Claude Cohen-Tannoudji explained how light can chill atoms

Person: Claude Cohen-Tannoudji
Lived: b. 1933
Field: Atomic physics
From: Algerian-born French

The story

Claude Cohen-Tannoudji was born in 1933 in Constantine, Algeria, into a Sephardic Jewish family whose name records its origins in Tangier after flight from Spain during the Inquisition. Trained at the École Normale Supérieure under Alfred Kastler, he spent decades studying how atoms and light interact, and in 1973 became a professor at the Collège de France.

At room temperature the atoms in a gas rush about at speeds of the order of a thousand metres a second, far too fast to measure precisely. In the 1980s physicists learned to slow them with laser light, a technique called optical molasses. In 1988 William Phillips in the United States found that sodium atoms could be cooled to about 40 millionths of a degree above absolute zero, six times colder than theory allowed. Cohen-Tannoudji’s group, with Jean Dalibard, explained why: in the pattern of laser beams, atoms keep climbing small hills of energy and losing energy at the top, like Sisyphus endlessly pushing his stone uphill. They named it Sisyphus cooling.

His group then went further. Between 1988 and 1995 they developed a method that steers the slowest helium atoms into a dark state in which they stop absorbing light, so they are no longer jostled by it. Using six laser beams they reached 0.18 millionths of a degree, below the limit once thought to be set by the kick of a single photon. He shared the 1997 Nobel Prize in Physics with Steven Chu and William Phillips.

The most important everyday use is in timekeeping. In a caesium fountain clock, laser-cooled atoms are tossed gently upward and fall back, giving about a second to measure their natural vibration instead of the tens of millionths of a second possible with a hot atomic beam. Fountain clocks of this kind serve as primary time standards at national laboratories such as NIST in the United States, the Paris Observatory, Britain’s National Physical Laboratory and Germany’s PTB, and contribute to International Atomic Time.

Why it matters

Laser-cooled atoms are the heart of the fountain clocks that define the second and keep international time, on which navigation satellites, telecommunications and the internet depend, and of new quantum sensors.

About Claude Cohen-Tannoudji

Algerian-born physicist who showed how laser light can chill atoms to a millionth of a degree, the basis of today’s most precise atomic clocks, and co-wrote a classic quantum textbook. Nobel 1997.

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Claude Cohen-Tannoudji

b. 1933 · Atomic physics

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