The French physicist Claude Cohen-Tannoudji, who helped develop techniques to cool atoms to barely above absolute zero using laser light, passed away on October 22, aged 93. His work revolutionised physics by giving scientists a way to manipulate atoms and probe their quantum properties, work for which he won a piece of the physics Nobel Prize in 1997.
As it happens, Cohen-Tannoudji also had many connections with physics in India, including acknowledging the work of a little-known — even in India — physicist who had died three decades before the prize was awarded.
Cohen-Tannoudji was born in French Algeria in 1933. He moved to Paris when he was 20 to study mathematics, before lectures by the physicist Alfred Kastler convinced him he should be studying physics instead. He soon joined a group led by Kastler and Jean Brossel in 1955 and then spent most of his career at the institution since called the Laboratoire Kastler Brossel.
Sisyphus cooling
Conventionally, lasers are good at delivering energy to objects, so the idea of using them to cool atoms down can seem counterintuitive. The key detail is that laser light also has momentum.
In one technique called Doppler cooling, scientists shine laser beams at atoms from different directions. The lasers are configured such that an atom moving towards one laser beam is more likely to absorb photons from that source than from the beams on other sides. When the atom has absorbed a photon, its own energy briefly rises before falling again as the atom spits the photon back out.
Here is the catch: the atom always absorbs photons in one direction but spits it out in random directions. So the kick provided by the exiting photon is cancelled by the kicks the atom is getting in multiple directions — whereas the reverse-kick happens in only one direction: in the direction of that one laser beam. So as the atom repeatedly absorbs and emits photons, its average kinetic energy, and therefore its temperature, drops.
Clever though it was, Doppler cooling had a lower limit: as the atom cooled, the lasers became less effective at slowing it further because they could no longer distinguish the direction of motion as easily. In 1988, the U.S. physicist William Phillips led a team that produced atoms colder than the Doppler cooling limit — and Cohen-Tannoudji and his peers, but especially Jean Dalibard, explained how this was possible. It had to do with the polarisation of light and the energy levels of the electrons in the target atoms.
Their explanation proved important because it led to a new technique called Sisyphus cooling, named for the doomed figure in Greek mythology who had to keep rolling a boulder uphill only for it to roll down again. Here, an atom’s kinetic energy is repeatedly converted to potential energy, which is ‘pumped out’ using a technique similar to Doppler cooling.
The name of the technique comes from a different way to visualise the atom’s experience, so to speak. Lasers are used to create a space where the atom, in motion, is forced to move from a part where it has lower potential energy to where it has more. The same thing happens when you climb a staircase: as you go from the ground floor to the first floor, you expend kinetic energy as you move but your body also accumulates more potential energy.
Once the atom is at the ‘top’, a process called an optical pump cools it down to have less energy than when it was on the ground floor. Then it is forced to climb another set of stairs. This way, repeatedly, the atom can be cooled to extremely low temperatures.
Cohen-Tannoudji’s group developed methods to cool atoms to a few nanokelvins — billionths of a degree above absolute zero — paving the way for advanced atomic clocks and atom interferometers.
Remembering Pancharatnam
In 1994, Cohen-Tannoudji contributed a three-page essay for a special issue of the Indian journal Current Science commemorating physicist S. Pancharatnam, who had worked with C.V. Raman in Bangalore (now Bengaluru) and later with George William Series in Britain. Unfortunately, Pancharatnam passed away in 1969, when he was only 35.
Pancharatnam did most of his work before tunable lasers became widely available, and in the same areas as Cohen-Tannoudji. In his article, Cohen-Tannoudji recalled Pancharatnam’s “series of beautiful papers” on optical pumping and remarked that recent developments in the field would have pleased him. He also acknowledged that some of the theoretical foundations of (then) contemporary laser cooling techniques had been considered well before the experimental proof could be devised.
The same issue of Current Science also featured an essay by the British physicist Michael Berry, who recalled visiting Bangalore in 1987 and discussing the work of Pancharatnam with Sivaraj Ramaseshan, which he said helped him appreciate the connection with Pancharatnam’s previous work and Berry’s own in the 1980s.
Cohen-Tannoudji visited Bangalore again in early 1999, where he participated in an Indo-French workshop at the Indian Institute of Astrophysics. He also visited the Tata Institute of Fundamental Research in Mumbai on the same trip, where he delivered the first Homi J. Bhabha Lecture, followed by a lecture in New Delhi.
He also co-authored a 2001 paper in Physical Review Letters with C.S. Unnikrishnan that reported an unusual phase of matter called a Bose-Einstein condensate in a thin gas of helium — a significant achievement in atomic physics.
Cohen-Tannoudji was widely regarded as a generous figure in a profession that more than rarely featured fierce competition, and competitive personalities. Alain Aspect, who won a share of the physics Nobel Prize in 2022, has said that while some senior physicists considered Aspect’s early work (on experimentally testing the Bell’s inequalities) to be a waste of time, Cohen-Tannoudji was encouraging and helped Aspect be taken seriously.
In one 2015 interview to Weizmann Direct, a publication of the Weizmann Institute in Israel, Cohen-Tannoudji recalled organising meetings bringing Israeli and Arab, including Palestinian, scientists together for week-long discussions. He signed a letter in 2021 calling for the release of doctor and human rights activist Binayak Sen. A tributary statement from the Laboratoire Kastler Brossel after his passing also recalled his willingness to discuss ideas with anyone who approached him.
mukunth.v@thehindu.co.in
Published – October 26, 2026 07:20 am IST
