Paul Cohen-Tannoudji

French physicist (1933–)

Cohen-Tannoudji was educated at the Ecole Normale Supéreure. In 1973 he was appointed professor of atomic and molecular physics at the Collège de France, Paris.

Cohen-Tannoudji, following on from the work of William Phillips and Steven Chu, sought to understand and improve the process of optical cooling of single atoms. He proposed that laser traps operate by a process of what has since been called Sisyphus cooling. The laser beams, he argued, produce a series of standing waves of light polarized in different directions. As the atoms pass through the various fields their energy levels and thereby temperature is lowered.

Early efforts at cooling had found that laser traps unfortunately also tend to agitate atoms causing them to move out of the beam. One solution proposed by Cohen-Tannoudji allowed helium atoms to be cooled to 0.18 microkelvins. The method exploited the fact that atoms could occupy a particular combination of two distinct quantum states with different velocities, in which they remain invisible to any additional photons. Thus, once in this state, their energy cannot be increased by any further photon collisions.

For his work in this field Cohen-Tannoudji shared the 1997 Nobel Prize for physics with Chu and Phillips.