October 2012 Press release Nobel Prize The Assembly of Professors of the Collège de France is pleased to announce the award of the Nobel Prize in Physics 2012 to Professor Serge Haroche, Director of the Collège de France The Nobel Prize in Physics 2012 has been awarded jointly to Serge Haroche in France and David Wineland in America for their research in the field of quantum physics. The Nobel Prize jury made the award for “ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems” and stated that these two researchers had “opened the door to a new era of experimentation with quantum physics by demonstrating the direct observation of individual quantum particles without destroying them”. The Assembly of Professors believes that this Nobel Prize pays tribute to the work of an exceptional researcher and his experimental successes. It honors the fundamental research which has been the mission of the Institution since its foundation. The Assembly would like to highlight the fact that this is the tenth Nobel Prize awarded to one of its professors, covering the widest range of disciplines after Harvard. Claude Cohen Tannoudji, winner of the Nobel Prize in Physics in 1997, an honorary professor of the Collège de France and Serge Haroche’s PhD supervisor, stated that the prize recognized “a man with remarkable scientific and human qualities”. Professor Serge Haroche has held the Chair in Quantum Physics at the Collège de France since 2001 and was appointed Director in September 2012. He began his career at the CNRS (French National Center for Scientific Research) and has carried out most of his research at the Kastler Brossel Laboratory (École normale supérieure/UPMC/CNRS/Collège de France), which specializes in the study of the interaction between light and matter. He heads the Electrodynamics of Simple Systems Group there. Serge Haroche was initially interested in mathematics, but soon turned to physics; “I was fascinated by the fact that nature can be understood using mathematical laws and I was quickly drawn to physics, which added a major constraint to mathematics – namely reality”. Atoms and light – thought experiments have become a reality As a specialist in atomic physics and quantum optics, Serge Haroche is a pioneer of cavity quantum electrodynamics, a field which uses experiments to cast light on the fundamental principles of quantum theory and to produce prototypes based on quantum information processing. Working with his teams, he has designed innovative experimental methods to isolate, measure and manipulate single quantum particles for relatively long periods of time (about one-hundredth of a millisecond). His research is aimed in particular at understanding the transition from the quantum world to the macroscopic world. This “decoherence” can now be observed using experiments with trapped photons, thus illustrating experimentally certain premises of quantum mechanics and fulfilling Albert Einstein’s dream of isolating and studying an atom or photon. Although some like to dream of the ultra-powerful and fast supercomputers or extremely accurate clocks which could one day be developed using this research, Serge Haroche prefers to point out that what takes precedence is the desire to understand the world around us, the importance of fundamental research and the fact that the applications facilitated by this fundamental research often take even scientists themselves by surprise. “The majority of modern technologies which very often have their basis in quantum physics, such as transistors, lasers and MRI imaging, are the outcome of chance and the convergence of fundamental research activities which were not focusing on that specific goal. Chance and time are also key elements of fundamental research.” Professor Serge Haroche’s lectures are available in audio or video format on the Collège de France website (www.college-de-france.fr). Press contact: Marie Chéron/Cécile Barnier: +33 (0)1 44 27 12 72 - [email protected] October 2012 Serge Haroche’s research field When thought experiments become a reality The world is made up of atoms which emit, absorb and diffuse light, the essential vehicle for the information which we receive about our environment. At the beginning of the last century, quantum theory uncovered the enigmatic laws obeyed by matter and radiation at a microscopic level, in a counter-intuitive world in which the notions of waves and particles are closely intertwined. Light is both a continuous stream and a collection of discrete photons. This enigmatic area of physics is based on the principle of superposition. A microscopic system can actually exist in several possible states simultaneously, suspended as it were between various classical realities. The founding fathers of quantum theory based their famous discussions on thought experiments, manipulating atoms and photons in the virtual realm. Experiments which for a long time were the stuff of dreams then are now being carried out at last. Juggling with atoms and photons and making them interact in a controlled environment is a now a thriving field of experimental research in which Serge Haroche is a pioneer. He has forced an atom to interact with several photons in a “photon box” whose walls offer an almost ideal degree of reflectivity, something of which Bohr and Einstein could only previously dream. He has thus been able to observe the atom-light interaction in its most basic form. Serge Haroche and the ENS team, which he currently leads with the co-authors of this article, were trail blazers in the field of cavity quantum electrodynamics, which has evolved significantly over the last thirty years. Although simple in theory, the ENS experiments are technically complex. Cavities, which are resonant in the microwave field, are comprised of superconducting mirrors which face each other; these are the highest quality mirrors currently available, off which light bounces several billion times before being absorbed or diffused. The photons therefore travel 40,000 kilometers in the confined 3 cm space between the mirrors, giving the experimenters 13 hundredths of a second to manipulate or observe them. The atoms which interact with these photons are also very unusual. They are atoms in which a single electron has been put into a highly excited orbit, with a radius (0.1µm) which is 2,500 times greater than that of the atom in its basic state. Extensive research has been carried out on these Rydberg atoms in the last thirty years. Serge Haroche was a pioneer of this research in the 1970s, demonstrating the extreme sensitivity of these atoms to microwaves and developing methods to prepare, manipulate and detect them. Using these ground-breaking tools, Serge Haroche and the ENS team have, for example, recently developed a revolutionary new method for counting photons. Standard detection methods (including the human eye) destroyed the photons they counted, but the team has perfected a “transparent” detection method in which photons interact with the counting equipment, without being absorbed. The experiment consists of making the field, which is trapped in the cavity, interact with atom “probes”. They pass through the cavity one by one carrying an imprint of the state of the field without absorbing light energy. Information relating to the number of photons is acquired progressively, as each atom is detected, making a partial contribution to determining the state of the field. When a photon subsequently disappears, absorbed by the imperfections in the mirrors, the energy in the field undergoes a sudden and discontinuous variation which is detected by the atoms. These quantum jumps, a fundamental quantum principle, had never been observed in relation to light prior to this experiment. The Zeno effect is another spectacular quantum phenomenon illustrated by these experiments. Adopting a paradoxical line of argument, the Greek philosopher Zeno denied the existence of the movement of an arrow, claiming that it was immobile because it was located in a specific place at any given instant. A succession of immobile states cannot constitute movement. This sophism is of course false in the macroscopic world, but it can become true in quantum physics where observation influences the object being measured. The ENS team has demonstrated that the evolution of a field which they attempt to inject into the cavity is frozen if the number of photons is counted repeatedly and non-destructively. Quantum physics therefore proves that Zeno was right, but the reasons advanced are far more subtle than those posited by the philosopher! Atoms and cavities can also be used to explore the quantum-classical boundary. In a key experiment, Serge Haroche and his colleagues monitored the state of a field containing several photons and a single atom. The field was in a quantum superposition of two radically different states. In practice, the atom controls the oscillation Press contact: Marie Chéron/Cécile Barnier: +33 (0)1 44 27 12 72 - [email protected] October 2012 phase, but it is equally valid and simpler to work on the basis that it controls amplitude. After interacting with the atom, the field is in a superposition of a state in which it oscillates strongly (high amplitude) and a state in which it does not oscillate at all (zero amplitude). This situation is impossible in the classical world, but falls within the laws of quantum physics. Such states are referred to as Schrödinger’s cat states, after a thought experiment in which an imaginary cat was sealed in a box with a radioactive atom and placed in the awkward position of being suspended between life and death in quantum terms. In the real world, a cat is either dead or alive! This is where “decoherence” enters the picture. When macroscopic objects are coupled with their environment their superposition of states rapidly disappears. Quantum ambiguity is replaced by the classical world of everyday experience.