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Atoms and Radiation

Jean Dalibard


Serge Haroche
Traduction de Liz Libbrecht

Texte intégral

1Light, in the form of electromagnetic waves with different frequencies, is the main channel through which we receive information from the world around us. Through it, we observe the Universe, we communicate, and we discover the core of the physical world. It thus plays a crucial role in Science. The play of light on the surface of paintings or the contours of sculptures also creates and conveys the emotions of Art. This pre-eminence of light in all forms of perception has been recognized since time immemorial. In the Book of Genesis, light is God’s first creation. In the beginning there was nothing, says the Bible, “the Earth was a formless void” and God said “let there be light, and there was light”. “There was still nothing, but you could see it a whole lot better”, quipped a comedian with little reverence for the sacred text. This joke illustrates an obvious fact: light could not exist without the matter from which it stems, which absorbs or diffuses it. The study of light is therefore very closely linked to that of the atoms with which it interacts. Today, we are inaugurating the Chair of Atoms and Radiation. As the name indicates, this is a vast domain that is crucial both to physics and way beyond it, to many other forms of knowledge.

2One might imagine that we know everything about light and its interactions with matter. Maxwell’s equation, formulated a hundred and fifty years ago, provides a highly precise description of the interaction between electric charges and electromagnetic waves. Quantum laws, discovered a hundred years ago, have allowed us to understand the nature of the processes through which atoms emit and absorb light. The modern version of electromagnetism, quantum electrodynamics, describes with extraordinary precision all the phenomena involving light at microscopic level, and these descriptions are remarkably often validated by experiments. Laser, magnetic resonance imaging, atomic clocks and the GPS, all of which are outstanding technological advances of the last fifty years, are direct by-products of quantum electrodynamics, from which we benefit on a daily basis.

3Being familiar with these everyday devices, we may think that light has in a way fallen into the public domain of acquired knowledge and that the forefront of science has moved on to more uncertain territory, relegating optics to the field of applied physics. We have often heard this argument, which turns atomic physics into a super technology at the service of the other sciences. Yet over the last fifty years it has regularly been refuted by the periodic revival of atomic physics and quantum optics as a fundamental science, which repeatedly opens unsuspected doors onto new fields of knowledge.

4Jean Dalibard, whom we welcome here today for his inaugural lecture at the Collège de France, is one of the physicists to thank for the constant renewal of the vitality of atomic physics and quantum optics. As a former student of the École normale supérieure, he did his PhD under the supervision of our colleague Claude Cohen-Tannoudji. He then entered the CNRS, where he has remained for his entire career. He is now Research Director at the Kastler Brossel laboratory of the ENS, where he heads a group of students and young researchers. In parallel, Jean Dalibard is also a professor at the École polytechnique. He is a member of the Académie des sciences, to which he was nominated at a very young age. He has received many French and international awards and distinctions.

5Dear Jean, I shall stop here with this enumeration of titles and honours, to continue on a more personal note. Over the last thirty years, I have valued not only your qualities as a physicist who promotes the ideas and ideals of science, but also your human qualities, which are evident in all your professional relationships and friendships. Listening to your inaugural lecture, everyone will be able to witness your exceptional qualities and your crucial contributions to cold atom physics, a rich domain of today’s physics that your research is spreading in France and throughout the world.

6I will not talk about your work now; you will do so much better than I could in your lecture. I would simply like to highlight a few facts which you will probably not mention, out of modesty. I would first like to make a general and irrefutable statement: you are one of the masters of cold atom physics. This science studies the properties and behaviour of atoms which the right arrangement of lasers can slow down, stop in their course and trap in controllable spatial configurations, for either fundamental research or possible applications. You are one of the most well-recognized experts on this research theme, which appeared some thirty years ago as you were working on it for your thesis, and which continues to develop rapidly throughout the world. In addition to this general observation, I shall mention three more specific points:

  1. The magneto-optical cold atom trap, the principle of which you laid down nearly thirty years ago when you were still just a young student, is one of the key ingredients of the hundreds of experiments on ultra-cold atoms performed daily by a very large number of research groups worldwide, in both fundamental and applied research.

  2. The theoretical Monte Carlo wave-function method, which you described twenty years ago with a Danish colleague, is used by many researchers to simulate atoms’ or photons’ behaviour in varied experimental situations. It very clearly shows how quantum indeterminacy manifests itself in the form of quantum leaps in these systems.

  3. The experiments on quantum whirlpools in cold atom gases which you initiated ten years ago have opened up a prolific field of study of the superfluidity of these systems. They illustrate very general behaviours of matter that had previously received little experimental attention. Your research draws on cold atom physics to explore universal phenomena which can be studied in ideal conditions using this physics. On a particularly well-controllable system, you simulate situations encountered in condensed matter physics, nuclear physics, and astrophysics.

7The few points that I have just mentioned illustrate the fecundity of your scientific approach. Your experiments rely on technological feats that implement elegant ideas and require remarkable insight. The analysis of the physical phenomena revealed by these experiments draws on profound ideas, and requires exceptional theoretical finesse. You master not only quantum physics in its most fundamental aspects, but also optics, statistical physics and condensed matter physics. The phenomena you study are at the interface between all these physics; they synthesize remarkably well the ideas and concepts spawned by different fields of physics, which combine and cross-pollinate one another, so to speak, in cold atom physics. You will be revealing this whole research field to us in your inaugural lecture, and then in the lectures that will follow year after year.

8I would like to conclude by saying that your arrival at the Collège de France is not only that of a professor, to teach. You will also be moving your research to the new laboratories currently being completed, and thus joining the teams of other physics Chairs, including my own, and the chemistry Chairs. You will thereby be contributing to the revival of dynamic experimental research in these disciplines at the Collège de France. Teams of young researchers will soon be associated with the teams of the physics Chairs, in a project incubator (HAP) currently being set up by the Collège de France in collaboration with the CNRS. Young researchers starting out will be welcome for a few years, to allow them to take their first steps as researchers before spreading their wings elsewhere. Your energy and charisma, dear Jean, will be an invaluable asset to ensure the success of this undertaking.

9In this new adventure, you will nevertheless remain part of the Kastler Brossel laboratory at the École normale supérieure, an environment that is highly conducive to the development of free and innovative research. The Collège de France and the ENS have signed an agreement allowing our teams of physicists to work in the new laboratories made available to them by the Collège, while remaining administratively and intellectually attached to the ENS. They will thus also remain close to a community of researchers and students whose initiative and qualities we value. I wish you good luck as you embark on this new stage of your already rich career, and I now leave the floor to you.

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