Claude Cohen-Tannoudji

In 1959, he began a thesis under the supervision of A. Kastler and J. Brossel, as a CNRS intern. The aim was to develop a rigorous quantum theory of optical pumping. This led to the discovery of new effects, in particular the displacement of levels by non-resonant light, which he was able to demonstrate experimentally. He defended his thesis at the end of 1962.

He was then recruited as an assistant professor, and later as a professor, at the Faculty of Science of the Paris University, which later became the Pierre et Marie Curie University. Here, he taught quantum mechanics at various levels. These lectures formed the basis of the books he later published.

In 1973, on Anatole Abragam’s initiative, he was elected Professor of Atomic and Molecular Physics at the Collège de France. He taught there for 30 years. He is currently “ Emeritus Professor ”. His lectures, given with chalk on the blackboard, always attracted large audiences, as did the seminar that followed. They provided a unique forum for atomic and molecular physics researchers from a wide range of backgrounds.

With his first thesis students, in particular Serge Haroche, he introduced the concept of the “dressed atom” to describe the system formed by an atom interacting with a radiofrequency field. This point of view makes it easy to visualize changes in the eigenfrequencies of the overall system, as well as the properties of the photons it may emit, in particular their correlations. Remarkable results include the prediction of new observable resonances on coherences, and their drastic modification with increasing radiofrequency intensity. The generalization of the “dressed atom” to laser-excited optical transitions, which he established with his student Serge Reynaud, has proved highly valuable in predicting the spectral properties of fluorescent light and the correlations between emitted photons.

The idea that light exerts a force on atoms by exchanging momentum with photons is a long-standing one. Lasers, which are more intense than conventional light sources, offer new possibilities for cooling and trapping atoms ². In the early 1980s, Claude Cohen-Tannoudji set up a research group in this field with Alain Aspect, Christophe Salomon, Jean Dalibard and, from 1993, Michèle Leduc. His group proposed several innovative cooling and trapping methods. In particular, he explains why the limiting temperatures obtained in “optical molasses”, formed by three pairs of counter-propagating light beams, are much lower than expected ². This cooling is due to the “Sisyphus effect”, which brings into play the polarization gradients of light within molasses: atoms are forced to keep climbing potential hills, with optical pumping changing their Zeeman sublevel as soon as they move away from a potential valley floor. The cooling limit is then of the order of the atom’s recoil velocity during spontaneous photon emission, corresponding to temperatures in the microkelvin range.

Claude Cohen-Tannoudji and his team are also proposing another cooling method, enabling temperatures to be reached in the nanokelvin range, i.e. below the recoil energy limit. This method, called “velocity-selective coherent population trapping”, is based on the notion of a black state, i.e. a linear combination of the atom’s Zeeman sublevels rendered insensitive to light by an interference effect. At zero velocity, the interference effect is complete, and the atom absorbs no photons. This is no longer true for a moving atom: it undergoes an optical pumping cycle, and the recoil of the emitted photon changes its velocity; through a random walk in velocity space, it can finally fall to zero velocity, where it remains indefinitely. Claude Cohen-Tannoudji and his group, in collaboration with Jean-Philippe Bouchaud, have proposed a detailed statistical analysis of this cooling, based on the notion of “Lévy flights”.

These contributions earned him the 1997 Nobel Prize, together with William D. Phillips and Steven Chu. This research led to spectacular progress in the field of cold atoms, which has since developed in many directions: Bose-Einstein condensation, simulation of N-body quantum systems, control of interactions between cold atoms, formation of fermionic atom pairs in analogy with the electronic Cooper pairs of superconductivity, application to atomic clocks whose accuracy has made spectacular progress…

Already during his thesis, Claude Cohen-Tannoudji had been asked by Jean Brossel to teach courses in statistical mechanics and relativity. He thus inaugurated a long series of courses that enabled students of all levels, as well as researchers, to familiarize themselves with new subjects. After learning quantum mechanics himself from Albert Messiah’s lectures at CEA-Saclay, he went on to teach the subject at third-year undergraduate and master’s level (at the time the second year of the master’s degree and the first year of postgraduate studies). This led to the publication of three volumes of “Quantum Mechanics”, written with Bernard Diu and Franck Laloë. These books were widely distributed after their publication in 1973, and translated into several languages.

At the level of what would now be the Master 2, he also gave an advanced course on a different subject each year, e.g. the resolvent, Feynman diagrams… Some of these courses gave rise to mimeographed written versions. From these lectures, and from those he gave at the Collège de France after his election in 1973, came two further volumes entitled “Photons and Atoms”, written with Jacques Dupont-Roc and Gilbert Grynberg, focusing on the quantum theory of the electromagnetic field and the various processes associated with its interaction with atoms. They were published in 1987-88, with an English version the following year. Another book, “Advances in Atomic Physics, an overview”, was published in 2011 with David Guéry-Odelin: it covers all aspects of cold atom physics addressed in lectures at the Collège de France up to 2004.

In addition to his lectures at the Collège de France, some of which have been given in Lyon, Grenoble and Bordeaux, and several appearances at the Ecole de Physique Théorique des Houches, Claude Cohen-Tannoudji has been invited to numerous summer schools all over the world. In this way, he has played an important role in making atomic physics and quantum optics an attractive and creative field of physics.