TThhee QQuuaannttuumm TTiimmeess APS Topical Group on Quantum Information, Concepts, and Computation
Spring 2007 Volume 2, Number 1
Quantum Information and Quantum Foundations I sense a new interest in quantum foundations among people Changes afoot… working in quantum information. There are at least two motives There are a few subtle (and for this. First is the recognition that the ideas one finds in the perhaps a few not-so-subtle) standard textbooks are not quite what we need for quantum changes that mark their appearance information. Understanding the time development of systems that with this issue. The first and are constantly interacting with experimental probes and the hopefully most aesthetically environment requires more than cross sections and perturbation obvious is that I have endeavored theory. Second is the desire to use quantum information theory to to make some changes to fonts for tackle and, hopefully, resolve the well-known conceptual headings and in the masthead. My difficulties of Copenhagen – I use it as a convenient albeit hope is that it not only looks more inaccurate abbreviation for what one finds in current textbooks – professional but is a bit easier on quantum mechanics. the eyes. I think both motives are valid. But I also worry that instead Another change that should be of cleaning up the conceptual mess of quantum foundations, we fairly obvious from the start is that may simply end up by importing it into quantum information, we have essentially settled into a building a new structure on top of old, flawed ideas, and quarterly schedule here at The confusing not only ourselves but also the computer scientists, Times, and thus have opted to drop electrical engineers, mathematicians, and others now interested in the specific month in favor of the our discipline. I already see signs that this is going on, and I am ‘season.’ As such, this is the concerned. Spring 2007 issue (my apologies to As someone who has worked for many years in quantum readers from the Southern foundations, let me admit that the field deserves some of the bad Hemisphere – I had to draw the reputation it has acquired in the broader physics community. line somewhere and the equator There has been lots of work but not all that much progress in seemed a good place to do it). resolving the central issues, most of which date back to the good Very roughly we expect to put out old days of the 1920s or 30s. Disagreement is more common than an issue toward the end of each agreement among the practitioners. Sometimes it seems more ‘season’ which is really no like academic philosophy – where some critics think we belong – different than we’ve been doing: than like physics. Despite this I believe there are useful lessons to May, July or August, November, be learned from previous work in quantum foundations. Even the and February. failures can teach us something, and there have been some You will notice that once again interesting ideas that are worth further development. Perhaps the we have a section for letters. I am first and most obvious lesson is that the foundational problems hoping that each issue contains a are not trivial. Do not let the failures of Einstein, Feynman, robust letters section so please Schrödinger and Wigner discourage you from attempting your write to me (e-mail is preferred). own attack on these matters, but do expect to have to give them Specific information on sustained, serious thought. submissions can be found on the One of the few points of widespread agreement in quantum last page of the newsletter. In foundations is that measurement, despite its presence in every addition, a section of short news textbook, is a most unsatisfactory foundation for interpreting items makes a permanent return quantum mechanics. Everyone thinks the Copenhagen approach (well, at least until we change our results in a nasty unsolved “measurement problem,'' even if there minds). is little agreement on what to do about it. No doubt the textbook approach assigns correct probabilities to measurement outcomes -Ian T. Durham, Editor (the “pointer positions'' in the archaic terminology of a discipline Department of Physics that predates the computer age). But what does the pointer Saint Anselm College (continued on next page)
1 position tell us about the microscopic system that nonlocal influences are needed. It is simply a was (supposedly) being measured? I call this the matter of statistical correlation, and obviously does first measurement problem, and will return to it not conflict with relativity. later. The second measurement problem has to do The quantum case can be worked out in a with finding a fully quantum mechanical similar way. Alice and Bob share an entangled description of a real measurement process carried singlet state of two spin- "0 = ( 00 # 11 ) 2 out in the laboratory using equipment made of half particles (qubits). Alice measures S for her atoms that (presumably) follow quantum laws. z particle, and if the pointer points up, concludes that There are compelling arguments that a consistent Sz was +1/2, corresponding to the state 0 just quantum description of real measurements is ! impossible within the Copenhagen framework of before the measurement took place. From this and wavefunction collapse . Yes, POVMs have the information provided by "0 she can infer that been considered, and no, they do not help. To my S = "1/2 for Bob's particle. This is a correct z ! mind the failure to solve the second problem is inference whether or not Bob carries out a particularly serious, as it indicates a basic measurement on his particle, as long as he does inconsistency in the textbook approach. The hope nothing to perturb !it s spin. If we understand the that things can somehow be salvaged usi!ng a wavefunction as somehow representing genuinely classical apparatus fades with each information, there is no danger of falling into the advance in creating more and more exotic nonlocality trap and supposing that Alice's entangled states in the laboratory. So I ask – and it measurement has an instantaneous influence on is a serious question – do we really want to Bob's particle. Instead, her measurement provides construct the edifice of quantum information her with information about the state of her particle theory with measurement as one of the axioms? before the measurement, and hence with And if not, what are the alternatives? information about the correlated state of Bob's There is at least one idea coming out of particle. In this respect the quantum case is no quantum information theory that I think could be different from its classical analog. But in what helpful in quantum foundations: the notion that a sense does " represent information? I use the wavefunction can represent information, rather 0 term pre-probability in Sec. 9.4 of my book  than physical reality . While this does not by (hereafter referred to as CQT). That is, , while itself solve the measurement problem, it could be "0 not itself a probability, can be used to calculate helpful in disposing of a somewhat different ! conceptual mess: the nonlocality ghost. It is probabilities, in particular the joint probability widely believed that when Alice makes a distribution of Sz values for Alice's and for Bob's measurement over here on her half of an entangled particles, respectively. This !is the counterpart of state there is an instantaneous, superluminal, the joint probability for colors of the two slips of nonlocal influence that somehow changes Bob's paper used in the classical analog discussed above. half, no matter how far away. There are theorems Thus information, in the broad sense of statistical showing that the nonlocality ghost cannot transmit correlation, is playing a similar role in both cases. information, which is to say that it can never In the background I hear someone shouting, manifest itself in experiments. The only thing it is “How dare you claim that Alice's measurement capable of doing is causing confusion, and for this revealed the value Sz had before the measurement reason it needs to be permanently laid to rest, was made. That just isn't good quantum especially as it has given rise to the notion that mechanics!'' Well, it certainly is not good quantum mechanics is incompatible with special Copenhagen (i.e., textbook) quantum mechanics, relativity. which is indeed very difficult to merge with a Can we deal with this problem using the notion of the wavefunction as information. But it notion of wavefunction-as-information? I think so. is good physics if we assume that Alice is a To see how, consider the classical analog in which competent experimentalist who knows how to Charlie places a red slip of paper in one opaque construct a piece of apparatus which will measure envelope, a green slip in another, and after mixing the z component of angular momentum of a them up mails one envelope to Alice in Atlanta and particle. Do you think those folks are crazy who the other to Bob in Boston. If Alice opens her take results of their particle detectors and envelope and sees a red slip she can instantly extrapolate the tracks backwards to a point where – conclude, as she knows the protocol, that the slip in so they claim – a collision occurred between an Bob's envelope is green, independent of whether he electron and a positron? Let me suggest, at the risk has opened or ever will open his envelope. No of losing some friends, that the problem is not with
2 the way experimentalists think about real reasoning. But what constitutes sound quantum measurements. What is wrong is a textbook reasoning? Here quantum foundations provides a approach that invokes measurements, but then useful idea. In 1936 Birkhoff and von Neumann cannot tell you the connection between the  proposed that quantum reasoning would work outcome and the measured property. better by modifying the rules of propositional If we can talk (in consistent quantum terms) logic. Now some folks treat any suggestion that about the Sz that Alice's apparatus has just rules of logic be changed as a frontal assault on measured, we are equally free to talk about z for Rational Thought, a grave threat to all of Western Bob's particle before he measures it, or whether or Civilization. If you feel that way, I suggest you not he measures it. It is important to notice that take a look at this paper, written by two of the great mathematicians of the 20th century. Reading it is "0 provides information (in terms of probabilities) about Bob's particle, and only not easy, but getting the general drift is not indirectly about the outcome of some future difficult. Irrational it surely is not. While the measurement. A wavefunction provides Birkhoff and von Neumann proposal has not (yet) ! information about what it is about, not about what caused the collapse of Western Civilization, it has it is not about. If you want to discuss in quantum also not (yet) solved the conceptual difficulties of terms the outcome of a future measurement, the quantum mechanics, despite a lot of effort in the discussion should include the wavefunction quantum foundations community. This failure may describing the apparatus. This can be done; see simply reflect the fact that we physicists are not Chs. 17 and 18 of CQT for a consistent analysis in smart enough to make full use of radical new ideas. fully quantum terms of how a (properly Perhaps in a few decades the artificial intelligence constructed) quantum apparatus functions the way of robots, which some of my colleagues assure me experimentalists think it should. If Bob is a good will soon exceed that of human beings, can do a experimentalist using the right equipment, he will better job. (And if that doesn't suffice, how about artificial quantum intelligence?) learn from looking at the pointer the value of Sz for his particle before the measurement took place, In the meantime it is worth considering a much something Alice already knows if she has less radical proposal by Omnès and me (see CQT) measured her particle. What if Alice decides to which allows one to talk in a consistent way about at least some aspects of the microscopic quantum measure Sx instead of Sz? In this case the same pre- world without falling into paradoxes. The basic probability " can be used to calculate a new set 0 idea is quite simple. Consider a spin-half particle of probabilities in the same fashion as before. See whose properties, such as S = +1/2 or S = +1/2, Chs. 23 and 24 of CQT for details and an explicit z x correspond to one-dimensional subspaces or rays in demonstration that neither the choice nor the the two-dimensional Hilbert space; each ray ou!tc ome of Alice's measurement has the slightest corresponds to a point on the surface of the familiar effect upon Bob's particle. This yields a very short Bloch sphere. In! c lassical phy!si cs whenever A and proof that the nonlocality ghost cannot transmit B are two properties of some physical system, the information: it does not exist. conjunction A AND B makes sense, though it may Why don't textbooks talk about properties of be something that is never true; e.g., A = “energy quantum systems? Why introduce measurements, less than 5 J'' and B = “energy more than 10 J.'' which our students immediately (and correctly) But consider the quantum conjunction recognize as a very odd way to do physics? The
same reason your parents insisted you be home by S = +1/2 AND S = +1/2 (1) nine in the evening: to keep you out of trouble. x z Quantum foundations research has uncovered all sorts of dangerous logical paradoxes hiding like What does it mean? It surely cannot correspond to alligators in the conceptual swamp underneath the some property of the particle, for every property is ! Copenhagen interpretation of quantum theory. To associated with a ray, and every ray (every point on the Bloch sphere) has the meaning for avoid falling prey to them, stay safely on the Sw = +1/2 classical, macroscopic side of the measurement some direction w in space, so there is none left over process: look at the pointer, and don't ask what it to represent (1). If, on the other hand, you assume means. that (1) is always false (so its negation is always Logical paradoxes arise through faulty true) you will soon be in lo!gi cal difficulties (Sec. 4 reasoning, and they are easily avoided by not of CQT) if you follow the usual rules – this is what reasoning: Shut up and calculate! A better Birkhoff and von Neumann were concerned about. approach is to replace faulty reasoning with sound For these reasons, among others, Omnès and I
3 consider (1) to be meaningless, in the precise sense References that Hilbert space quantum mechanics assigns it no meaning. The rules of sound quantum reasoning,  The essential difficulties are discussed in E.P. according to us, require the use of meaningful Wigner, Am. J. Phys. 31, 6 (1963). For a statements, which means, in particular, avoiding thorough analysis, see P. Mittelstaedt, The combinations, made using AND or OR, of Interpretation of Quantum Mechanics and the incompatible propositions, those for which the Measurement Process (Cambridge University corresponding operators do not commute. This is Press, Cambridge, 1998). an example of what we call the single framework  C.M. Caves, C.A. Fuchs and R. Schack, J. rule. In some sense it is just the logical counterpart Math. Phys. 43, 4537-4559 (2002); quant- of the well-known result in quantum theory that ph/0104088. when A and B are operators representing physical  (CQT) R.B. Griffiths, Consistent Quantum quantities, the same is not true of AB unless it is Theory (Cambridge University Press, 2002) equal to BA. and http://quantum.phys.cmu.edu. Though the single framework rule seems  G. Birkhoff and J. von Neumann, Annals of restrictive, it is much less so than Copenhagen. We Math. 37, 823 (1936); John von Neumann can talk about what a measurement measures, i.e., Collected Works, edited by A. H. Taub what the pointer position is telling us. (Macmillan, New York, 1962), Vol. IV, p. Wavefunctions can provide information about 105. microscopic quantum properties, not just about  R.B. Griffiths, “Quantum Mechanics Without outcomes of future measurements. On the other Measurements,” quant-ph/0612065. hand, the single framework restrictions are very effective in getting rid of the alligators. They quickly starve when you stop feeding them meaningless quantum nonsense. A number of paradoxes are studied in Chs. 19 to 25 of CQT, and in every case the supposed inconsistency arises Bits, Bytes, & Qubits from some violation of the single framework rule, Quantum news and notes i.e., from faulty quantum reasoning. To summarize, I think quantum information 0 Theodore Harold “Ted” Maiman, 1927 – will advance more rapidly with fewer difficulties, and be much more accessible to people coming to 2007. As we began assembling this issue of the subject from outside physics, if we replace the The Quantum Times, word came that Ted Maiman, creator of the world’s first working internally inconsistent and confusing measure!m ent framework of current quantum textbooks with laser, passed away on May 5 at his home in something better. A more extensive discussion of Vancouver, British Columbia from what I think is better will be found in . If you complications due to cancer. A native of Los can do even better than that, so much the better. I Angeles, he received his undergraduate degree would be delighted to see courses in standard from the University of Colorado in 1949 quantum mechanics taught from an information before moving on to Stanford where he theory perspective provided measurements are put received his master’s in 1951 and his doctorate in their proper place: processes to which the same in 1955. Ted then moved on to Hughes quantum laws apply as to everything else, and not Research Laboratories (now HRL – see page unanalyzable axioms as in the Copenhagen 9) where a laser he built, based on a synthetic tradition. Finally, the wavefunction as information ruby crystal that was grown by Ralph is a good idea if we make it clear that it provides Hutcheson, first became operational on May information about whatever the wavefunction is 16, 1960. This capped nearly a decade of about, not (primarily, at least) about the outcomes work by the likes of Willis Lamb, Alfred of a future measurement that may or may not take Kastler, Charles Townes, Arthur Schawlow, place. and Gordon Gould, who first coined the acronym LASER (Light Amplification by –Robert B. Griffiths Stimulated Emission of Radiation) in a Department of Physics conference paper in 1959. Gould also fought a Carnegie Mellon University protracted legal battle with Maiman, Hughes, Bell Labs, and the US Patent and Trade Office that lasted into the 1980s. Maiman left
4 Hughes soon after building his laser to work at is, and this is what the new Toshiba system Quantatron, whose laser assets were purchased does. In essence it produces weaker decoy in 1962 by Union Carbide in order to form the pulses that enables the transmitter (Alice) to Korad corporation with Maiman as head. detect the siphoning by the eavesdropper Selling his stake in Korad to Union Carbide in (Eve). This is because the decoy pulses are 1968, Maiman founded his own company. He weaker on average and thus will rarely contain was twice nominated for the Nobel Prize in two or more photons. Toshiba has reportedly Physics for his work on the laser, won the demonstrated a one-hundred-fold increase in Wolf Prize in Physics in 1983, was awarded the rate at which keys could be securely the APS’ Oliver E. Buckley Prize in transmitted over a 25 km-long fiber to 5.5 condensed matter physics in 1966, received kbits/sec. The work was actually part of an the 1987 Japan Prize, and was a member of EU initiative to build a secure communication both the National Academy of Sciences and network based on QKD. the National Academy of Engineering. Look for a short history of the now ubiquitous and 1 0 Call for nominations. The 2007 Quantum indispensable laser to appear in the next issue Information Processing and Communication of The Quantum Times. (QIPC) Young Investigator’s Award, sponsored by Qurope (QIPC in Europe), is 1 0 John Backus, 1925-2007. Who amon!g us accepting nominations. The award will be over the age of 30 (maybe even younger?), presented to an outstanding young researcher whether engineer, physicist, or mathematician in the field during the QIPC conference, in (and most certainly computer scientist), hasn’t Barcelona, in October 2007. The award ! been exposed to Fortran at some point in our consists of a diploma and a lump sum of careers? Sadly, the man who developed the 3000€. Eligible researchers must be less than formerly ubiquitous programming language 35 years old on the 1st of October 2007. passed away in March at the age of 82 at his Nominations, including self-nominations, home in Oregon. In its obituary in March, The should be submitted to L. Theussl New York Times referred to Fortran as “the ([email protected]), the QUROPE first successful higher-level language.” Administrative Officer, by 31 July 2007. The Despite flunking out of the University of nominations should include a short CV of the Virginia and then being drafted in 1943, candidate, a letter containing a one page Backus went on to earn a master’s degree in summary of the candidate's achievements, a mathematics at Columbia in 1950 (thanks in list of key publications, and at least two letters part to high scores on his Army aptitude tests). of endorsement. The letter should be prepared It was in New York that he landed a job at in pdf-format. Candidates are encouraged to IBM when he wandered into corporate also submit an oral presentation at the QIPC headquarters and a tour guide asked him what conference in Barcelona in autumn 2007. he was studying. In addition to Fortran, Additional information can be found on the Backus co-developed with Peter Naur a Qurope website at http://www.qurope.net. notation for describing the structure of programming languages, somewhat akin to 1 0 Deutsch’s algorithm via cluster states. A grammar for spoken languages. joint project of Queen’s University in Belfast and the University of Vienna has successfully 1 0 Loophole closed. Toshiba announced in created a quantum computer using highly late February that they were able to clo!s e a entangled multi-partite quantum states known loophole in practical quantum key distribution as cluster states which is more practical and (QKD) systems. Previously, the weak laser efficient than the standard logic-gate approach ! diode that was used to create the photons used that is similar to classical computing networks. to distribute the keys would sometimes Deutsch’s algorithm is a specific case of the produce pulses that contained multiple more general Deutsch-Josza algorithm used to photons, making it possible for an solve what is known as Deutsch’s problem eavesdropper to siphon off one of these extra which very roughly involves querying a photons, thereby obtaining at least a portion of register about the result of some calculated the key. While guaranteeing single photon function. pulses is not feasible, fooling the eavesdropper
5 1 0 2007 Gödel Prize recipients announced. Conference Announcement The 2007 Gödel Prize, awarded for Quantum error correction “outstanding journal articles in theoretical computer science” by the European ! Quantum error correction of Association for Theoretical Computer Science decoherence and faulty control operations (EATCS), has been awarded to Alexander A. forms the backbone of all of quantum Razborov and Steven Rudich for their paper information processing. In spite of entitled “Natural Proofs,” that appeared in the remarkable progress on this front ever since Journal of Computer and System Sciences in the discovery of quantum error correcting 1997 and was first presented in 1994 at the codes a decade ago, there remain important Twenty-sixth Annual ACM Symposium on open problems in both theory and Theory of Computing in Montréal. In short, applications to real physical systems. In their paper provides fairly strong evidence that short, a theory of quantum error correction no natural proof exists that separates P and NP that is at the same time comprehensive and computing problems since, if such a proof did realistically applicable has not yet been exist, it would violate the conjecture that discovered and thus remains a very active pseudorandom number generators exist. area of research. Briefly, P and NP refer to computing problems The First International Conference on that can be solved quickly on a classical Quantum Error Correction, hosted by the computer (P problems) and problems whose USC Center for Quantum Information solutions can be quickly checked on a classical Science & Technology (CQIST) and computer (NP problems). One of the most organized by Daniel Lidar (Chair), Todd important unresolved questions in theoretical Brun, and Paolo Zanardi, will bring together computer science is whether P and NP are a wide group of experts to discuss all aspects identical or not. More information on the of decoherence control and fault tolerance. award and the winning paper can be found on At this point in time the subject is mostly the EATCS website at http://www.eatcs.org. theoretical, but the conference will include A brief but accessible discussion of the role talks surveying the latest experimental quantum computing may play in these sorts of progress, and will seek to promote an problems can be found on pages 40-42 of interaction between theoreticians and Quantum Computation and Quantum experimentalists. Information by Michael A. Nielsen and Isaac A list of many of the topics that will be L. Chuang (a book I am sure most readers are considered can be found on the conference well-acquainted with). website, http://qserver.usc.edu/qec07/, that 1 also contains full details of the conference itself. In brief, the conference will take place during the week of Dec. 17, 2007. It will start with a series of tutorial lectures by Dave The lighter s!i de Bacon (Washington), Daniel Gottesman (Perimeter Institute), Raymond Laflamme (Waterloo), and Lorenza Viola (Dartmouth). It will feature keynote talks by David Cory (MIT), John Preskill (CalTech), Peter Shor (MIT), and David Wineland (NIST). Registration is now open and the number of spots is limited, so hurry!
–Daniel Lidar Departments of Electrical Engineering and Chemistry University of Southern California
My kids in APS shirts – oh, if you only knew…
Editorial Entropy Random thoughts & updates Quantum Communication Letters to the editor From the classroom to Denver and back In our last issue I wrote an editorial that Further comments on our name touched on some pedagogical issues and noted As one of the original organizers of GQI, I think that I was teaching a course in quantum it would be a big mistake to change the name so as to mechanics this semester. The course caused only include quantum information. When we sent me to think extensively not just about how best around the petition for people to sign, we advertised to teach it but what to teach since it has been ourselves as an inclusive organization, for people my opinion since graduate school that the “old” working on all fundamental aspects of quantum way of teaching (including some of the topics) mechanics, since there was no place else in the APS was outdated. Luckily this first batch of for such people to go. There are people in the group students were receptive and flexible, helping who are working on all sorts of fascinating topics that me feel out a more modern approach basically have nothing to do with information. For appropriate for undergraduates. example, there are many people working on the I had numerous conversations at the March various interpretations of the theory, on using down Meeting in Denver about these and related conversion to perform experiments on the uncertainty issues that were both useful and enlightening, principle, teleportation, entanglement swapping, the and I intend to continue to facilitate such AB effect, etc. There are people interested in discussions, hopefully leading to some concrete entanglement as such, such as Werner and GHZ accomplishments potentially including web- states, entropy of entanglement, purity of based resources for teachers of quantum entanglement, and entanglement as a resource mechanics. I will include a more detailed exclusive of its use as an information resource. update in a future issue, but other ideas being There are experiments in mesoscopic physics, such as pursued include a workshop or conference the diffraction of large molecules. There are all kinds dedicated to discussing these issues (an idea of questions in BEC and gravity that impinge on that first surfaced during my interview of Bill quantum concepts. And of course there are people Wootters last summer for The Times) and, who work on all aspects of Bell-type theorems. potentially by spring of 2009 (and despite the In short, quantum information is just one facet of fact that some of my colleagues disagree with a very big field, and to consider it as our only source me on the need for this), a new textbook that of strength is sort of like the tail wagging the dog. transforms the actual pedagogy while being Right now it is a very hot topic. and lots of people are open-ended enough to suit a variety of teaching working in it. But it has also led to new ways to styles, topics, etc. analyze quantum theory and it may very well lead to new insights that have little relevance to information Hey, I needed to put something here… as such, as we perceive it today. Personally, I would In the November 2006 issue, in a rather like to see more people enter the group, so that silly article, I noted that my father – a retired someday we might even aim for divisional status. high school English teacher – might qualify as That way we can raise much more money for our having an Erdös-Bacon number. My father’s own awards, and recommend more of our own people Erdös number was never in question (see the for APS fellowships, and have more influence in the November issue), but since Bacon numbers physics community, etc. It makes no sense to drive require acting in a film and not onstage, there people out of the group because they perceive us as was some question. But, apparently, he really not being relevant to their research. It is also a did appear in an independent (though never- breach of trust to the people who originally signed released) film with his former student David the petition. Moreland (who was in Donnie Darko among I think we should bury the issue. It isn't even other things) and so his Erdös-Bacon number is fruitful to talk about it. It's only divisive. at least as legitimate as Hank Aaron’s (the Erdös portion of which was “earned” by Sincerely, signing the same baseball as Erdös). Dan Greenberger, CCNY [email protected]
7 cheap plastic solar cells. He illustrated the Back to Baltimore significance of this discovery by showing off a quantum-related acronyms gather working prototype with roughly the thickness and flexibility of a sheet of glossy paper. CLEO, QELS, and PhAST meeting report In Tuesday's sessions on Entanglement and The joint CLEO, QELS, and PhAST Squeezing, Ben Brown (JQI/NIST) described an conferences were held 6-11 May 2007 in atom-interferometer based on number-squeezed Baltimore, Maryland. Conference-goers were states in an optical lattice. Dzmitry Matsukevich favored by warm, sunny weather all week, (Georgia Tech) outlined recent progress towards allowing pleasant reprieves from the over-air- the realization of quantum repeaters based on conditioned Baltimore Convention Center when a quantum ensembles, and described the few moments could be spared between talks. entanglement of atomic qubits separated by more This year's Quantum Electronics and Laser than five meters. During the Cold Atoms session, Science (QELS) conference offered up an Tanya Zelevinsky (JILA/Colorado) described astounding array of experimental and theoretical experiments on coherent manipulation and topics, ranging from new technologies based on precision measurement of ultracold atoms in quantum devices to deeper conceptual issues in optical lattices. Kurt Gibble (Penn State) asked quantum information theory. Faced with the what the difference between a photon's momentum impossible task of giving a comprehensive and an atom's recoil should be, and surprised many description of the many talks presented, I will members of the audience with the revelation that instead provide a "sample platter" based on my they are not the same! Interestingly enough, an own interests. In no way should this be construed atom absorbing a photon from a laser receives a as an endorsement for the talks I have included – smaller momentum kick than (naively) expected, or, more importantly, as a judgment against those I due to the Gaussian profile of the beam. have omitted – rather, the selection process may be Wednesday morning, CLEO plenary speaker considered representative only of my own William Phillips (JQI/NIST) provided a beautiful capricious nature. [Editor’s note: Capricious explanation of how photons can be made to carry nature of author not verified.] For more detailed orbital angular momentum, and provided several information, visit the conference website at examples of the demonstration of this http://www.cleoconference.org. phenomenon in the lab. QELS plenary speaker Sir Jeff Kimble (CalTech) kicked off Monday's John Pendry (Imperial) gave the QELS plenary on Cavity QED sessions by conjecturing on the use of the realization of negative-index metamaterials quantum networks to distribute specially prepared and recent experimental progress in the field. quantum states, and the eventual realization of a During Wednesday's Symposium on "quantum software" industry. In his hour-long Degenerate Fermi gases, Hrvoje Buljan (Zagreb) tutorial, he provided an extensive overview of discussed experimental realizations of Bose gases recent advances in quantum network technologies, confined to one dimension in the Tonks-Girardeau from measurement-induced entanglement to (i.e., "faux fermionic") regime, and recent reversible state transfer between light and atoms. advances in solving these systems numerically. In the Integrated Nanophotonics session, Luke Later, during the Entanglement session, Carlton Bissell (Inst. of Optics) gave a fascinating Caves (New Mexico) described methods for description of a room-temperature single-photon reaching the Heisenberg measurement limit for source based on a quantum dot submerged in a different experimental setups, then provided a liquid crystal. During Quantum Key Distribution, general proof of the limit, independent of the Hideo Kosaka (Tohoku/CREST-JST) discussed the experimental realization. demonstration of the coherent transfer of photonic Thursday was a busy day for quantum qubits to an electron spin for use in semi-conductor information. The Quantum Information session quantum repeaters while Taehyun Kim (MIT) began with Patricia Lee (JQI/NIST) speaking on described the experimental realization of an the experimental implementation of a square-root entangling probe attack on the BB84 protocol. At SWAP gate in a double-well optical lattice. This Quantum Dots, Eric Gansen described the photon- was followed by my own talk on a proposal to use number-resolving capabilities of a quantum dot a double well potential to prepare entangled atoms photodetector. for use in a loophole-free Bell inequality test. In the evening, Alan Heeger (UCSB) gave a Eugene Polzik (NBI/Copenhagen) spoke about CLEO plenary speech on the use of Bucky Balls experimentally teleporting the quantum state of a and semiconducting polymers in the production of
8 pulse of light onto that of an atomic ensemble at room temperature. Position Announcement During Quantum Communication, David Quantum error correction Fattal (HP Labs) spoke on a proposal to use a micropillar cavity coupled to a waveguide for a The Computational Physics Group at HRL quantum repeater system with high operating Laboratories has an immediate opening for a efficiency, despite realistic imperfections. Christian Research Staff Member in the area of quantum Bonato (BU/Padova) discussed the use of Earth-to- information processing. The candidate will be satellite links for global quantum communication responsible for the analysis and design of fault networks and considered compensation systems for tolerant quantum error correction methodologies imperfections in the satellite pointing mechanism. for semiconductor-based quantum information In Quantum Computing, Manny Knill (NIST) gave processing. This will primarily involve the an overview of the status of quantum computing simulation and analysis of physically motivated with special attention paid to tolerable error Hamiltonians, decoherence-free subspaces and thresholds. He took a balanced position on the fault tolerant quantum error correction circuits. question of what an acceptable error threshold The ideal candidate will have proven should be, warning against both overly optimistic experience in these areas and, additionally, and pessimistic predictions. Interestingly enough, experience with the analysis of time dependent he also expressed skepticism about verifications of spin Hamiltonians, construction of Hamiltonians two-qubit gates that rely on tomographies, claiming for low-dimensional semiconductor systems, he wasn't satisfied that anyone had demonstrated a modeling of semiconductor nanostructures, two-qubit gate yet! Julio Gea-Banacloche symbolic computation and Mathematica (Arkansas) gave an interesting talk on the programming. He or she will also have a working limitations encountered when "recycling" the knowledge of the theory of quantum error electromagnetic fields for quantum gates, correction and fault tolerance, solid state explaining how error probabilities scale worse than implementations of quantum information one would superficially expect as the number of processors, basic semiconductor physics, physical reuses increases. modeling and simulation, discrete mathematics, Post-deadline QELS sessions ran Thursday and numerical methods for scientific computing. evening from 8:00pm to 10:00pm, including such A Ph.D. in Physics, Electrical Engineering, topics as high sensitivity magnetometry using an Applied Mathematics or Computer Science is alkali vapour cell, a CNOT gate using linear optics required and postdoctoral or equivalent work in the telecom band, a fibre-based entangled experience is desirable. photon source, and slow-to-fast light switching in a HRL Laboratories, LLC is a corporate R&D quantum well semiconductor, just to name a few. laboratory owned by Boeing and General Motors. Friday's session on the Dynamics of Dots, HRL provides custom R&D and performs Wires and Tubes included interesting talks on spin additional R&D contract services for its LLC relaxation times in dots by Evegy Zibik (Sheffield) Members, for the U.S. government, and for other and Yasuaka Masumoto (Tsukuba), as well an commercial entities. Overlooking the Pacific intriguing presentation by Dawei Wang (Queen’s Ocean in the coastal community of Malibu, [Canada]) on the advantages of solving the California, HRL Laboratories provides an ideal semiconductor Bloch equations in the exciton environment for you to apply your scientific basis. Shortly after, I was lured away to the session knowledge and abilities. Our organization offers a on Miscellaneous Nonlinear Optics for a talk by competitive salary and benefits package. Bahram Jalali (UCLA) on "Energy Harvesting in If you are interested in applying for this Silicon Photonics," in which he described the use position, please go to our website at: of nonlinear effects in silicon to obtain a two- http://www.hrl.com, go to Careers, Open photon photovoltaic effect. By this point, my brain Positions, and then click on 0750B. Alternately, was full and could fit no more, and so a very busy depending on your version of Adobe Reader, you week of Quantum Electronics and Laser Science may click here to be connected directly to the came to a close. appropriate page.
U.S. citizen or permanent resident status is –Nathan Babcock required. Institute for Quantum Information Science University of Calgary
Quantum Mecha nics in Denver 2007 APS March meetin g visits mile-high city
Stockyards, Steak, and Stuff even greater challenge to summarize each and I’d been to Denver before but, oddly, never every session. With the specter of simultaneous downtown. My impression – both first and last – sessions an even greater possibility as the group was that it (and some of the nearby neighborhoods) grows, the need for multiple contributors becomes reminded me quite a bit of the downtown and more pressing. This year the focus of these nearby areas of my hometown of Buffalo (minus summaries is weighted heavily toward sessions I the big lake). This is not necessarily a bad thing personally attended and took an interest in with the (before you inundate me with e-mails, you should exception of one (many thanks to Sergio Boixo of know Buffalonians are a notoriously loyal breed the University of New Mexico and LANL for his that borders on the obsessive). In any case, it was a contribution). I am hoping additional contributors pleasant surprise in some senses including a will volunteer next year in order to broaden our distinct lack of the bland ubiquity that seems to coverage to include all the sessions while also define most places these days. It had a somewhat providing a greater diversity of viewpoint. gritty feel to it and was less populated in the In addition to the session summaries provided evenings than I expected. One might think that the here, brief contributions from our two student smell of the stockyards wafting down 14th Street paper award winners are included. As someone would entice me to order a non-meat product for who teaches undergraduates I was excited to see dinner, but, alas, the bison meatloaf just looked too one win one of the awards. good to pass up. Of course, none of this has anything to do with Monday, March 5 physics but part of the fun of any conference is The GQI-sponsored portion of the conference spending a little time in the host city. Though there began early Monday morning with the focus were no karaoke stories to tell as far as I know (see session Quantum-Limited Measurements that the November 2006 issue if you don’t get the featured an invited talk given by JM Geremia (New reference), the culinary aspects of the trip were Mexico) on the role entanglement plays in generally excellent (with the exception of the meal metrology and in quantum parameter estimation as that gave me food poisoning the night before I was a means of achieving the fundamental limits of to chair an 8 AM session). But there was hardly uncertainty dictated by quantum mechanics. In enough time to do much cavorting, with the various particular he discussed a recent proposal for dinner meetings and receptions taking up most improving such measurements by making use of evenings (though I’m still kicking myself for not multi-body quantum interactions. The idea of sending back that RSVP to the IOPP reception using such multi-system interactions was expanded since they gave out nifty, programmable digital ID on later in the session by the aforementioned Steve tags – my kids would have loved them). Flammia in conjunction with Geremia, Sergio Perhaps the most notable of these for a variety Boixo, and Carl Caves (all at New Mexico with of reasons was the business meeting of the GQI, Boixo also at LANL). Specifically, they developed held on Tuesday evening (March 6). The evening generalized bounds for quantum single-parameter included honoring GQI members recently elected problems where the coupling to this parameter is as APS Fellows, financial and membership described by these multi-system interactions. updates, continued discussion of the group name, Among the many other interesting papers and general merriment (including, of course, food presented in that session was one by Aashish Clerk and drink). Further discussions took place the and Dian Wahyu Utami of McGill University that following evening at the Executive Committee particularly intrigued me given my interest in meeting where planning for next year’s meeting in statistical mechanics. They show, at least New Orleans already began. theoretically, how one can extract the photon Along those lines, GQI sponsors a full slate of number statistics of a driven, damped oscillator at a sessions, sometimes even sponsoring more than finite temperature from the dephasing spectrum of one session during a single time-slot. It is a some two-level system that is dispersively coupled testament to the rapid growth of the group as well to the oscillator (previously only purely thermal or as the field as a whole. However, it makes it an zero-temperature driven cases had been
10 considered). By assessing the fidelity they are able the temporal analogue of Bell’s inequalities. This to show how the initial number statistics are occurs when the accuracy with which the represented by the measurement itself. measurement is made is unrestricted (i.e. in the The midday session was the first of two limit of increasing accuracy). The second aspect is sessions on Quantum Foundations (I) (including that, in the opposite situation, when measurements my own paper, discussed below, since I usually are coarse-grained (i.e. the accuracy is restricted in end up in one of the foundations sessions). Rob some way), even Newtonian physics emerges from Spekkens (Cambridge) kicked things off with an classical physics. Ultimately this summed up my invited talk based on the idea that classical theory own personal philosophy on the quantum-classical plus limited knowledge is almost quantum theory. contrast which is based on the aggregate behavior Measurement results, then, are not reality, per sé, of probabilities: determinism and irreversibility but rather “states of knowledge.” This is even true gradually take form as systems become larger and for the vacuum state as demonstrated by applying larger. This was pointed out by Arthur Eddington the Elitzur-Vaidman bomb test to his toy theory. in the 1920s (a point I spent some time discussing in my doctoral thesis a few years back). Surprisingly, Chris Fuchs (Bell Labs) did not discuss his Bayesian approach this time round. Rather he discussed the possibility of quasi- orthonormal bases for density operators, emphasizing the need for a good coordinate system to be introduced, though the states of knowledge of the system are technically coordinate free (again, this rings faintly of aspects of Eddington’s so- called ‘fundamental theory’). Again, following on some of Brukner’s ideas, Jeff Tollaksen (George Mason) discussed non- contextuality and the effort to find a time- symmetric reformulation of quantum mechanics Ho w did Spekkens get this through security? (see Sergio Boixo’s summary of the second foundations session). This included a presentation Matt Leifer (Perimeter Institute) then spoke on of the rather odd ‘three-box’ paradox which certain aspects of de Finetti’s theorem which got produces a basic failure of the product rule (e.g. PA me thinking tangentially about whether this = 1, PB = 1, but PAPB = 0). continuing quest to axiomatize quantum theory Time also played a role in Jan-Åke Larsson’s runs into a little problem by the name of Gödel. (Linköping) talk in more ways that one. In a Leifer’s work, done in conjunction with Howard somewhat humorous bit of timing, the talk began Barnum (LANL), Jonathan Barrett (Perimeter with a Skype call from his wife. Like the true Institute), and Alexander Wilce (Susquehanna), professional that he is he promptly hung up on her builds on a generalized probabilistic theory (she reportedly has forgiven him) before making developed by Barrett that actually includes both the the important general observations that 1.) classical and quantum theories as special cases. ‘theorems,’ per sé, can’t technically be violated and The Gödel question resurfaced in the very next talk that 2.) ‘loopholes’ are really experimental in which Frank Schroeck gave a brief overview of problems (take that, you fiendish experimentalists! his phase space version of quantum mechanics and – oh, that’s not what he meant, was it?). He then discussed the notion of informational proceeded to analyze the time-dependence of completeness. He also, curiously, indicated that, Bell’s inequalities by looking at the timing in the based on his results, Shor’s theorem would need to measurements. This provides a way to compare be reformulated. local realist models with non-local realist models. Continuing with the theme of exploring the In addition he makes the note that the Clauser- quantum-classical contrast, Caslav Brukner Horne inequalities from 1974 (CH74) are the (Vienna) discussed a new approach to macroscopic ‘mother-of-all’ Bell-type inequalities since they realism and classical physics within the bounds of deal more specifically with probabilities. I had a quantum theory. There were two aspects of this hunch about this a few years ago and even found a that I found interesting, the first being that funky set theoretic way to relate the CH74 macrorealism can be violated for large systems by inequalities to the generalized uncertainty violating the Leggett-Garg inequalities, viewed as
11 principle, but no one took me seriously (note to self: do not include picture of clown in next paper). Several other papers found ways to poke at the ever annoying problem presented by mixed states and highlighted the differences in measurements between mixed and pure states (and one by New Mexico’s Matt Elliott discussed a really neat graphical description of Clifford groups with definite pedagogical use). This all led up to the most stunning talk of the entire conference given by some supremely annoying little fellow by the name of Ian Durham (Saint Anselm). Hey! Don’t be so quick to agree! Quite seriously, though, I discussed work Qu antum Foundations II (recently revised) in which I derived the Cerf- The second foundations session started with Adami inequalities from the second law of a ta lk delivered by Alioscia Hamma (USC), in thermodynamics and the Markovian postulate plac e of Dan Lidar (USC), on Adiabaticity in (which, in itself, is really part of the second law) Ope n Quantum Systems. The adiabatic theorem and showed a link to the uncertainty relation state s that an eigenstate of a slowly varying (entirely separate from the set theoretic one I clos ed Hamiltonian will remain an eigenstate at mentioned above – in the revised version I made later times, and is a key ingredient of adiabatic use of a phase space relation Frank Schroeck quan tum computation. Nevertheless, real showed me to enhance this). Oddly enough, it quan tum systems are open, and, surprisingly, wasn’t until weeks after the conference that I adia baticity has not been previously studied in discovered that Caslav Brukner and some this context. For the crudest form of the adiabatic collaborators had derived entanglement from the theo rem, write the time-dependent Schrödinger third law of thermodynamics. Having chatted with equa tion in the instantaneous diagonal basis, plus Caslav more than once at the conference, I was a p erturbation that updates this basis. If the surprised he didn’t mention it. Ham iltonian changes slowly, the perturbation is The midday session on Monday centered on negl igible, and the eigenspaces remain decoupled. the topic of Ion Traps for Scalable Quantum This intuition is carried to the open systems Computing. Scalability is, of course, one of the pictu re by considering the Lindblad operator as a primary hurdles in the drive to develop practical supe ropertor (matrix). The problem is that quantum computers, both in terms of increasing the eige nspaces of the closed system do not number of qubits as well as reducing the size. This corr espond to eigenspaces of the Lindblad session included presentations by MIT’s Isaac supe roperator. Furthermore, the superoperator Chuang and NIST’s Dave Wineland, among others. mig ht not even be diagonalizable. The closest Unfortunately, hunger got the better of me and I struc ture would be a Jordan block decomposition, missed this session. and the open adiabatic theorem deals with the Monday closed out with a second session on deco upling of the Jordan blocks. Another Quantum Foundations (II), summarized in the imp ortant difference is that the eigenvalues might box to the right. have imaginary parts, and adiabaticity might brak e down even for slowly varying interactions. Tuesday, March 6 David Craig (LeMoyne) talked about the Tuesday morning came bright and early unce rtainly principle in the context of the beginning with the first of four focus sessions on cons istent histories approach to standard quantum Superconducting Qubits (I). This began with an theo ry, pioneered by Griffiths, Gell-Mann and invited talk given by Jay Gambetta (Yale) on the Hart le. This approach starts with the observation application of continuous-in-time measurement that probabilities can be assigned to a sequence of theory to circuit QED in order to obtain a quantum mea surements results, which can be viewed as a trajectory description of the qubits. Schrödinger path or history. Once there, the underlying inner would likely have been pleased. Unfortunately, I (continued in box on next page) had a meeting and missed the session.
12 I was back in time for the two concurrent (con tinued from previous page) midday sessions. The first, on Quantum prod uct between histories is taken as the primary Computing in AMO Systems, was co-sponsored obje ct. This is often called the decoherence by DAMOP. Among the interesting talks in this func tion. Probabilities can be assigned to sets of session was one by Fernando Cucchietti (LANL) histo ries only under certain consistency who demonstrated how a Loschmidt echo can be cond itions, which boil down to the orthogonality used to measure such things as the fidelity of the (or decoherence) between different histories. As quantum simulation and even the intensity of some one would expect, non-commuting observables external potential (including gravity). What is a gene rate incompatible histories. Craig proposed Loschmidt echo you ask? In a sense it is a measure to interpret the uncertainty principle as of irreversibility (there it is again!) in quantum inco nsistence among histories. Kicheon Kang systems, or, conversely a measure of the time (Cho nnam University) proposed an experiment reversal of the evolution of a quantum system. for quantum erasure in electronic Mach-Zender Mathematically, it takes the following rather inter ferometers. An electronic MZ is a two-path elegant form, elec tron interferometer that works like an optical
2 MZ, with quantum point contacts (QPTs) as beam M (t) = 0 exp(it(H + S) exp("itH) 0 split ters. Kang proposes to use another QPT for
which-path information, and that completes the nece ssary elements for quantum erasure. Ken where S is the perturbation and H is the Hamiltonian and specifically looks at the Wharton (San Jose State) talked about a Time-
! attenuation in some localized density excitation. A Symmetric Quantum Mechanics interpretation,
nice basic discussion (that includes a nifty picture accomplished by applying two consecutive
of a stamp with Loschmidt on it) can be found at boundary conditions onto solutions of a time-
http://www.lanais.famaf.unc.edu.ar/loschmidt/. symmetric wave equation. Michael Clover
René Stock, in collaboration with Nathan (SAIC) argued about a possible local
Babcock (see the QELS article) and our very own interpretation of quantum mechanics, but his
Barry Sanders (all of IQIS/Calgary), presented an interpretation of some classical equations was
interesting paper in which they devised entangling somehow criticized.
operations using Yb and Sr atoms in which certain Just some brief comments on the other talks. Edw ard Floyd talked about quantum Young's electron transitions are forbidden thus resulting in low decoherence. They use this scheme to experiment, nonlocality and trajectories, and
investigate the rapid measurement of clock-state showed some nice diagrams with temporal
qubits in a Bell-test that manages to avoid the retrograde motion. Joel Maker replaces the
detection loophole for spacelike-separated general covariance in the Standard Model and
entangled qubits. gets azimuthal trifolium. Shantilal Goradia
The second midday session was the second (Gravity Research Institute) argued, among many
focus session on Superconducting Qubits (II) and other things, that particles have barcodes.
included a series of talks on Cooper-pairs. Finally, the much talked about Fourier
Notably, Ofer Naaman and José Aumentado (both transforming purple bacteria were also in this
of NIST) presented work in which they session.
spectroscopically measured narrow-band -Sergio Boixo De pt. of Physics and Astronomy, U. New Mexico microwave radiation that was emitted by a single- Cooper-pair transistor (SCPT) electrometer that and Los Alamos National Laboratory
was biased in its subgap region. It never ceases to
amaze me how small a system we can now the development of a scalable quantum computer: manipulate (and, yes, I am aware that single- that they (the qubits) be tunable, that the coupling electron tunneling [SETs] transistors have existed itself be tunable, and that the qubit is capable of since 1987). information storage. Later in the session D-Wave A bit later in this session, Matthias Steffen, in systems was slated to present experimental results collaboration with Frederico Brito, APS GQI Vice- for a system of four coupled qubits under adiabatic Chair David DiVincenzo, and Roger Koch (all evolution, but I got hungry again. from IBM), presented IBM’s tunable flux qubit in The first of two afternoon sessions was the both two and three junction versions coupled to a jointly sponsored DCMP/GQI Prize session that harmonic oscillator. The IBM qubits exhibited included presentations by Irfan Siddiqi (Berkeley), three features that the authors consider essential for Bill Wootters (Williams), Huanqian Loh (Data
13 Storage Institute), Hugh Churchill (Harvard), and (IQC/Waterloo) that gave a general overview of the Fazley Bary Malik (Southern Illinois). topic, describing the latest achievements to date. Siddiqi discussed using Josephson bifurcation He was followed by Travis Hime (Berkeley) amplifiers as a means of measuring quantum speaking on solid-state qubits and SQUIDs, systems since the inherent non-linearity provides Andrew Houck (Yale) describing the generation for fast, sensitive detection and the lack of and measurement of single photons in circuit QED dissipation reduces decoherence problems. So far systems, Matthias Steffen (UCSB/IBM) detailing these amplifiers have been successfully used to the use of state tomography to directly measure the read the states of superconducting qubits and future entanglement of two superconducting qubits, and applications potentially include single molecule Hans Mooij (Delft) discussing the readout method magnets (how cool is that?). applied to systems of flux qubits. Alas, I only Wootters, winner of the Prize for Research at poked my head in a few times in between papers in an Undergraduate Institution, gave an overview of the other morning session, Quantum his work on discrete Wigner functions (see the July Measurement. 2006 issue of The Times for an interview with Bill That particular session got rolling with a paper in which he discusses this work in some depth). by New Mexico’s Sergio Boixo (see box above) This particular discrete Wigner function developed who, in collaboration with Rolando Somma by Wootters and his students (there are other such (LANL), described a quantum circuit that estimates functions) is particularly well-suited for work in operators at the optimal Heisenberg limit. This is quantum computation and quantum cryptography achieved using a general unitary operation for due to its suitability for binary objects. In these multi-parameter estimation when the operator acts analyses, the Wigner function provides an alternate on a set of qubits. Achieving the optimal method for determining probability distributions Heisenberg limit essentially means reaching a (as opposed to methods employing state vectors or sensitivity for determining the parameters in the density operators). While the Wigner function estimation that scales as 1/N where N is the number itself can’t be interpreted as a probability of times the unknown unitary operator is applied. distribution since it can have negative values, its The circuit contains an ancilla (extra qubit) that is integral along any axis in phase space is the initially in a pure state with the system qubits probability distribution of an observable along that initially in a mixed state. Basically, any unitary axis. transformation on some arbitrary number of qubits The second of the afternoon sessions was the can be built from elementary quantum gates (like a third focus session on Superconducting Qubits Hadamard gate for instance) and measurements are (III), a session I only poked my head into a couple then performed on the ancilla. of times. While the second superconducting qubits Another fascinating paper appeared courtesy session focused on Cooper-pairs, this one focused of Mark Keller and Neil Zimmerman (both of on phase qubits. One talk that did catch my eye in NIST) working in collaboration with Ali this session was given by a group out of UC Santa Eichenberger (METAS). They asked the question: Barbara that is working to develop a CHSH Bell- is the charge carried by the discrete quanta in type experiment using Josephson phase qubits. single-electron circuits exactly e? In order to The experiment is unique in that it places high answer this apparently non-trivial question they demands on most qubit performance measures (e.g. actually utilized some good, old-fashioned E&M: fidelity, energy relaxation time, decoherence time, they placed a known number of SET charge quanta etc.). This is a novel approach to Bell-testing onto a known capacitor and measured the voltage which, these days, is most often performed using across it (the capacitor)! Hey, even my lasers or atom trapping. introductory physics students would at least get the Tuesday closed out with the business meeting, basic idea (assuming they remember that C = Q/V). discussed above. In any case, as simple as that sounds it did require a Josephson voltage standard (something my intro. Wednesday, May 7 students likely would not be familiar with). Their Highlighting just how many sessions GQI results put the equivalence of the SET charge 6 sponsored or co-sponsored this year, Wednesday quantum and e at one part in 10 which is about morning began with yet another pair of concurrent 100 times better than any previous result and they sessions. The first, Progress in Superconducting expect their results to improve to about 3 parts in 7 Quantum Computing, was co-sponsored with 10 sometime in the near future which would then DCMP and consisted entirely of invited talks. It provide some potentially useful information on began with a talk by Frank Wilhelm possible corrections to the Josephson constant, KJ =
14 2e/h (which is the inverse of the magnetic flux one-qubit Hadamard, phase, and π/8, and the two- quantum, Φ0). qubit controlled-phase. This was followed by Later in the session, Constanze Metzger (LMU Emily Pritchett (Georgia) presenting a procedure Munich) described an interesting project designed for two-qubit gate realization utilizing a small set to realize laser-cooling of macroscopic mechanical of primitive operations whose Makhlin invariants resonators. The system described employed are then compared to that of the target gate (i.e. the passive optical cooling of the Brownian motion of set of operations is tweaked until the Makhlin a cantilevered micromirror to cool an 11 pg (yes, invariants match). Examples included several new that’s pico-gram) mass (the mirror itself) which is CNOT gates. CNOT gates were later analyzed the smallest mass cooled thusfar. from a slightly different perspective by Gabriel Toward the end of this session, Alexander Colburn (Colorado School of Mines). In particular, Korotkov (UC Riverside) discussed work with the feasibility and minimal implementation of these Andrew Jordan (Rochester) in which they propose gates from specific model Hamiltonians was that it is possible to actually undo a quantum discussed. measurement! This is, of course, a very unique idea since, if you’re a Copenhagen adherent Thursday, May 8 (personally, I’m agnostic) you would immediately Well, if you’re still with me, you’re in for be suspect since wavefunction collapse is a chuckle (or perhaps a cringe). I had to arise rather generally irreversible process. On the other hand, early to chair one of the morning (i.e. 8 AM) if you adhere to a non-collapse interpretation, sessions after having been up late with food perhaps there’s nothing surprising here. In any poisoning! My, life is an adventure, isn’t it? case, Korotkov and Jordan suggested potential In any case, we again had two concurrent experimental realizations of this using quantum sessions and since I chaired one I could not attend dots or superconducting qubits. Personally, I’ll be the other, which was the first of two on Quantum very interested to see the results of any Cryptography and Quantum Communication experiments of this type. (I). While the papers in my session were generally The midday session was our fourth interesting and of a high caliber, I did miss the Superconducting Qubits (IV) session and it chance to hear Anton Zeilinger speak on one of my began with a series of papers from the Yale favorite topics: long-distance, large scale quantum contingent discussing their proposal of a new type communication! I suppose I am simply jealous of superconducting qubit called the “transmon” that Anton gets to spend time beaming giant lasers that consists of a Cooper-pair box shunted by a over the Canary Islands, but it’s an interesting and large capacitance. As interesting as the concept fun topic nonetheless, particularly when applied to was (and it really did sound interesting), I shuffled quantum cryptography (it brings out my “inner off to lunch. spy”). The rest of this session consisted of invited The day closed with a session that bridged a talks as well, including the Beller Lectureship lot of the topics already discussed by considering Recipient Talk. Physical Implementations of Qubits. The first The session I was actually at that morning, two talks dealt primarily with ion-traps while the Quantum Algorithms, Simulation, and Error third, from the Vienna group (Robert Prevedel, et. Correction, was actually probably just as al.), presented work on a physical realization of interesting. In particular, Ari Mizel (Penn State) high-speed linear optics quantum computing in proved the equivalence of adiabatic quantum which randomly induced measurement errors are computation and the usual circuit model of classically fed forward and then corrected by quantum computation. This work actually relates adapting the basis of subsequent measurements. In to the work discussed at the very beginning of the a sense, this is a physical realization of quantum second session on quantum foundations (see box error-correction where the change in basis for above), which is no surprise considering Dan Lidar subsequent measurements is, in conjunction with (USC) had a hand in both. Ari was followed by the feed-forward step, the error-correction Peter Love (Haverford) who gave a really cool talk operation. With current technology, the feed- presenting quantum cellular automata as a means to forward step can be performed in less than 150 ns. address questions about quantum dynamics. In the next talk, Frank Gaitan (Southern Specifically Peter discussed a particular unitary Illinois) presented results of simulations that class of automata. Cellular automata in general suggest a particular class of non-adiabatic rapid have their origins in game theory and, in particular, passage sweeps from NMR should be able to in perhaps the first person to get hooked on implement a series of quantum gates including the computer “games,” Stanislaw Ulam who was
15 working with John von Neumann at Los Alamos in In all honesty, after Som’s talk I went to lunch the 1950s. The basic rules were codified in 1970 and returned for the afternoon session on by John Conway in what has now become known Quantum Entanglement for just long enough to as the Game of Life. The “game” Ulam was listen to Barry Sanders (IQIS/Calgary) to discuss playing (and that Conway codified) involved entangled Gaussian states. In particular, the major pattern repetition through replication and von revelation in their work was that all tripartite Neumann began to consider what would happen if entangled Gaussian states achieved through three- something (a universal constructor) could be mode squeezed light are actually su(1,1) of the type programmed to make itself. Ulam persuaded von first developed by Sebaweh Abdalla. This suggests Neumann to consider cellular automata as a potential ways to generalize both theories and possible mechanism since they are self-replicating applications of multipartite Gaussian states. by nature. Quantum cellular automata don’t After Barry’s talk, I realized that my brain had exactly self-replicate but they mimic each other in curdled, perhaps a result of the combination of four a sense. The idea was first developed at Notre days of physics and a night of food poisoning, all at Dame in the early 1990s and has taken off from over 5000 feet in altitude (thinking back on it, I’m there. surprised I didn’t start hallucinating). As such, I The ubiquitous Professor Lidar discussed called it quits that day and, since I had a morning quantum error correction a bit later in the session flight out on Friday, I also missed the final two (see the conference announcement on page 6) and GQI-sponsored sessions: Quantum Information adiabatic quantum computing made its return at the AMO/Condensed Matter Interface, co- immediately following in an interesting talk by sponsored by DAMOP, and Decoherence and William Kaminsky (MIT) in which was presented a Quantum Control. The chances are pretty good I general approach to determining the asymptotic wouldn’t have made much sense of them anyway scaling of resources in random instances of NP- due to the state of my brain, which is too bad complete graph theory problems in adiabatic because I am sure they were very interesting. quantum computational resources. An important Nonetheless, my daughter’s birthday party was conclusion was that adiabatic quantum computers coming up and I simply could stay no longer based on quantum Ising models are much less regardless. likely to be efficient than those based on Finally, we close out this summary of the very Heisenberg or quantum rotor models. successful 2007 March Meeting with summaries The midday session was the second dedicated from our two student paper award-winners. The to Quantum Cryptography and Quantum award for theory is graciously sponsored by the Communication (II) and was particularly notable Perimeter Institute and includes a cash prize of in that it contained one of our student award- $500 and this year’s recipient was Gleb Axelrod of winning papers, a summary of which is presented the University of Urbana-Champaign (UIUC). The by the paper’s author, Gleb Axelrod (UIUC) after award for experiment is graciously sponsored by this general conference summary. Other the Institute for Quantum Computing (IQC) in interesting papers in this session included Jan-Åke Waterloo and likewise includes a cash prize of Larsson’s discussion of security aspects of the $500 given to this year’s winner Frank Koppens of authentication process in QKD in which he the University of Technology in Delft (TU Delft) pinpoints a security weakness in the authentication in the Netherlands. We would like to thank the process. A bit later, Som Bandyopadhyay IQC and Perimeter Institute for sponsoring these (Montréal) presented work that showed the awards. Without their generous financial qualitative link between local distinguishability and assistance this level of award would not be entanglement lies at the level of stochastic rather possible. We would also like to thank all the than deterministic processes (an example presented student presenters, the faculty who nominated included someone’s prized goat and a Ferrari). If I them, and those who agreed to serve as judges for recall, this is what led to a multi-party hallway the competition. Finally, we extend a big thanks to discussion in which Rob Spekkens asserted his Chris Fuchs who organized everything surrounding conviction that entanglement was in some way these awards. related to the exclusion principle (clearly so since The following two articles are summaries of the latter is tied up in distinguishability). I have yet these student paper awards given by the winners to convince anyone that the uncertainty principle whom we also thank for spending the time to write and the second law of thermodynamics are also newsletter-length summaries. related (see my summary of my own talk above).
2007 GQI student paper award for 2007 GQI Student paper award for theory experiment Numerical Modeling and Optimization Coherence and control of a single of Type-I Entangled-Photon Sources electron spin
One of the most robust ways of generating A single electron spin is an exemplary object entangled photons is by the process of spontaneous for studying quantum phenomena like coherence parametric down-conversion. In this scheme, a and entanglement in a solid-state environment; a high-frequency pump photon is incident on a pair rich field which has become experimentally of nonlinear crystals, and can split into two low- accessible during the last decade due to the frequency photons which are entangled in their achieved high level of coherent control of a variety polarization. Developing such sources that are of isolated quantum systems. bright and high-purity is crucial for quantum key The progress in the field of manipulating distribution, quantum teleportation, and tests of confined spins in quantum dots was substantial nonlocality. However, the purity of these sources is since Loss and Divincenzo’s  proposal on reduced because not all of the produced photons electron spins as quantum bits. Important examples pairs are indistinguishable. Due to imperfect phase reflecting this progress are the realization of single- matching in the crystals and the finite bandwidth of shot read-out of a single electron spin , and the pump laser, down-converted photons are coherent coupling of two electron spins . produced in a range of wavelengths and directions. However, the final step to produce a real quantum Also, because the down-conversion crystals are bit, namely the possibility to rotate the spin of a birefringent, each polarization component of the single electron in a quantum dot, remained beyond pair acquires a different phase. As a result, each reach for a long time. pair of photons becomes distinguishable, which results in effective decoherence and lowers the purity of the source. In order to quantify this decoherence, and develop ways of minimizing it, we have developed a numerical model of our sources. This model takes into account the properties of both uniaxial and biaxial down-conversion crystals, the pump laser bandwidth and spatial modes, and photon collection irises and filters. The result of the calculation is a density matrix that represents the collected two-photon polarization state, which we can use to determine the relative effect of each experimental parameter on the quality of the state. To verify the model, the predicted states were compared with experimentally obtained quantum state tomography data, showing good agreement.
Using the model we have also designed spatial and Cohe rent oscillations of a single electron spin. The current temporal phase compensation crystals to reduce the flow (vertical axis) reflects the spin direction, and the spin phase decoherence and improve the brightness and rotation angle is controlled by the duration of the oscillating purity of our sources. This code will be freely field burst (with amplitude B1). Curves offset for clarity. Inset : scanning-electron-microscope image of metallic gates available to the quantum optics community as a on to p of a two-dimensional electron gas. Quantum dots resource for designing and characterizing (indic ated by white dotted circles) are formed by applying optimized entangled-photon sources. negative voltages to the gates.
-Gleb Axelrod Recently, this control of the quantum state of a in collaboration with Joseph Altpeter, single electron spin has been realized via electron- Michael Goggin, Jaime Valle, Joseph Yasi spin-resonance (ESR) . One of the main and Paul Kwiat difficulties was the presence of unwanted electric Department of Physics fields coming together with the on-chip generated University of Illinois at Urbana-Champaign oscillating magnetic field. This made it hard to
17 rotate the electron spin and read it out at the same References time. However, it was possible to get around the  D. Loss and D.P. DiVincenzo, Physical side effects. By confining a second electron in Review A, A57 120-126 (1998). another quantum dot alongside the first one (see  J. M. Elzerman, R. Hanson, L. H. Willems figure inset), it is possible to read out the spin van Beveren, B. Witkamp, L. M. K. direction of the first electron in a very robust way. Vandersypen & L. P. Kouwenhoven, Nature Namely, for two electrons confined in a quantum 430, 431–435 (2004). dot, the Pauli principle tells us that the energy is  J.R. Petta, A. C. Johnson, J. M. Taylor, E. A. higher if the spins are the same, rather than Laird, A. Yacoby, M. D. Lukin, C. M. opposite spins. By looking whether the electron Marcus, M. P. Hanson & A. C. Gossard, can move to the other electron in the adjacent Science 309, 2180–2184 (2005). quantum dot, it is possible to read-out the spin  F.H.L. Koppens, C. Buizert, K.J. Tielrooij, state. Applying bursts of the oscillating magnetic I.T. Vink, K.C. Nowack, T. Meunier, L.P. field allowed full control over the rotation angle of Kouwenhoven & L.M.K. Vandersypen, the spin and subsequent detection of the spin Nature 442, 766–771 (2006). direction revealed coherent (Rabi) oscillations as  A.C. Johnson, J.R. Petta, J.M. Taylor, A. shown in the figure. Yacoby, M.D. Lukin, C.M. Marcus, M.P. Once this control over a well-defined quantum Hanson & A.C. Gossard, Nature 435, 925– object is gained, the important and fundamentally 928 (2005). interesting question is how long a superposition  F.H.L. Koppens, J.A. Folk, J.M. Elzerman, R. state is preserved. This can be measured via a Hanson, L.H. Willems van Beveren, I.T. Ramsey-type experiment where the spin is rotated Vink, H.P. Tranitz, W. Wegscheider, L.P. from an eigenstate to a superposition state and after Kouwenhoven & L.M.K. Vandersypen, a short free evolution time rotated back to an Science 309, 1346–1350 (2005). eigenstate. The probability to find the same eigenstate again reflects to what extent the coherence is preserved. When averaged over many GQI Executive Committee experimental runs, we found that on average the Chair coherence was lost already after 30 ns. Not Carlton Caves, University of New Mexico surprising, however, because the nuclear spins in Chair-elect the semiconductor lattice couple collectively to the Lorenza Viola, Dartmouth College electron spin, leading to an uncertainty in an Vice-chair effective magnetic field [5,6] (so-called David DiVincenzo, IBM Corporation Overhauser field). Current research is focused on Secretary-Treasurer different ways to reduce this uncertainty in the Barry Sanders, IQIS/University of Calgary Overhauser field, while the coherence loss due to Past-chair the nuclear field could already be reversed to a Charles Bennett, IBM Corporation large extent via a spin-echo technique. We found Members-at-large that coherence was preserved for at least 500 ns, Christopher Fuchs, Bell Labs which is promisingly long for future experiments. Raymond Laflamme, University of Waterloo
-Frank Koppens Department of Applied Physics GQI Website University of Technology, Delft http://units.aps.org/units/gqi/
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