Alfred Kastler

Alfred Kastler was born on May 3, 1902, in Guebwiller, Alsace, which was then part of the German Empire. At the start of World War I, his family fled the fighting and took refuge near Colmar, where the young Alfred attended high school and learned French as a foreign language. Although very attracted to literature —he would continue writing poetry in German throughout his life— he eventually chose to study science in his final year of school. In 1921, he was admitted to the École normale supérieure, where he specialized in physics under the guidance of Henri Abraham and Eugène Bloch. The latter, a pioneer in teaching quantum theory in France, introduced him to the fundamental role of angular momentum in atomic physics, a concept that would underpin his later discoveries.

In 1924, while still a student, Alfred Kastler married Élise, a historian. After ranking first in the physics agrégation in 1926, he began a five-year career as a physics teacher in high schools in Mulhouse, Colmar, and Bordeaux. It was during this time that he discovered what he described as “a passion for teaching,” which would remain central to his academic career.

Starting in 1929, while still a high school teacher, Alfred Kastler began working at the University of Bordeaux’s laboratory. Professor Pierre Daure offered him an assistant position, enabling him to undertake a doctoral thesis on a topic of his own choosing. Building on his reflections on the transfer of angular momentum by light waves, he focused his research on the exchange of angular momentum between atoms and light, a topic that foreshadowed optical pumping. Alfred Kastler specifically studied the polarization of fluorescence emitted by a mercury vapor when atoms underwent successive excitations at different wavelengths. He demonstrated that the exchanges between the atoms in the cell and the absorbed and re-emitted light followed the universal law of angular momentum conservation.

After defending his thesis in 1936, Alfred Kastler was immediately appointed assistant professor at the University of Clermont-Ferrand, before returning to Bordeaux as a professor in 1938. In 1941, Georges Bruhat invited him to Paris to join the ENS, succeeding Pierre Auger. There, he held a chair at the University of Paris assigned to ENS. Until the late 1940s, his research focused on Raman spectra of single crystals and the emission of sodium in the sky. It was during this time that he met Jean Brossel, then a student at ENS, who would become his lifelong collaborator.

As post-war French laboratories suffered from severe shortages, Alfred Kastler advised Jean Brossel to pursue his research training in the United Kingdom and the United States. The two physicists remained in correspondence until Brossel’s return to France.

Meanwhile, Alfred Kastler’s lectures at ENS gained an excellent reputation. The publisher Masson entrusted him with revising two textbooks by Georges Bruhat. In Optics, he added a new section on atomic spectroscopy, which had seen significant developments. In Thermodynamics, he completely rewrote the content. These revisions led to the renowned Bruhat-Kastler textbooks.

Optical Pumping

Alfred Kastler and Jean Brossel worked together on double resonance, a technique combining optical and radio-frequency resonances to precisely measure some atomic energy levels. This research, an extension of Kastler’s thesis, led to the development of optical pumping, an application of these principles to atomic ground states. The method involves using circularly polarized light to align atomic spins in their ground state through angular momentum exchange. Alfred Kastler proposed the concept in 1950, and in 1952, Jean Brossel and Jacques Winter, then a student in the group, conducted the first experimental demonstration on a sodium atomic beam. The sodium atoms were illuminated for a few milliseconds along their trajectory by an intense sodium lamp, and their spin polarization was measured optically via fluorescence.

Optical pumping was soon applied to atomic vapors confined in glass cells, using special coatings to reduce depolarization from wall collisions. This breakthrough led to numerous discoveries in the following years, particularly at the laboratory: nuclear spin orientation of certain atoms, relaxation mechanisms due to atomic collisions, coherence transfer between excited atoms leading to prolonged lifetimes, multi-quantum transitions, energy level shifts, etc. Optical pumping is also considered a decisive step in the discovery of the laser in 1960. Moreover, Alfred Kastler had already explored atomic recoil effects induced by laser pulses, which later became fundamental in the physics of cold atoms, a field where the laboratory would excel after his death.

Optical pumping also resulted in significant technological applications, which Kastler regarded as particularly important. Optically pumped magnetometers, highly accurate in weak magnetic fields, were used on the Voyager spacecraft and became standard in geophysics. Another major application was atomic clocks, which, by the end of Kastler’s lifetime, had already achieved a precision improvement of three orders of magnitude. Indeed, in 1958, he became the director of the newly established Atomic Clock Laboratory.

In 1951, Alfred Kastler and Jean Brossel founded the Laboratoire de spectroscopie hertzienne de l’ENS, under challenging post-war conditions. Their unique partnership was instrumental in co-directing the lab: Kastler focused on fundamental principles and theoretical concepts, while Brossel handled experimental aspects.

Although the first five young researchers (Jacques Blamont, Jacques Winter, Jean-Claude Pébay-Peyroula, Jean-Pierre Barrat, Jean Margerie) left the laboratory after their thesis, the laboratory began to expand in the early 1960s thanks to permanent positions. Bernard Cagnac ¹, Marie-Anne Bouchiat, and Claude Cohen-Tannoudji were the first to benefit. Cohen-Tannoudji, who would be awarded the Nobel Prize in 1997, developed theoretical tools for studying the interactions between light and atoms, which would be used by all the laboratory’s teams. By 1966, when Alfred Kastler was awarded the Nobel Prize, the laboratory already had a staff of around thirty.

In 1967, the expansion of the laboratory into premises on the new campus of the Paris Faculty of Science at Jussieu opened up further development opportunities. In the 1970s, wavelength-tunable dye lasers were adopted by a large number of teams on both sites to conduct experiments of unprecedented precision. From his office at ENS rue Lhomond, Alfred Kastler kept abreast of all the laboratory’s scientific activity and continued to imagine new research themes, which he presented at the Saturday morning seminar. His appointment to the CNRS in 1968 enabled him to cope better with the fatigue associated with the many demands made on him after winning the Nobel Prize. He remained co-director of the laboratory until his retirement in 1972, before moving to Bandol in 1983, where he died in January 1984, followed by his wife Élise a few months later.

Alfred Kastler had a very endearing personality: modest and exquisitely polite, always ready to listen to others, he was nonetheless firm in his commitment to social and political life. In 1958, during the Algerian war, he criticized the declarations of the Committee of Public Safety in Algiers and called for resistance, which led to his apartment being plastered. He was active in pacifist movements from an early age, and was one of the “thirteen citizens of the world” who launched an appeal against nuclear weapons in 1966, before joining the Pugwash movement founded by Albert Einstein and Bertrand Russel. In December 1981, he helped found the Committee for Nuclear Disarmament in Europe.

He also defended human rights and rallied to support Jewish physicists from the USSR, the “refuseniks”, who were forbidden to emigrate from their country. At the French Academy of Sciences, he is in charge of the association for the support of refugee scientists. He is committed to helping developing countries, notably by helping to organize the International Center for Theoretical Physics in Trieste.

Recognition

Alfred Kastler played a major role in the advancement of research in France. He became a member of the French Academy of Sciences in 1964, President of the French Physical Society in 1954, a member of the Comité consultatif and the Conseil de l’enseignement supérieur in 1947, and Chairman of the Board of the Institut d’Optique in 1962.

His international influence was considerable. He was a member of more than a dozen science academies worldwide, and won the Félix-Robin prize in 1946, the Holweck prize in 1954 and the Wilhem Exner medal in 1979. He also gave his name to the Franco-German Gentner-Kastler Grand Prize. He was awarded the CNRS Gold Medal in 1964, before the Nobel Prize in 1966. However, he always regretted that the latter was not shared with Jean Brossel.

In 1985, a year after his death, a major international symposium was held in Paris to honour his memory. Some one hundred prestigious speakers from all over the world testified to the scientific influence of his work. Today, a square in Paris bears Alfred Kastler’s name, as do several high schools in Cergy-Pontoise, Talence, Guebwiller, Stenay and Casablanca.