As it is impossible to copy a quantum state without Entangled states with about twenty qubits have been destroying it, quantum error correction is more tested and this is the current record. Processors of a delicate. If we had virtually perfect physical qubits, few tens of qubits are planned, which could also make correction would only require about ten physical it possible to carry out certain tasks more efficiently than a classical computer. A few small companies Daniel Estève is Director of Research and qubits per logical quantum bit. But the existing qubits require a far more fault-tolerant architecture. The spun off from major research centres (such as head of the Quantronics group at CEA’s CHALLENGES & PROSPECTS only known one, called the “surface code” is inspired Innsbruck university in Europe) are tackling the Condensed State Physics Department Fig.1: Top: Schematic diagram of a quantum by the work of the Russian-American physicist Kitaev challenge, although integration on a very large scale (Saclay Institute of Matter and Radiation). computer. A qubits register (systems with two and requires a very large number of physical qubits would appear to be difficult. quantum levels) is equipped with a set of quantum logic gates enabling all possible Exploring evolutions to be carried out. Each quantum bit can be read accurately. The probability of

QUANTUM BITS obtaining a 0 or 1 result corresponds to the

RESET UNIVERSAL SET OF LOGIC GATES HI-FI READING weight of the register state with this value on Quantum Computing the bit measured. Patrice Bertet is a researcher at the Quantronics group at CEA’s Condensed ? 1 1 ? Even if we do not yet have a real quantum computer technology, numerous 0 1 Gate 0 1 Bottom: Basic example of a quantum State Physics Department 0 2 bits 0 algorithm: the Grover’s search algorithm on (Saclay Institute of Matter and Radiation). avenues are currently being explored and, despite the difficulties, research has four objects. An operator marking a particular never been so active. basis state of the register must be found. Gate1 bit Gate1 bit Starting from the register in state I00>, we algorithm. It was however the invention by Peter Shor apply gates to a quantum bit which superposes From quantum entanglement (1994) of a quantum algorithm for factorisation of it on all the basis states of the register. to quantum processors numbers that was exponentially faster than the known PREPARATION CALL ORACLE O DECODING READING SOLUTION We then apply the O operator to be discovered, classical algorithms that triggered a race for physical then the decoding which provides the state Shortly after the entanglement of photon pairs was Denis Vion is a researcher at the Quantronics systems allowing quantum information to be 0 1 00 marked by the oracle, given by the final demonstrated experimentally (1982), the status of the 0 Y ϖ/2 i=00 i=01 X ϖ/2 group at CEA’s Condensed State Physics processed. The operating principle of a quantum 01 read-out. Despite a success rate that was concept of entanglement went from a strange quantum computer handling a register of qubits is schematically iSWAP Department (Saclay Institute of Matter and ? 11 limited by decoherence and 60% read-out phenomenon to a useful resource. When, in 1985, he represented on Figure 1. The criteria to be met for its 0 Y i=10 i=11 X 0 1 Radiation). ϖ/2 ϖ/2 10 wondered whether a computer using quantum physics correct working are those of DiVincenzo, named after errors, the device produced in 2012 at the SPEC could beat classical computers, David Deutsch took a the physicist who explained them. They are relatively demonstrated the quantum acceleration of basic classification problem to show how the handling hard to meet all at the same time and few attempts Grover’s algorithm over a classical algorithm 00 1 i 1 O i i of a register of quantum bits (qubits) enabled this task have succeeded in actually producing anything 2 Σ 2 Σ i which would require at least two invocations of i i to be carried out more efficiently than with a classical resembling an even rudimentary quantum computer. the discrimination function.

32 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 33 QUANTUM ENGINEERING 2.0 COMPUTING COMPUTING QUANTUM ENGINEERING 2.0 / CEA © P.Stroppa

Superconducting qubits based on Josephson 2012, built a basic processor with two qubits, or four junctions are currently the most advanced electrical basis states. This rudimentary processor utilises a Semi-conducting qubits: A highly qubits. The Quantronics team at the Condensed State true quantum algorithm, Grover’s algorithm for integrable architecture Physics Department of CEA-Saclay – which had searching for an object in a base with only one For a long time now, semiconducting nanostructures have demonstrated the quantum behaviour of a Josephson discrimination function. For four objects, this junction (a thin tunnel barrier between two reached a condition in which a very small number of electrons Making superconducting quantum algorithm ideally finds the solution with a single superconducting electrodes) coupled with a resonator (or holes) is controlled in small island isolated in bits in an aluminium evaporation invocation of this function, which demonstrates – in 1996 proposed the superconducting circuit bidimensional electronic gases or in nanowires. The first machine: introducing a sapphire quantum acceleration over a classical algorithm. This 1 known as the Cooper-pair box to produce a quantum results with these “2D gases” in heterostructures of gallium wafer used for supporting circuits. bit (Fig. 2). A team from NEC in Japan was the first was proven with the tested device, even though it was arsenide were disappointing owing to the decoherence caused 0 to achieve this in 1999, with the CEA team imperfect. Since then, more complex circuits have by the magnetic noise of the nuclear spins of the material. demonstrating the first functional electrical quantum been built (Fig. 3) and more advanced quantum More recently, the production of islands in silicon containing bit in 2002 and carrying out all fundamental protocols have been demonstrated. Circuits with few nuclear spins, and even isotopically purified to eliminate An alternative strategy: quantum operations on this unique object. The about twenty qubits developed by Google, IBM or them completely, paved the way for spin qubits with excellent Cooper-pair box is still the most widely used Rigetti Computing, which do not however have logic potential quantum coherence. The spin quantum bit of a hole the production of robust qubits superconducting quantum bit, but in its optimised gates between all the qubits, did not demonstrate a in the island of a CMOS transistor produced at CEA Grenoble To escape the complexity of the surface codes, a different “transmon” version developed at Yale University in quantum algorithm resolving a difficult problem. The in 2016 (see p. 36) showed that a quantum bit, and thus potentially a very large number, can be produced by an solution would be to have qubits with better coherence. This 2006. Coupling of the box with a microwave cavity in very recent announcements by IBM and Google about which it is placed enables the qubit to be measured industrial manufacturing line. The integration capacity of this would make error correction easier and enable small superconducting circuits with 50 to 70 qubits aim to via the small change in the frequency of this cavity, CMOS technology makes it a platform of choice for processors without error correction to be used. At Yale, prepare quantum error correction on a single qubit which is dependent on the state of the qubit. The implementing the surface codes. Since then, QuTech in Delft Michel Devoret and Robert Schoelkopf are setting up an but also to prove – with a specific problem – the analogy between this system and cavity quantum has run a small processor with two qubits made in a 2D architecture proposed by Mazyar Mirrahimi from Inria: electrodynamics developed by Serge Haroche’s team quantum supremacy of the quantum computer over silicon gas, a simpler technology but also one that is less Qubits based on entangled states of a microwave resonator “The spin at the “École normale supérieure” gave its name to the classical computer. This transmon-based integrable. The integration race is thus on for semiconductor with a very high figure of merit, for which the only quantum bit of a Fig. 2: Top: Quantronium version of the the field, circuit quantum electrodynamics (circuit architecture cannot yet however be sufficiently qubits, but it will first be necessary to develop the building decoherence mode is the detectable loss of a photon. In Cooper-pairs box developed at CEA in 2002. QED). In 2009, the CEA team demonstrated one-shot integrated to obtain circuits with a sufficient number blocks, the logic and read-out gates, that are compatible with addition, the coherence of these qubits can be maintained hole in the island With an electrical circuit, this superconducting high-fidelity read-out of the transmon and then, in of logical qubits to tackle an interesting problem. integration. actively by gently pumping the system so that it self-corrects of a CMOS device was able to reproduce the fundamental continuously. Their coherence is in the process of exceeding experiments of quantum physics. the already excellent coherence of the initial resonator and transistor The quantum bit is formed by the two lowest bringing the Yale team closer to the eternal quantum bit. A produced at CEA energy states, built from two charge states Fig. 3: Superconducting quantum processors logic gate with two qubits has even been produced, but the use of these original qubits is however highly complex. Grenoble in 2016 differing in a Cooper-pair in the island of the box, showed that a situated opposite the control grid visible at the The CEA Quantronics team is following a different path based bottom left. This island, connected by two small on the electronuclear spin levels of charged impurities placed quantum bit, Josephson junctions in parallel with a common in an insulating crystal such as diamond or silicon. The and thus electrode, has a very low capacitance (fF). quantum coherence of these impurities can reach hours and The quantum bit is manipulated by applying a the problem is to control them, manipulate them and couple potentially a very resonant microwave signal to the grid electrode. them. This research to obtain more robust qubits is supported large number, by an ANR industrial chair, partnered by Atos (see p. 43). It Bottom: Transmon version of the Cooper-pair box currently concerns the control of a very small number of can be produced currently used in superconducting quantum electronic spins coupled with a superconducting resonator by an industrial processors. A Josephson junction (cross) connected with a high figure of merit. The preliminary results in manufacturing to a far higher capacitance constitutes an Processor with four transmons (CEA, 2014). electronic paramagnetic resonance are described on page 28 anharmonic oscillator whose two lowest energy The line (connected at the bottom) performed multiplexed of this issue. line.” © D.Guillaudin / CEA levels form the quantum bit. This oscillator is read-out of the qubits by coupling to individual non-linear coupled to a microwave resonator whose relatively resonators. A resonator coupled to all the qubits enables different resonance frequency depends on the logic gates to be built between all the qubits. state of the quantum bit, which enables it to be The close-up on the left shows a quantum bit/non-linear measured by microwave reflectometry. read-out resonator block. † Conclusion Even if we do not yet have a real quantum computer technology, many avenues are nonetheless being explored today. All are faced with difficulties for which there is no viable solution. But as history has shown that all obstacles considered Processor with 19 transmons (Rigetti 19Q, Dec. 2017) to be impossible are one day overcome, research is more active than ever. At the European level, this research will be with individual read-out resonators. significantly aided by the quantum technologies flagship. CEA is involved in fault-tolerant architectures based on silicon The diagram above shows the coupled bits. CMOS technology qubits and on the development of highly robust qubits.

34 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 35 QUANTUM ENGINEERING 2.0 COMPUTING COMPUTING QUANTUM ENGINEERING 2.0

As ambitious as this coding might appear, this was The idea was then to use a fixed electron on a site • nonetheless conceived of and achieved. and code the information on its spin which can take This achievement is based on the observation that, two values depending on an axis given by a magnetic CONTRIBUTORS in our smallest transistors, the average number of field. This concept had been successfully studied in (Technological Research Division, electrons in the transistor channel can be one. The gallium arsenide based quantum dots. The Fundamental Research Division) transistor channel is a conducting wire which opens advantage of these dots is that there is less disorder or closes according to the charge that builds up on than in the channels of silicon-based transistors. the control gate (the “information”). At ambient Unfortunately, the atoms of arsenic and gallium temperature it is impossible to precisely define this carry nuclear spins. These nuclear spins fluctuate “CEA recently succeeded number of electrons, because it fluctuates and interact with the electron’s spin, thus disturbing continuously. This temporal fluctuation is necessary the spin quantum bit. in producing silicon layers for a current to be able to circulate in the wire (and on wafers 300 mm in possibly charge the gate capacitor of another Maud Vinet is head of the Logical On the other hand, silicon is an atom which does not 29 downstream transistor, thus enabling the carry a nuclear spin, except for its minority isotope diameter, with a Si Components Integration Laboratory (Silicon “information” to be propagated from one gate to the 29Si (present at 4.7% in natural silicon). It can also Components Department) at CEA/Leti. concentration of less than other in the processor). be purified to eliminate this isotope which is 0.006 %.” problematical for electronic spins. But at low temperature, when the thermal energy kBT (kB being the Boltzmann constant) is lower than the CEA recently succeeded in producing silicon layers energy e2/C, where C is the capacitance of the on wafers 300 mm in diameter, with a 29Si transistor channel (the smaller the channel the lower concentration of less than 0.006%. Macroscopic it is, down to a few tens of attoFarad), the number of measurements on very large spin populations have APPLICATIONS electrons in the channel is frozen. It is then possible shown that the spin relaxation time in 28Si is very to control the electrons in the channel one by one. Marc Sanquer is head of the CEA long at low temperature [6]. The electron spin in a Quantum Electronic Transport and 28 CEA pioneered this discovery. In 2003, single crystal of Si thus has a “spin vacuum” around it. Superconductivity Laboratory electron control was for the first time obtained in a (Nanosciences and Cryogenics Institute). controlled manner in a channel of an MOS transistor [4]. On these bases, the silicon spin qubit was developed The Search for This control had previously been obtained by several in two stages: in 2012, the first silicon spin qubits were [4] F. Bœuf, T. Skotnicki, X . Jehl and groups (including CEA) in rudimentary silicon published in Australia [7] and the USA [8]. Then, in M. Sanquer “Controlled single-electron effects transistors and/or stochastically. One could then 2014, a team in Sydney showed spin qubits with very in non-overlapped ultra short silicon field imaging coding the information on the number of long coherence times in silicon isotopically enriched to effect transistor”, IEEE transactions on the Integrable Qubit electrons in the channel of a nano-transistor at low 99.92% with 28Si [9]. The same year, two quantum bits Nanotechnology, vol2, No3, p144 (2003). temperature. This charge quantum bit has actually been coupled in the same silicon were obtained by the same [5] quantum cellular automaton CEA teams are working on transforming the iconic device of silicon technology proposed and even produced on this principle [5]. Australian group [10]. These publications show the Shi, Z. et al. Coherent quantum oscillations Silvano de Franceschi is a researcher – the field-effect transistor – into a quantum bit that could potentially be Unfortunately, this charge quantum bit, which renewed interest in the silicon spin qubit worldwide, in the CEA Quantum Electronic Transport implies coherent electron transfers between several for example INTEL, which announced that it was and echo measurements of a si charge qubit. and Superconductivity Laboratory integrated on a large scale. sites, is not stable enough. embarking on this path in 2015. Phys. Rev. B 88, 075416 (2013). (Nanosciences and Cryogenics Institute). [6] Witzel, W. M., Carroll, M. S., Morello, A., he manipulation and measurement of a small capacitors are those of the control grids of our Cywin´ski, Ł. & Das Sarma, S. Electron spin single electron spin in a field-effect field-effect transistors, the iconic component of our decoherence in isotope-enriched silicon. Phys. transistor is an astounding discovery processors. Making superconducting qubit chips: Rev. Lett. 105, 187602 (2010). which opens the door to the construction electron lithography and reading device. [7] Pla, J. J. et al. A single-atom electron spin Tof a quantum processor based on components used The electron charge is thus the basis of our qubit in silicon. Nature 489, 541/545 (2012). everywhere for classical processors. This result, electronics, but the electron has another attribute, which draws on several fields in science and its spin. Postulated about 100 years ago [2], its [8] Maune, B. M. et al. Coherent singlet-triplet technology (isotopic purification, CMOS discoverers describe [3] that a certain degree of oscillations in a silicon-based double technologies, quantum mechanics of electronic daring was required to postulate this quantum quantum dot. Nature 481, 344–347 (2012). circuits, etc.) constitutes a real feat and opens up property of electrons, because there was no classical [9] Muhonen, J.T. et al. Storing quantum many prospects. counterpart. information for 30 seconds in a nanoelectronic [1] i.e V=ne/C=1.6 10^-19 C * 10^4/10^-15 F device. Nature Nanotechnology doi:10.1038/ of about 0.16V. Manipulating large populations of spins – the nnano.2014.211 (2014) ; Veldhorst, M. et al. The bases of silicon spin qubit forgotten variable of electronics – is the domain of [2] Uhlenbeck & Goudsmit, Nature An addressable quantum dot qubit with spintronics, a rapidly growing discipline. One could All information is stored and handled on a physical fault-tolerant fidelity, Nature Nanotechnology 1926,117,264. also quite naturally imagine coding information on medium. In classical processors, the information is doi:10.1038/nnano.2014.216 (2014). [3] S.A. Goudsmit. La découverte coded by a voltage of about 1 V (or 0V) produced by the spin of a single electron. This could be seen as an du spin de l’électron. Journal de Physique, several tens of thousands of electrons that have built ultimate coding in terms of energy cost and even [10] M. Veldhorst et al., Nature 526, 410–414 1967, 28 (1), pp.123-128. up on capacitors of about one femto Farad potentially a quantum coding, because it involves the (2015).

(including stray interconnection capacitors)[1]. The quantum attribute of a single elementary particle. / CEA © P.Stroppa

36 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 37 QUANTUM ENGINEERING 2.0 COMPUTING COMPUTING QUANTUM ENGINEERING 2.0 © D.Guillaudin / CEA

been defined with a transverse cross-section of CEA’s CMOS qubit 10 nm×20 nm, covered with a grid 30 nm long. Two There are several options for the spin qubit on transistors of this type were placed in series to allow silicon: The electrons can be confined at the detection of the spins (Fig. 1). interface between the silicon and its oxide, as in This device consists of two field effect transistors CMOS technology, or around the dopants, or at a relatively isolated from the highly doped zones that buried interface between the silicon and the are the source and the drain, to avoid the dopants germanium. In the first case, a solid silicon crystal from disturbing the core of the device. The source can be used, or a fine layer of crystalline silicon on and the drain are doped with acceptors, because we an insulator, etc. Each option has advantages and are working in the valence band of silicon (with anti- “Various spin echo drawbacks and there is no guarantee that a CMOS Installation of a high-power refrigerator for cooling quasi-electrons called holes) rather than with quasi- field effect transistor – optimised for operation at silicon quantum bits and their environment manipulations, electrons in the silicon conduction band [12]. The Fig. 1 : The spin qubit with CMOS technology, ambient temperature – could, even after adaptation, of reading/control to 0.35 K (around - 273°C). spin of a hole is sensitive to the electrical field owing inspired by become a good qubit. Yet this is the approach we coloured electron microscope cross-section image to a relativistic effect called spin-orbit coupling, chose in Grenoble. electronic (after Maurand16]. The silicon channel of the while the spin of electrons is mainly influenced by transistor is shown in yellow. The two gates are in It should be recalled here that the CMOS field effect the magnetic field. In field effect transistors, we only paramagnetic blue. The source and drain contacts of thickened transistor is a remarkable object which switches in use the electrical field, which is why it is preferable resonance, allow silicon heavily doped with acceptors are in dark less than a picosecond, with an extremely low energy to also manipulate the spin with an electrical field. voltages again so that the (2,0)➝(1,0) transition is extended to several qubits, while not losing sight of grey. The red zones represent the gate oxides: in cost of a few tens of attoJoules at ambient permitted. If the (1,1)➝(2,0) transition takes place, the target of an architecture and a variability measurement cross-section, they enclose the yellow silicon wire temperature. Our challenge is to prove that the Manipulating a quantum bit we transfer a hole, otherwise there is no transfer. compatible with the manufacture of several on three faces and they can be seen in dark grey transistor technology - not its principle - can be By sufficiently repeating this initialisation / thousand or even several million qubits on the same of the spin “under” the wire, in fact behind it. adapted and thus extremely useful for developing The principle of manipulating the quantum bit is on manipulation / detection cycle, we thus transform chip. The next steps are: coherence time The purple zones are the silicon nitride spacers the quantum bit. We are therefore starting from this the whole as follows: DC voltages are applied to the the spin orientation information under gate 1 into a To measure the spin more rapidly using the grid designed to keep the heavily doped source and technology rather than from a blank page. two grids to attract a hole under each grid. More measurable drain-source current. reflectometry technique which does not use the and enable it to be specifically, we ensure that the configurations with drain-source current measurement, which is too drain away from the channel zones under and In 2016, CEA produced the first CMOS technology Various spin echo manipulations, inspired by compared with the a hole on each gate (1,1), two holes on gate 2 (0,2) slow; between the gates. The light grey background based qubit [11]. This was a field effect transistor electronic paramagnetic resonance, allow and (1,0) are equivalent in terms of energy. In this Rabi frequency.” represents the silicon oxide which encapsulates based on Grenoble’s FD-SOI (Fully-depleted Silicon- measurement of the spin coherence time and enable To obtain coherent interaction between several case, a hole current circulates from the drain (slight the device. The scale bar is equal to 50 nm. on-Insulator) technology in which a nanowire had it to be compared with the Rabi frequency. We quantum bits, first of all neighbours and then at positive polarisation) to the source by the following observe a coherence time of 300 ns for a Rabi longer range; sequence: (1,0)➝(1,1)➝(2,0)➝(1,0), at least when frequency of 80 MHz, in other words we can turn To use isotopically purified silicon to increase the the spin orientations are anti-parallel in state (1,1). the spin 24 times before losing the quantum coherence time; If they are parallel, in state (2,0), they must occupy coherence of the spin. the same orbital (the channel is so small that only To adapt the technology and the architecture, to one orbital is energetically available), which is ruled be able to electrically address a large number of out by the Pauli exclusion principle. This puts an The problems to be resolved qubits. We know how to address each of these end to the hole transfer between drain and source. points, but the road to achieving a quantum We can thus be sure that the spins of the two This proof of concept of a spin qubit in a device processor is a long one, whichever basic quantum entirely built using FD-SOI technology must now be bit is envisaged. trapped holes in (1,1) have parallel orientation if we wait a little (“initialisation” phase). We then rapidly Gate 1 Gate 2 change the DC voltages so that state (1,0) is energetically prohibited, states (1,1) and (2,0) [11] A CMOS silicon spin qubit, R.Maurand, remaining equivalent and, now, isolated X. Jehl, D. Kotekar Patil, A. Corna, (“manipulation” phase). We then add an RF voltage † Conclusion H. Bohuslavskyi, R. Laviéville, L. Hutin, Source Drain to gate 1. Thanks to the spin/orbit coupling and to Despite their qualities, the performance of CMOS transistors and processors are today saturated and S. Barraud, M. Vinet, M. Sanquer, Channel the correct choice of microwave frequency (about new means of processing information must be found in order to continue to progress. S. De Franceschi - arXiv:1605.07599 Nature 20GHz), the spin of the hole under gate 1 turns as a Communications 7, Article number: 13575. function of the length of the RF pulse applied with Quantum information is one of these means. Naturally, the power of classical processors has already a frequency called the Rabi frequency (about set the bar very high. Quantum computing, which is based on radically new principles, offers hope [12] We’re talking about quasi-electrons 80MHz in our best case). When the two spins are that this bar can be exceeded. The spin qubit in silicon, the newcomer in this vast field of quantum because they are operating 50nm parallel, the transition (1,1)➝(2,0) takes place. engineering research, is progressing fast and can draw on a widely developed technology. in silicon and not in a vacuum. Finally, in the “detection” phase, we adjust the gate

38 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 39 QUANTUM ENGINEERING 2.0 COMPUTING COMPUTING QUANTUM ENGINEERING 2.0

APPLICATIONS z 2 θ ψ • QUBIT CONTRIBUTORS The Benefits of A qubit can be viewed as a unit-length (Technological Research Division, Q6 Q1 vector, pointing from the origin towards a Fundamental Research Division) y point at the surface of a Bloch's sphere. Quantum Computing φ The values of a conventional bit Q5 Q4 Q3 x correspond to North and South poles If quantum computing today arouses such high hopes and leads to so many and the state superpositions correspond Q7 Q2 announcements from the IT industry, it is that the construction of machines to all the other points of the surface . applying these principles could allow the exploration of problems that are H H Christian Gamrat is scientific difficult or impossible to resolve with classical computers. What are these manager at the Architecture Design and Embedded Software Department problems? Or, to put it another way, which problems are today limited by the gigantic computing capacity of our most powerful computers? 0.4 (DACLE) at CEA/List. 100 0.2 © MuncherOfSpleens, Creative Commons CC BY-SA 3.04 BY-SA © MuncherOfSpleens, Creative Commons CC 50 0 0 Among the leading fields of application considered The situation is very similar for nuclear physics and, -0,2

for quantum computing are physical problems in general, for any system based on quantum -0,4 considered to be difficult using classical approaches. physics. -0,6

This difficulty is often linked to the number of (hartree) Energy Christophe Calvin is head of the IT, variables which is excessively high or which grows Machine learning & optimisation -0,8 numerical simulation and exponentially with the size of the problem posed. If it is often possible to tackle these problems with -1,0 supercomputing sector at CEA/DRF. The technologies related to artificial intelligence are atomistic simulation approaches, although the -1,2 currently enjoying rapid growth, more particularly precision of the results and the computing times are 0 1 2 3 4 deep learning. Of the many aspects of this field, rapidly constrained by the limited capacity of Interatomic distance (Å) learning is often mentioned as a potential classical computers. application for quantum computing. Learning Quantum chemistry techniques usually consist in minimising an error Fig. 1: Calculation of an elementary molecule function between the desired output states obtained (H2) by a quantum computer: energy versus Simulation in chemistry is a prime example of this. by descending a gradient in a very large dimension interatomic distance (experimental results: black states space and adjusting a considerable number of Most computational chemistry problems consist in dots – quantum simulation: red curve) [7]. determining molecular orbitals, calculating their parameters. Moreover, this minimisation must be spatial and energy distributions and the properties of carried out for a number of configurations (the examples to be learned) which is also gigantic: Often he foundations of quantum computing classical computers – with which it would be the fundamental states of a molecule. From the several million. This leads to highly iterative were laid by Richard Feynman in 1982 possible to build a “universal quantum computer” calculation of these values we can then deduce and learning algorithms for which the execution times [1], in an article in which he wondered [2]. He remarked that such a construction would predict the stability and reactivity of a molecule and [1] R. P. Feynman, “Simulating physics can be calculated in days or even weeks, depending about the possibility of simulating offer remarkable characteristics that would be all the important properties for explaining its effects with computers,” Int J Theor Phys, vol. 21, in a wide variety of applications (pharmacology, on the complexity of the problems, even on Tphysical phenomena, time, probabilities and impossible to reproduce with any Turing machine, catalysis). The quantum wave function (Schrödinger extremely powerful classical machines. In this type nos. 6-7, pp. 467–488, 1982. quantum mechanics with a computer. In order to do by introducing quantum parallelism: “A method by function) enables this information to be obtained, but of problem, the quantum approach could allow a [2] D. Deutsch, “Quantum theory, the this, he wondered whether it were possible to which certain probabilistic tasks can be performed [5] P. W. Shor, “Polynomial-Time this is a calculation that is impossible with a classical more efficient exploration of the states space by Church–Turing principle and the universal produce a probabilistic simulation of a quantum faster by a quantum computer than by any classical Algorithms for Prime Factorization approach other than by means of a certain number of utilising quantum superposition. quantum computer,” Proc. R. Soc. Lond. A, system using a classical computer? He commented: restriction of it”. Deutsch concluded his article by and Discrete Logarithms on a Quantum “If I try my best to make the equations [of quantum approximations. Moreover, the size of the problem vol. 400, no. 1818, pp. 97–117, Jul. 1985. noting that the existence of such a machine implied We can see that machine learning is a particular Computer,” Aug. 1995. mechanics] look as near as possible to what would increases with the complexity of the molecule and the the existence of programmes capable of performing problem of optimisation with respect to an error [3] D. Deutsch and R. Jozsa, “Rapid be imitable by a classical probabilistic computer, greater these approximations then become. We are [6] C. S. Calude, “De-quantizing the any physics experiment. This statement was to be function on the desired output. However, there are solution of problems by quantum I get into trouble” [1]. He therefore concludes that typically in a classical modelling approach where, as solution of deutsch’s problem,” enhanced later on by Deutsch and Josza [3] who computation,” Proc. R. Soc. Lond. A, vol. the best way of simulating a quantum system would the real problem cannot be resolved in all its numerous very useful applications, sometimes used Int. J. Quantum Inform., vol. 05, no. 03, introduced one of the first examples of a quantum 439, no. 1907, pp. 553–558, Dec. 1992. be to do it using a device which itself obeyed the complexity, it is necessary to employ a phase with a on a daily basis, which are faced with a similar pp. 409–415, Jun. 2007. algorithm running exponentially faster than any laws of quantum mechanics, thus producing what simplified representation of the problem (the model) optimisation problem on a set of constraints: Traffic [4] D. P. DiVincenzo and IBM, classical deterministic algorithm. David DiVincenzo [7] A. Kandala et al., “Hardware-efficient he called a “universal quantum simulator”. to find an approximate solution. Owing to its timetables, traffic management, personnel “The Physical Implementation of Quantum was subsequently to set out the first conditions for potential computing power and exponential capacity timetables, problems of scheduling and planning in variational quantum eigensolver for small Computation,” Fortschritte der Physik, vol. Three years later, in 1985, David Deutsch proposed the practical construction of a quantum computer, for storing information, quantum computing means general. As for other applications, the quantum molecules and quantum magnets,” Nature, 48, nos. 9-11, pp. 771–783, Sep. 2000. a quantum generalisation of the computing model in the form of five criteria known as the “DiVincenzo that direct simulation of these complex molecular approach could prove to be extremely useful once vol. 549, no. 7671, pp. 242-246, Sep. 2017. of the “Turing machine” – which is the basis of criteria” [4]. systems can be envisaged. the size of the problem increases exponentially.

40 - Révolutions Quantiques Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Révolutions Quantiques - 41 QUANTUM ENGINEERING 2.0 COMPUTING COMPUTING QUANTUM ENGINEERING 2.0

Called "Atos Quantum Learning Machine", this Shor algorithm and access large amounts of simulator is based on an universal language propelled Cryptography confidential data relating to electronic commerce, by a new ultra-compact supercomputer. among others. The quantum computer thus made a When Peter Shor published an article showing how it much trumpeted entrance into the world of would be possible to carry out the prime factorisation computing and cyber-security. Faced with this of integers in a polynomial time with a quantum potential risk, the NIST (American organisation computer [5], particular attention has been focused whose role includes standardising cryptography on the quantum computer. The principle of breaking systems) launched an initiative at the beginning of “The owner of a down a sufficiently large integer into its prime factors 2017 to call for studies on “post-quantum” is the basis of the RSA encryption algorithm cryptography algorithms, in other words which are quantum computer commonly used to exchange confidential data on the capable of withstanding attacks by future quantum could “break” the internet. The owner of a quantum computer could machines. Such algorithms do exist, showing that the RSA code in a “break” the RSA code in a reasonable time using the quantum computer also has its limitations. reasonable time • using the Shor Programming a quantum computer BY algorithm and access © Atos PHILIPPE DULUC large amounts of The effective creation of the applications mentioned If everything goes well, the final measurement will (ATOS) above obviously implies that one could have an ope- give a solution to the problem with a probability that confidential data rational quantum computer and the ability to pro- increases with the extent to which the fidelity of the gramme it effectively. For the time being, a method quantum calculation has been conserved throughout relating to electronic for programming an application on a quantum com- the series of steps. It is however clear that to in- commerce, among puter could be schematically represented by the crease the reliability of the final result, this series of following steps (Fig. 2): steps must be repeated a sufficient number of times. others.” 1. Prepare a set of qubits in an initial state appro- We can see that this series of steps implies a part priate to the problem to be solved; given over to classical computing for the preparation Philippe Duluc is Chief Technology 2. Apply a sequence of quantum operations to this and measurement phases, such that the execution Officer “Big data & Security” at Atos. set of qubits; of a quantum computer programme would necessa- 3. Measure a (classical) probability of the final state. rily be a hybrid: its quantum part can thus be seen as an accelerator and its classical part as an opera- Quantum Computing tions supervisor. The overall performance of the Implementation quantum computer will then be limited by the per- formance of its classical part. This is a quantum version of the famous Amdahl‘s law (principle set for Tomorrow? Initialisation out by Gene Amdahl in 1967 indicating that the per- formance of a parallel code is always limited by its The potential of quantum computing needs no further demonstration, but the serial part). “Until now, the Allocation of jobs road ahead is a long one. In collaboration notably with CEA, Atos is investing The operating mode of quantum computing is radi- heavily in developing tomorrow’s quantum technologies. computing power Preparation of initial cally different from classical Von Neumann compu- doubled every ting. For example, on a classical computer, the result quantum states ndustry and research always need more inversion, list search or combinatory optimisation is deterministic, whereas for a quantum computer, computing power. Until now, this power algorithms. 18 months in Conventional it is probabilistic. Another notable difference is that doubled every 18 months in accordance with Quantum calculations parallel code one cannot copy the value of a qubit. Quantum pro- accordance with Moore’s law. Today however, technology can Atos, the European computing leader, provides its gramming is thus completely different from a clas- Ino longer keep up, with chip etching now reaching customers with the best technology and permanently Moore’s law. sical programming approach and the resolution of Measurement of the states 5 nanometres at best. The miniaturisation of silicon anticipates the opportunities and challenges to the initial problem must be looked at in a completely come. This is why the group was the first in Europe Today however, Probabilities chips, with a crystalline lattice of 0.3 nm, would take new way. to launch an internally-funded dedicated us into the atomic dimension, where the quantum technology can no programme so that its customers could benefit from Paradoxically, when the mode of resolving problems properties which had hitherto been constructive will Interpretation of results a disruption that will probably be a major one. Atos longer keep up.” is reviewed to adapt them to quantum computing, become destructive. To go further, Atos has already Quantum was thus launched at the beginning of classical algorithms are also improved [6] with their delivered the BullSequana, which will be capable of 2016 by Thierry Breton. It covers two areas. The Final result limits being pushed back slightly further. Conventional Quantum achieving one exaflop/s (1 billion operations per main one, in the short term, aims to provide a second), as of 2020. For the next generation, new In short, we could envisage accelerating certain programming, optimisation and emulation platform identified problems using carefully produced quan- architectures will be needed. Quantum technologies for quantum algorithms, through a simulator called Fig. 2: Schematic execution of a quantum application: the quantum steps (in yellow) tum circuits and appropriate programming means. could provide a part of the answer: The possibility Atos Quantum Learning Machine (Atos QLM). The are integrated into a classical execution sequence (in gray). However, the quantum computer – still hybrid – is of acceleration has been demonstrated not only for Atos QLM was developed in 18 months and not yet ready to replace the classical computer. factorisation, but also on machine learning, matrix delivered to the first customers at the end of 2017.

42 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 43 QUANTUM ENGINEERING 2.0 COMPUTING OUTLOOK

These include the American Oak Ridge research qubits…) and business applications. There can be no centre. It is accelerating the development of quantum development without simultaneous quantum algorithms by enabling them to be progress in these three fields. The software and optimised and tested up to 40 qubits by emulation application fields remain insufficiently developed, a on a specially modified in-memory computer. The true terra incognita, where specialists are few and eventual goal is to offer simulation capability that is far between. We are aware of the physical limits • as realistic as possible, according to the quantum encountered by hardware technologies with regard BY hardware used. To do this, we will incorporate to quantum decoherence: Reducing the depth of a ROLAND LEHOUCQ “A quantum computer specific characteristics into the simulator quantum circuit brings down the execution time and (connectivity, gates design, topology, etc.) along thus the noise. This can be done by software, hence (Fundamental Research Division) comprises three parts: with noise models, which offers an incalculable the importance of optimisation and algorithm Hardware (physical qubits, advantage for the software and hardware innovations. developers. Explosion of a massive star (supernova) ejecting Some algorithms benefit from no quantum lasers, cooling, control its nucleosynthesis products at high speed. The second area is preparing for the post- acceleration. Consequently, the universal quantum electronics, etc.), software BullSequana period and aims to provide quantum computer loses some of its attractiveness. The future (algorithms, programming, acceleration capacity applicable to certain computing platforms will tend more to offer algorithms. As with supercomputers from Intel or heterogeneous capacity employing several Roland Lehoucq is an technologies (CPU, GPU, FPGA and quantum testing, optimisation, ARM for example, Atos will rely on QPU supplier astrophysicist (Institute for Research partners, depending on whoever supplies what we accelerators). This evolution also constitutes a major into the Fundamental Laws control of qubits…) want, and will build its own machines. At CEA, this challenge for the software. For any application, one of the Universe) at CEA and teaches and business applications. concerns the Quantronics teamv (Fundamental will have to pick and choose from a library of The Universe Research Division) whose experience and skills are algorithms, some being accelerated by quantum or at the École Polytechnique. There can be no quantum unanimously recognised and with which an others by GPUs. An application could be broken down © Laurence Honnorat / Utopiales 2014 development without Industrial Chair has been set up, as well as the Leti into several modules addressing different technologies, (Technological Research Division) on silicon qubits with new data exchange and synchronisation problems is all quantum simultaneous progress which constitute a more recent and promising that the developers will have to tackle. It is clear that technological avenue (see p. 36). the arrival of quantum computing on hybrid platforms in these three fields.” is a major challenge in itself, a new frontier, for the The Universe has a history and we can trace its main steps. That is certainly one A quantum computer comprises three parts: software and the system, and one in which Atos of the greatest discoveries of the 20th century. It reveals a considerable surprise: Hardware (physical qubits, lasers, cooling, control intends to play a leading role. For CEA, this means a electronics, etc.), software (algorithms, truly complementary approach enabling us together to There was a time when the atoms of which we are made did not exist and their programming, testing, optimisation, control of cover the entire quantum computing field. production is the result of quantum phenomena.

bout 13.8 billion years ago, the Universe primordial elements, hydrogen and helium. The was filled with light and elementary emergence of the heavier nuclei owes much to “Without stellar © Belga particles: Protons, neutrons, electrons, quantum physics. nucleosynthesis and neutrinos. Its expansion cooled this very At the end of the 19th century, physicists were unable hotA gas, allowing the existence of atomic nuclei, supernova, our Universe to explain how the Sun could have continue to shine stable combinations of protons and neutrons. This for more than a billion years – the age that the nucleosynthesis period lasted only a few minutes, would be an extremely geologists then gave the Earth – at the rate being leaving behind only the lightest ones, those which observed. In 1920, the British physicist Arthur uninteresting place and contained few protons and neutrons. For the Eddington (1882-1944) published an article entitled formation of the heavier nuclei, it was necessary to The Internal Constitution of the Stars in which he would only contain two wait until favourable conditions reappeared, with far wondered about a source of energy capable of primordial elements, more time being given over to the nuclear reactions. explaining the considerable age of the Sun’s activity. Thierry Breton, CEO of Atos, during the launch of the Atos Quantum It was the heart of the stars, nuclear reactors in Using the very recent result obtained by his hydrogen and helium. program on 4 November, 2016. which matter is confined by gravitation, that was the compatriot Francis Aston (1877-1945), who had just crucible for nuclei, ranging from carbon to iron. The shown that the mass of a helium-4 atom was 0.8% The emergence of the most massive stars, when they explode violently, less than that of four hydrogen atoms, and bearing in Scientific Sterring Committee of the Atos Quantum Program eject their nucleosynthesis products at high speed. mind the equivalent between mass and energy heavier nuclei owes much Thierry Breton, CEO of Atos, with Serge Haroche, Prix Nobel de These supernovae gradually enriched the interstellar demonstrated by Albert Einstein (the famous E = mc2!), to quantum physics.” Physique, Cédric Villani, lauréat de la Médaille Fields, Daniel Estève, medium with heavy nuclei which were to be used in Eddington showed that the stars could shine for a Alain Aspect, David DiVincenzo, Artur Ekert and the members the composition of future stars. Without stellar long time owing to the energy given off by the of the leading team of Atos Quantum, during the launch of Atos nucleosynthesis and supernovæ, our Universe would transformation of four hydrogen nuclei into a Quantum program, on 4 November, 2016. be a boring place and would only contain two nucleus of helium-4. © Atos

44 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 45 OUTLOOK OUTLOOK

To accept once and for all that fusion reactions were the nuclear attraction. Thanks to quantum at work, one must also understand how two nuclei, mechanics, Gamow showed that the alpha particle carrying a positive electrical charge, can get had a slight but non zero probability of crossing this sufficiently close together to merge, despite their energy barrier. Thanks to this “tunnel effect”, he electrical repulsion. Crossing this energy barrier is calculated a relationship already observed possible provided that the nuclei are fast enough, in experimentally and linking the lifetime of the other words, hot enough. The problem is that at the radioactive nucleus to the energy of the alpha particle “In a few tenths temperature at the heart of the Sun, about 15 million emitted. He thus showed that the tunnel effect plays degrees, the average thermal energy is 1,000 times a role in nuclear fusion, by significantly lowering the of a second, the less than the height of the barrier to be crossed. No energy actually needed to cross the electrical matter reaches matter! After all, not all gas particles have the same repulsion barrier. The work was completed in 1939 energy: Some have more, others less, and the by the German physicist Hans Bethe (1906-2005), the incredible density temperature of the gas, give or take a constant, is who explained the transformation of one of the simply the measurement of their average energy. In protons into a neutron via weak interaction, making of one million tonnes George Gamow (1904-1968) practice, the number of particles with a given energy the deuterium nucleus, the product of the fusion of per cubic centimetre, decreases so fast (exponentially) that none of the two protons, the first step towards the production of George Gamow, born Gueorgui Antonovitch particles of the Sun has the energy required to cause helium-4. the equivalent of Gamov (Георгий Антонович Гамов), nuclear fusion… In classical physics, the fusion The evolution and the fate of a star depend primarily is an American-Russian theoretical reaction proposed by Eddington is therefore on its mass. This is what sets the central an oil platform physicist, astronomer, cosmologist and impossible in solar conditions! thermodynamic conditions and thus the makes it compacted into the Subrahmanyan Chandrasekhar popularizer of science. possible for thermonuclear fusion reactions to take (1910-1995) He has proposed the theory of Quantum physics enters the scene place in the heart of the star. During most of its life, volume of a thimble!” Subrahmanyan Chandrasekhar is an a star makes up for its radiative energy bleed by alpha-radioactivity with quantum astrophysician and a mathematician The answer was to come from quantum physics and drawing on its resources of nuclear energy. The mechanical tunneling. He has worked on the American physicist of Russian origin George depletion of hydrogen triggers the gravitational of Indian origin. He won the star formation, stellar nucleosynthesis, Gamow (1904-1968) who studied the emission of a contraction of the core and the energy given off takes Nobel Prize for physics in 1983. elementary nucleosynthesis following the helium-4 nucleus by a radioactive nucleus, called over from the nuclear source. The heating and He is known for having defined over what Big Bang, cosmic microwave background, alpha decay. According to classical mechanics, this compression resulting from this contraction allow the limit a white dwarf becomes unstable and genetics at molecular level. spontaneous emission is impossible because the beginning of a new fusion cycle which burns the under certain conditions and collapses alpha particle is incapable of leaving the nucleus nuclei produced by the previous cycle. Contraction in a neutron star, initiating the supernova owing to the great amount of energy needed to escape then stops because the energy given off by the nuclear reactions means that the star is once again at This situation occurs in the least massive stars, such process. equilibrium. The more massive a star, the more its as our Sun, which could only achieve the fusion of This limit is now named core temperature will reach high values after its hydrogen to helium-4 followed by that of helium-4 to gravitational contraction and the heavier the nuclei carbon-12. However, stars with a mass at least eight Chandrasekhar mass. that could merge will be. times that of the Sun will be able to trigger a series of Average star Red giant Planetary nebula White dwarf fusion cycles leading to the formation of iron. The appearance of iron by the fusion of silicon marks the Quantum physics is back beginning of a process which will lead to the However, gravitational contraction does not destruction of the star. As the iron nucleus is the most necessarily lead to the beginning of a new fusion cycle bound of the stable nuclei it is energetically sterile because the electron gas in the central regions may and its fusion will not compensate for the energy the undergo a quantum effect which will stop the star continuously radiates from its surface. The star contraction and heating up of the core. When the is then once and for all short of nuclear fuel. density is high enough for the average distance between electrons to be smaller than the – quantum When gravity “The Chandrasekhar mass Neutron star – wavelength associated with them (known as the de Broglie wavelength), the description of the gas must overcomes quantum effects is reached when the average take account of quantum effects. In the case of an In 1930, the Indian astrophysicist Subrahmanyan speed of the electrons tends electron gas, the exclusion principle proposed by the Chandrasekhar (1910-1995) studied the gravitational towards that of light, in other Austrian physicist Wolfgang Pauli (1900-1958), stability of a star with no internal energy source and stipulates that two electrons cannot occupy the same supported by the pressure of a degenerate electron words when the density quantum state. In other words, two electrons cannot Stellar nebula gas. He showed that the star became unstable when of the electron gas is such that simultaneously have the same position, the same its mass exceeded a limit, now known as the its Fermi energy is approximately energy and the same spin state. In this situation, the Chandrasekhar mass. This limit is reached when the electron gas is said to be degenerate and even totally average speed of the degenerated electrons tends that of the mass energy degenerate when all the quantum states are occupied towards that of light, in other words when the density of an electron.” up to an energy known as the Fermi energy, and of the electron gas is such that its Fermi energy is Massive star Red super giant Supernova Black hole empty beyond. Contraction then stops because the approximately that of the mass energy of an electron. pressure of a degenerate gas is far higher than that of The electron pressure then becomes unable of a usual gas in the same conditions. supporting the weight of the outer layers of the star:

46 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 47 OUTLOOK OUTLOOK

“We are all made of stardust. We could also add that we are all the children of quantum physics.”

Gravity overcomes the quantum effects. As the fusion In passing it heats the matter to temperatures in of silicon progresses, the mass of iron increases excess of a billion degrees and causes fusion reactions inexorably and ends up attaining the Chandrasekhar producing heavy elements, in particular radioactive mass, thus causing the gravitational collapse of the nickel and cobalt. When the shockwave reaches the heart of the star. The conditions are such that the surface, the temperature suddenly rises and the photons break the iron nuclei, thus precipitating the entire star explodes, ejecting its component elements collapse of the core, stimulated by electron capture at speeds of up ten several tens of thousands of by the nuclei transforming the protons into neutrons. kilometres per second. This event, called a “type II This nuclear reaction is accompanied by an emission supernova”, marks the death of a massive star, but its of neutrinos, which carry the phenomenal quantity of influence on the interstellar medium will be felt for potential gravitational energy given off by millions of years: The explosion propels the nuclei contraction. In a few tenths of a second, the matter synthesised during the life of the star, as well as those reaches the incredible density of one million tonnes produced by the passage of the shockwave. per cubic centimetre, the equivalent of an oil platform Supernovae gradually enrich the interstellar medium compacted into the volume of a thimble! with new nuclei which will enter into the composition of future stars and any planets they may have. They New nuclei to enrich the Universe were absent at the beginning of the Universe and today represent only 2% of the nuclei: From the If you want to find out more about quantum The heart of the star, now consisting of neutrons, is nuclear viewpoint, the Universe has evolved very mechanics: Follow the on-line reduced to a small sphere about ten kilometres in little. The nuclei encountered on Earth all come from masterclass by Étienne Klein and Roland Lehoucq! diameter, with a mass about that of our Sun: A stellar nucleosynthesis and iron only comes from neutron star is formed and the remains of the supernovæ. This astounding fact has led to the This masterclass was recorded at the École Polytechnique on 29th May 2018 (only in French) collapsing star crash onto its rigid surface. The accurate statement that we are all made of stardust. resulting violent compression produces a shockwave We could also add that we are all the children of cea.fr/go/masterclass-physique-quantique which rises out through the outer layers of the star. quantum physics.

48 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 49 INFORMATION / SUBSCRIPTION INFORMATION / SUBSCRIPTION

#67

COMING IN DECEMBER 2018

#66 THE MEDICINE OF JUNE 2018 THE FUTURE

Clefs CEA N° 66 - June 2018

Review published by CEA Communication Division Head office 91 191 Gif-sur-Yvette Cedex - FR #66 Tél. : (+33) 1 64 50 10 00 Executive Publisher Xavier Clément

Editor in Chief Laetitia Baudin [email protected]

Editorial Board Cécile Castille, Etienne Klein, Sophie Martin, Eric Proust, Gérard Sanchez, Malgorzata Tkatchenko

Iconography Thinkstock, Wikipedia

Subscription If you want to subscribe or order an issue: Subscription (printed French version) (print only in French) to Clefs CEA is free. Requests for Send your surname, name, postal and e-mail addresses to: subscriptions should be addressed, preferably via Internet, by using the form [email protected] available at the site: www.cea.fr #65 #64 or by fax to: 01 64 50 20 01 Translated into English by AéroTraduction.

ISSN 1625-9718 With the exception of illustrations, all information in this issue of Clefs CEA may be freely reproduced, in whole or in part, subject to agreement by the editors Clefs CEA (Web) Find out this issue and the previous ones on: and mention of the source. Production & Design www.cea.fr/english/Pages/resources/clefs-cea.aspx This issue may not be sold. Agence Heidi www.agence-heidi.fr © 2018 CEA RCS Paris B 775 685 019 Print Head office: Bâtiment Le Ponant D, Imprimerie de la Centrale - Lens 25 rue Leblanc, 75015 Paris Printed on recycled paper.

50 - Quantum Revolutions Les voix de la recherche - #66 - Clefs Clefs - #66 - Les voix de la recherche Quantum Revolutions - 51 To find out more or discover all our thematic dossiers

www.cea.fr