The Cohen family from Tangier
Claude Cohen-Tannoudji was born on April 1, 1933, in Constantine, Algeria, then part of France, to Abraham Cohen-Tannoudji and Sarah Sebbah. His Sephardic Jewish family, he explained, had fled Spain during the Inquisition, settled in Tangier and then, in the sixteenth century, moved on to Tunisia and Algeria. The name simply means the Cohen family from Tangier. Algeria’s Jews had received French citizenship in 1870.
His parents lived modestly, and their great concern was their children’s education. His father was self-taught but intensely curious about biblical and Talmudic texts, philosophy, psychoanalysis and history, and he passed on to his son a love of study, discussion and debate. Claude later described these as the fundamental features of the Jewish tradition: studying, learning and sharing knowledge with others.
Spared by history
He counted himself lucky to have escaped the century’s worst events. When American forces landed in Algeria in November 1942, he wrote, they saved his family from the Nazi persecutions spreading across Europe. He finished school in Algiers and left for Paris in 1953, before the Algerian War and the turbulent years that led to independence.
That year he entered the École Normale Supérieure, the elite school that selects France’s top students. He attended mathematics lectures by Henri Cartan and Laurent Schwartz, but the physics lectures of Alfred Kastler were so stimulating, and Kastler so attractive a personality, that he switched to physics. In 1955 he joined Kastler’s small, intensely enthusiastic group, which was developing optical pumping, a way of using light to put atoms into chosen states, the work for which Kastler would win the Nobel Prize.
In the summer of 1955 he spent two months at the Les Houches summer school in the French Alps, where about six lectures a day were given by physicists such as Julian Schwinger, Norman Ramsey and Wolfgang Pauli. He then passed the agrégation, the competitive examination for teaching posts, in which Kastler helped train the candidates in how to organise and present a lecture, a skill that later showed in his own teaching.
Light shifts and the dressed atom
In 1958 he married Jacqueline Veyrat, a high school physics and chemistry teacher. His studies were then interrupted by 28 months of military service, lengthened by the Algerian War, part of which he spent on rocket experiments that released sodium clouds into the upper atmosphere. In 1960 he returned to the laboratory for his doctorate under Kastler and Jean Brossel. Calculations with Jean-Pierre Barrat predicted a surprising effect: light shining on atoms should slightly shift their energy levels. On Christmas Eve 1960 he saw it for the first time in the laboratory. Kastler called it the lamp shift; it is now known as the light shift, and it later became essential to laser cooling. He received his doctorate in 1962.
In the following years his group developed the dressed atom, a way of describing an atom together with the light around it as a single system. It gave new physical insight into how atoms and light interact and predicted new effects that were then observed.
With Serge Reynaud he applied the dressed atom to atoms in intense laser light, explaining the triple-peaked spectrum of light they scatter and predicting new correlations between the photons they emit, which were later observed in collaboration with Alain Aspect at the Institut d’Optique. In 1976 he spent a sabbatical in the United States, lecturing at Harvard and as a visiting professor at MIT.
Chilling atoms with light
In 1973 Cohen-Tannoudji became a professor at the Collège de France, an institution founded in 1530 where lectures are open to all and professors must lecture on a new topic each year. In the early 1980s he chose to lecture on the forces that light exerts on atoms, then a new field. In 1984 he invited Alain Aspect to join him and Jean Dalibard in a new experimental group on laser cooling and trapping, and Christophe Salomon joined a year later.
At room temperature the atoms and molecules of the air rush about at some 4,000 kilometres an hour, far too fast to study precisely. Laser cooling slows them by bombarding them with light. In 1988 William Phillips in the United States found that sodium atoms could be cooled well below the limit theory then predicted. Cohen-Tannoudji’s group, prepared by decades of work on optical pumping, light shifts and dressed atoms, explained why. In a pattern of laser beams, atoms are continually made to climb small energy hills and lose energy at the top, like Sisyphus endlessly rolling his stone uphill. The Paris group named this Sisyphus cooling.
In the mid-1980s the Paris group also showed that atoms could be channelled into the nodes or antinodes of a standing light wave, the first demonstration of confining neutral atoms in regions as small as a wavelength of light. This was an early step toward optical lattices, egg-crate patterns of light that hold atoms in regular arrays and are now used in the most accurate optical clocks and in quantum simulators. Working with Phillips, his group showed that caesium atoms could be cooled in optical molasses to about two millionths of a degree, roughly ten times the limit set by a single photon’s kick.
They then went further. Between 1988 and 1995, using helium atoms, they developed a method that puts the slowest atoms into a dark state in which they stop absorbing light, so they are no longer jostled by it. With six laser beams they cooled atoms to 0.18 millionths of a degree above absolute zero, below the limit once thought to be set by the kick of a single photon, so that the atoms crawled along at about two centimetres a second. In 1997 he shared the Nobel Prize in Physics with Steven Chu and William Phillips for the development of methods to cool and trap atoms with laser light.
Clocks, sensors and new matter
Laser cooling changed physics and technology. Its most direct everyday impact is on timekeeping. Atomic clocks measure time by the steady vibration of atoms, and moving atoms blur the measurement. In a fountain clock, laser-cooled caesium atoms are tossed gently upward and fall back under gravity, giving about a second to measure them instead of the tens of millionths of a second possible with a hot beam of atoms. Fountain clocks of this kind serve as primary time standards in national laboratories such as NIST in the United States, the Paris Observatory, Britain’s National Physical Laboratory and Germany’s PTB, and they help define the international time scale. That time scale, in turn, keeps global navigation satellite systems, telecommunications networks and financial markets synchronised. Even more precise optical clocks now under development also depend on laser-cooled atoms.
Laser-cooled atoms are also used in atom interferometers that measure gravity and rotation with extraordinary precision, with uses in navigation and geophysics, and they made possible the creation in 1995 of the Bose-Einstein condensate, a new state of matter now used to study quantum physics and to build quantum simulators. Colleagues from his Paris group, among them Jean Dalibard and Christophe Salomon, became leaders in these fields.
A teacher of quantum mechanics
Alongside research, Cohen-Tannoudji is renowned as a teacher. From 1964 he taught quantum mechanics at the University of Paris, and his lectures grew into the two-volume textbook Quantum Mechanics, written with Bernard Diu and Franck Laloë and first published in French in 1973. Translated into English, it became a popular text used by physics students around the world. He also wrote, with Jacques Dupont-Roc and Gilbert Grynberg, two influential books on the interaction of light and atoms. His annual courses at the Collège de France, open to anyone, shaped the field of atomic physics in France and beyond.
He has described the Collège de France rule that every year’s course must cover a new subject as very difficult and demanding but deeply stimulating, and said that without it he would not have started many of the research lines his group explored, including laser cooling itself, which grew out of his lectures on radiative forces.
Family and faith in learning
Cohen-Tannoudji has described his Jewish heritage with warmth and precision: a Sephardic family whose very name records its exile from Spain and its life in North Africa, and a father whose love of texts and debate he regards as the heart of the Jewish tradition. He was the first Nobel laureate in physics born in an Arab country.
His wife Jacqueline shared, he wrote, all the difficult and happy times of a researcher’s life while pursuing her own career as a teacher and raising their three children, Alain, Joëlle and Michel. Their eldest son, Alain, died in 1993 after a long illness, aged 34. Cohen-Tannoudji dedicated his Nobel lecture to his memory.
Legacy
He has received many honours, including the CNRS Gold Medal in 1996 and the Legion of Honour, and in 2015 he signed the Mainau Declaration on climate change with other Nobel laureates. He remains attached to the École Normale Supérieure, where the Laboratoire Kastler Brossel carries on the tradition of Kastler, Brossel and their students.
His legacy lies in the clocks that keep the world’s time, in the ultracold atoms now studied in laboratories everywhere, and in the students who learned quantum mechanics from his book. It is, in his own terms, a life of studying, learning and sharing knowledge with others.
Sources & further reading
Claude Cohen-Tannoudji, Biographical, NobelPrize.org · Press release: The Nobel Prize in Physics 1997, NobelPrize.org · NIST-F1, Wikipedia · Laser cooling, Wikipedia · Claude Cohen-Tannoudji, Wikipedia · Researched from the sources above and checked against them. Spotted an error? Tell us.
A life of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.