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.
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. 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. 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. A tributary statement from the Laboratoire Kastler Brossel after his passing also recalled his willingness to discuss ideas with anyone who approached him.
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. Cohen-Tannoudji visited Bangalore again in early 1999, where he participated in an Indo-French workshop at the Indian Institute of Astrophysics. 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.
