The physicist shared the 1997 physics Nobel for methods that cool and trap atoms with laser light. His explanation of unexpectedly low temperatures helped researchers cool atoms further.
Physicist and Nobel laureate Claude Cohen-Tannoudji, whose theoretical work explained how laser light could cool atoms beyond an expected limit, died in Paris on 22 October 2026, aged 93. Physics World said in its 28 October account that he used that understanding to devise methods for reaching still lower temperatures.
He shared the 1997 Nobel Prize in Physics with Steven Chu and William Phillips “for development of methods to cool and trap atoms with laser light”. Chu and Phillips carried out pioneering experiments in the 1980s that cooled atomic gases to well below one millikelvin, or one thousandth of a kelvin. Cohen-Tannoudji helped explain why experiments could go below the temperatures predicted by the initial theory, known as Doppler cooling.
In Doppler cooling, two opposing laser beams have their light tuned just below a frequency that an atom absorbs. The Doppler effect—the change in frequency seen by a moving atom—makes the atom absorb more photons, or particles of light, from the beam opposing its motion. Repeated absorption and randomly directed re-emission reduce the atoms’ random motion; using beams along three directions cools the whole cloud.
The basic Doppler model treats an atom as having just two internal energy levels. It predicts a minimum temperature where the slowing effect balances heating from the random momentum kicks that atoms receive when they emit photons. That minimum is the Doppler limit.
In 1988, Phillips’s experiments reached 40 microkelvin, below the Doppler prediction. Cohen-Tannoudji and Jean Dalibard explained the unexpected cooling by accounting for atoms’ more complex internal structure and changes in the laser light’s polarization—the orientation of its electric field.
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Cooling below the Doppler limit still leaves a lower threshold: the photon-recoil temperature, associated with the kick from emitting a single photon. Cohen-Tannoudji, Ennio Arimondo and Alain Aspect developed a different method, velocity-selective coherent population trapping, using atomic “dark states” that do not absorb the cooling light. The technique crossed the recoil limit in one dimension in 1988 and was extended to three dimensions in 1995.
Physics World said Cohen-Tannoudji, working with Aspect and other collaborators, cooled helium gas to 180 nanokelvin in 1995. That is 180 billionths of a kelvin above absolute zero.
By reducing atoms’ random thermal motion, laser cooling reduces motion-related Doppler shifts in spectroscopy. This enables high-precision measurements and instruments such as optical clocks.
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