Claude Cohen-Tannoudji, Nobel Physicist, Dies; Devised a Way to Catch Atoms

Claude Cohen-Tannoudji, a French physicist who shared the Nobel Prize for creating a way to trap and study individual atoms, leading scientists to a better understanding of how light and matter interact and paving the way for advances in measurement and technology, died on Thursday. He was 93.

His death was announced by the Laboratoire Kastler Brossel, an institute where he worked for many decades that is part of the French university École Normal Supérieure. The statement did not provide further details.

Trapping atoms is a difficult feat because in a gaseous state at room temperature, they move in random directions at speeds of about 2,500 miles per hour. Cooling atoms into a liquid or solid, so that they slow down, then renders them too close to each other to analyze individually.

In the late 1960s, Arthur Ashkin, while working at Bell Labs in New Jersey, created what he called optical tweezers to trap microscopic objects in a laser beam, a discovery for which he was awarded the Nobel Prize in 2018. Scientists soon realized that Dr. Ashkin’s discovery might be used to trap subatomic particles.

Light is composed of tiny particles called photons, which can be absorbed and emitted by atoms. By bombarding atoms with laser light, the atoms briefly absorb the photons before emitting new ones, each time recoiling slightly, a process that slows them down and cools them. The blasts of laser light therefore act like a thick liquid, which scientists call optical molasses.

(The tails of comets, which are bombarded by photons from the sun, are large examples of the recoil and cooling effect.)

Optical molasses are used to lower the temperature of atoms to as close as possible to absolute zero, the lowest possible temperature, at which motion comes to an almost complete stop. In the 1970s, scientists thought that they should be able to cool atoms to about 240 millionths of a degree Kelvin above that temperature (or about -459.67 degrees Fahrenheit).

Working with sodium atoms, Steven Chu and some colleagues at Bell Labs, including Dr. Ashkin, achieved that goal in experiments they ran in 1984 and 1985.

A few years later, William Daniel Phillips and a group of scientists at the National Institute of Standards and Technology in Maryland were able to cool sodium atoms to 40 millionths of a degree Kelvin above absolute zero, much lower than had been previously thought possible.

Starting in 1988, Dr. Cohen-Tannoudji and colleagues at the École Normal Supérieure in Paris began experiments to see if they could further lower the temperature of trapped atoms, working this time with helium. In 1995, they achieved a milestone: They cooled the helium to 0.18 millionths of a degree above absolute zero, a temperature at which the atoms were moving at less than an inch per second.

Dr. Cohen-Tannoudji, Dr. Phillips and Dr. Chu were awarded the Nobel Prize two years later.

In announcing the prize, the Nobel Institute explained that the physicists’ work has been used to build better atomic clocks, to improve atomic interferometers (for making extremely precise measurements), and in atomic lithography (used in making the tiniest semiconductors).

Their research had also made possible the creation of the first Bose-Einstein condensate in 1995, a state of matter predicted in 1925 by Albert Einstein and Satyendra Nath Bose in which microscopic quantum-mechanical phenomena become visible on a macroscopic scale. Eric A. Cornell, Wolfgang Ketterle and Carl E. Wieman were awarded the Nobel Prize in 2001 for creating the condensate.

Claude Cohen-Tannoudji was born in Constantine, Algeria, on April 1, 1933. He was the oldest of three children of Abraham and Sarah Cohen-Tannoudji.

Dr. Cohen-Tannoudji wrote in his Nobel autobiography that his family was Jewish and had fled the Spanish Inquisition in the 16th century, settling in Tangier, Morocco. After a while, the family had to flee again, this time to Tunisia. There, they became known as Cohen-Tannoudji, which in the Berber language spoken in North Africa means “the Cohen family from Tangier.” Jews continued to be persecuted there and the family eventually relocated to Algeria.

During World War II, Dr. Cohen-Tannoudji was in school in Algeria, which was under control of the Nazi-collaborating Vichy government. The allied invasion of North Africa in November 1942, which freed the region, spared the family from persecution and deportation.

Dr. Cohen-Tannoudji entered the École Normale Supérieure in Paris in 1953.

He was initially interested in studying mathematics, but he found the lectures of the physicist Alfred Kastler so stimulating that he switched majors. Dr. Kastler, who was awarded the Nobel Prize in 1966, was known for his work in optical pumping, in which light is used to raise the energy level of electrons in atoms — a precursor to the development of lasers.

After finishing his undergraduate studies, Dr. Cohen-Tannoudji spent 28 months in required military service. During that time, he met and married Jacqueline Veyrat, who became a high school physics and chemistry teacher. Together they had three children, Alain, Joëlle and Michel. Alain died at 34. A list of his survivors was not immediately available.

Dr. Cohen-Tannoudji returned to the École Normale in 1960 to pursue a doctorate. He worked in a laboratory with Dr. Kastler, Jean Brossel and others on studying and understanding optical pumping and Zeeman effects, which is the splitting of light into spectra in the presence of a magnetic field.

After receiving his Ph.D. in 1962, Dr. Cohen-Tannoudji accepted a position at the University of Paris. There, he set up his own research group to continue to study the interactions of light and matter.

In 1973, Dr. Cohen-Tannoudji was appointed a professor at the Collège de France. It was there, in 1984, that he created a new experimental laboratory group to study laser cooling and the trapping of atoms. Among those that he recruited to join his laboratory was Alain Aspect, who shared the 2022 Nobel Prize for his work on proving and exploring the weird phenomenon of quantum entanglement.

One of the techniques that Dr. Cohen-Tannoudji’s group developed, which was a fundamental part of the work that led to the Nobel Prize, was how to trap an atom and cool it over and over again, essentially causing it to progressively lose potential energy. They called the method Sisyphus cooling.

Dr. Cohen-Tannoudji wrote more than a half-dozen books, including two on quantum physics that are still widely used in French universities.

In addition to the Nobel Prize, Dr. Cohen-Tannoudji received dozens of honorary degrees and awards, including the Legion of Honor of France.

In a February 2004 interview with Etienne Klein for the Fondation Hugot du Collège de France, Dr. Cohen-Tannoudji expressed concern about the pressures put on researchers to produce immediate results, particularly by governments. Pragmatic uses from discoveries always come much later, he explained, and they are often unexpected.

“Trying to program research with fixed objectives makes no sense,” he said. “That is always the problem with politicians: They have short-term objectives. They want results during their terms of office of four or five years. They set objectives hoping to solve problems like cancer, like AIDS, energy in four years. That is not how things happen. It is first necessary to have a fundamental discovery to unlock something in our understanding.”

Alex Traub contributed reporting.

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