Claude Cohen-Tannoudji

Claude Cohen-Tannoudji was born in Constantine, Algeria, on 1 April 1933, into a Sephardic Jewish family whose name records their origins in Tangier and Tunisia. The family survived the Vichy years in Algeria, where he was expelled from school under the antisemitic laws, and he came to Paris in 1953 to study at the École Normale Supérieure. There he attended Alfred Kastler's lectures, took his doctorate under Kastler and Jean Brossel, and stayed within their optical-pumping tradition — the French school that had been manipulating atoms with light since well before lasers existed.

He is known to every physics student for a second reason: the textbook. Quantum Mechanics, written with Bernard Diu and Franck Laloë and published in 1973, is two enormous volumes that have taught the subject to generations, and his later Atom-Photon Interactions with Jacques Dupont-Roc and Gilbert Grynberg did the same for quantum optics. He held the chair of atomic and molecular physics at the Collège de France from 1973.

His decisive contribution was theoretical. When William Phillips measured the temperature of optical molasses in 1988, he found it far below the Doppler limit — the floor that theory said random photon re-emission imposed. The result was, on the accepted theory, impossible. Cohen-Tannoudji and Jean Dalibard worked out the mechanism: in the polarisation gradients of the interfering beams, an atom repeatedly climbs a potential hill, is optically pumped to a different sublevel at the top, and must climb again, losing kinetic energy each time. They named it Sisyphus cooling, and it explained an entire order of magnitude of unexpected cold.

He shared the 1997 Nobel Prize in Physics with Steven Chu and William Phillips. His group went further still, developing velocity-selective coherent population trapping, which parks atoms in a dark state that cannot absorb light and pushes below even the single-photon recoil limit — into nanokelvin. He remains emeritus at the ENS and the Collège de France.