Brossel asked him to teach quantum mechanics at a graduate level and a small group was established where every year a new student would join and do a post-graduate thesis or a Ph.D. In 1967, he taught quantum mechanics at a lower level. The book “Quantum Mechanics” originated from this teaching experience and was done in collaboration with Franck Laloë and Bernard Diu. Understanding atom-photon interactions in the high intensity limit where perturbative treatments are no longer valid was one of the main goals of the research group. They developed a new approach to these problems where one considers the “atom + photons” system as a global isolated system described by a time-independent Hamiltonian having true energy levels. This system was called the “dressed atom”. The dressed atom approach was useful in providing new physical insights into atom-photon interactions. New physical effects, which were difficult to predict by standard semi-classical methods, were appearing in the energy diagram of the dressed atom when examining how this energy diagram changes when the number of photons increases.
The dressed atom approach was introduced in the radio-frequency range while Nicole Polonsky, Serge Haroche, Jacques Dupont-Roc, Claire Landré, Gilbert Grynberg, Maryvonne Ledourneuf, Claude Fabre were working on their thesis. One of the effects that was predicted and observed was the modification, and even the cancellation of the Landé factor of an atomic level by interaction with an intense, high frequency radio-frequency field. This effect presents some analogy with the g-2 anomaly of the electron spin except that it has the opposite sign: theg-factor of the atomic level is reduced by virtual absorption and reemission of RF photons whereas the factor of the electron spin is enhanced by radiative corrections. A lot of effort was devoted to the interpretation of this change of sign and this led, with Jacques Dupont-Roc and Jean Dalibard, to propose new physical pictures involving the respective contributions of vacuum fluctuations and radiation reaction.
The dressed atom approach has also been useful in the optical domain. Spontaneous emission plays an important role as a damping mechanism and as a source of fluorescence photons. Serge Reynaud and Cohen-Tannoudjii applied this approach to the interpretation of resonance fluorescence in intense resonant laser beams. New physical pictures were given for the Mollow triplet and for the absorption spectrum of a weak probe beam, with the prediction and the observation of new Doppler free lines resulting from a compensation of the Doppler effect by velocity dependent light shifts. The picture of the dressed atom radiative cascade also provided new insights into photon correlations and photon antibunching. New types of time correlations between the photons emitted in the two sidebands of the Mollow triplet were predicted in this way and observed experimentally at the Institut d’Optique in collaboration with Alain Aspect.
In 1973, Cohen-Tannoudjii was apointed a Professor at the Collège de France, created in 1530 by King François I to counterbalance the overly rigid scholastic approach of the Sorbonne. The college is renowned for its flexibility and the professors are free to choose their lecture topics. These lectures must change and address different topics yearly. This stimulates him to explore new fields and to challenge himself, leading to many of the research lines that have been explored by his research group. This teaching experience has incited the two books on quantum electrodynamics and quantum optics written with Jacques Dupont-Roc and Gilbert Grynberg.
In the early 1980s, Cohen-Tannoudjii lectured on radiative forces, a new field for that time while also trying with Serge Reynaud, Christian Tanguy and Jean Dalibard to apply the dressed atom approach to the interpretation of atomic motion in a laser wave.
In 1984 he was given the opportunity to appoint an Associate Director for his laboratory. Alain Aspect accepted and then joined Cohen-Tannoudjii in forming, with Jean Dalibard, an experimental group on laser cooling and trapping. A year later, Christophe Salomon joined this group. They began to investigate a new cooling mechanism suggested by the dressed atom approach and that resulted from correlations between the spatial modulations of the dressed state energies in a high intensity laser standing wave and the spatial modulations of the spontaneous rates between the dressed states. As a result of these correlations, the moving atom is running up potential hills more frequently than down. The scheme was named “stimulated blue molasses” because it appears for a blue detuning of the cooling lasers, contrary to what happens for Doppler molasses that require a red detuning. It was the first high intensity version of what is called now “Sisyphus cooling” a denomination introduced in 1986. Shortly thereafter they observed the channeling of atoms at the nodes or antinodes of a standing wave. It was the first demonstration of laser confinement of neutral atoms in optical-wavelength-size regions.
In 1988, when sub-Doppler temperatures were observed, they were prepared with their background in optical pumping, light shifts and dressed atoms, to find the explanation of such anomalous low temperatures. They were resulting from another (low intensity) version of Sisyphus cooling.
Along with Alain Aspect and Ennio Arimondo, the group explored the possibility of applying coherent population trapping to laser cooling. By making such a quantum interference effect velocity selective, the group was able to demonstrate a new cooling scheme with no lower limit, which can notably cool atoms below the recoil limit corresponding to the recoil kinetic energy of an atom absorbing or emitting a single photon. These developments opened the microKelvin and the nanoKelvin range to laser cooling, and allowed several new applications to be explored with success.
Claude Cohen-Tannoudjii expresses his gratitude to all those who have helped him live such a great adventure: his family, his teachers, his students and fellow colleagues all over the world. He dedicates his Nobel Lecture to the memory of his son, Alain.
Adapted from “Claude Cohen-Tannoudji – Autobiography”. Nobelprize.org http://www.nobelprize.org/nobel_prizes/physics/laureates/1997/cohen-tannoudji-autobio.html