Jean Brossel

Brossel returned to France in 1951 to defend his doctoral thesis on double resonance, with a jury that included Kastler, and settled in the Physics Laboratory of ENS. Later that year, he and Kastler established the Hertzian Spectroscopy Laboratory at ENS, which was renamed the Kastler-Brossel Laboratory during Brossel’s lifetime (and with his consent) in 1994, when Michèle Leduc became the laboratory director. The two men co-directed the laboratory until Kastler’s retirement in 1972, after which Brossel continued as the sole director until 1984.

Brossel’s academic career was brilliant. After starting as a research associate abroad in 1945, he became a research scientist at CNRS upon his return to France in 1951. He was appointed a lecturer in physics at the Faculty of Science at the University of Paris in 1955, where he taught alongside Pierre Aigrain in the PCN program, and by 1957, he was teaching atomic physics. He eventually became a professor, first without a chair and then with a position at the Faculty of Science in Paris (1960-1985). He served as president of the Société Française de Physique (1968-1969), where he vigorously promoted membership among young researchers. Brossel quickly took on numerous significant roles in the academic world, including as a member of the CNRS National Committee (1971-1980) and the University Advisory Committee (1970-1976). In addition to directing the Hertzian Spectroscopy Laboratory (1967-1985), he also held the position of director of the Physics Laboratory at ENS for twelve years (1973-1985), succeeding Yves Rocard in that demanding role. He was elected to the Académie des Sciences in January 1977.

Jean Brossel was the recipient of numerous prestigious awards: the Robin Prize from the Société Française de Physique and the Arsonval Prize from the Collège de France (1954), the Henri de Parville Prize (1955), the Jaffé Prize (1965), and the Grand Prix Ampère (1974) from the Académie des Sciences, as well as the Holweck Prize from the Physical Society (1960) and the Prix des Trois Physiciens (1975). His crowning achievement came in 1984 with the award of the CNRS Gold Medal. The State honored him by naming him an Officer of the Légion d’Honneur and a Grand Cross of the National Order of Merit. An international symposium in tribute to Jean Brossel was organized at the Ministry of Research in 19xx, followed by another event in 2004 at a different location.

He passed away on February 4, 2003, and was buried privately in Périgueux, his hometown, which has a large square named in his honor, surrounded by university institutions.

The direction of the laboratory at ENS with Kastler: renaissance of atomic physics

Once back in France, Brossel began an exceptional collaboration with Alfred Kastler, leading to a flurry of results. They immediately focused on recruiting physicists to develop original themes and establish research teams. Together, they attracted many brilliant young physicists passionate about the field, including Jacques Winter, Jean-Claude Pebay-Peroula, Jacques Blamont, Bernard Cagnac, Marie-Anne Bouchiat, Jean-Pierre Barrat, Jean Margerie, and Claude Cohen-Tannoudji, to name just a few.

The first experiment conducted by Brossel at the ENS Physics Laboratory was the demonstration of optical pumping. This followed the double resonance observed at MIT, which involved manipulating atoms using both optical and hertzian waves: atoms were resonantly excited to an excited state with light, allowing for the observation of hertzian resonance (such as magnetic resonance) by inducing transitions between sub-levels of that excited state. This experimental work carried out at MIT was published in 1949 in the proceedings of the Académie des Sciences. The experimental demonstration of optical pumping was soon conducted by Brossel at ENS in collaboration with Jacques Winter, then a young student. Initially, it was performed on a sodium atomic beam irradiated by yellow light (similar to that from street lamps) that was circularly polarized. The degree of spin orientation of the atoms in the ground state was measured optically via fluorescence. This historic experiment, conducted by Brossel in August 1952, was published in the Journal de Physique in December of the same year. Following this, they learned to detect magnetic resonance phenomena between Zeeman sub-levels of the ground state. The optical pumping method was then adapted for use in glass cells rather than in a beam, opening up a wide range of applications.

Brossel was passionate about the various studies pursued in his laboratory, building on the initial research. His physical intuition often provided explanations for the observed phenomena: relaxation in all types of collisions, level crossings, multi-photon transitions, light scattering, and more. In the second half of the 1960s, a diversification of research topics began with the advent of the laser, the first of which at ENS was built by Bernard Decomps and Michel Dumont. Brossel closely followed all these developments, ultimately overseeing the completion of numerous theses. Beyond his role as director, Brossel attended all the thesis defenses in the laboratory, somewhat surprised to encounter an increasing number of young doctoral and postdoctoral researchers who did not speak French in the hallways over the years. Nevertheless, he was the driving force behind the remarkable development of the laboratory, significantly contributing to the expansion of facilities at the university center of Jussieu, located at the site of the former Halle aux Vins, in the late 1960s.

Alfred Kastler, especially after receiving the Nobel Prize in 1966, enjoyed considerable worldwide fame due to his humanist stances in politics. This reputation somewhat overshadowed the significant contributions of Brossel, who dedicated all his talent and energy to research, dramatically advancing knowledge in atomic physics. In reality, Kastler and Brossel formed a highly complementary duo: while Kastler spent much time promoting ideas within the national and international community, Brossel effectively led the group. He actively monitored and stimulated the theses being conducted, particularly encouraging women, who were nearly absent from French physics research in the 1960s but were relatively numerous in his laboratory (including Marie-Anne Guiochon-Bouchiat, Françoise Grossetête, Colette Julienne-Galleron, Françoise Boutron-Hartmann, Nicole Polonsky, and Michèle Leduc).

Brossel was one of the few “big names in physics” who actively engaged in experimental work. For example, he was the one who, with a torch in hand, filled the glass cells containing the gases to be studied (alkalis, mercury, helium, etc.) on his pumping bench. He was the only one who knew how to completely degas the cells and coat them with paraffin to prevent the relaxation of spins oriented by collisions with the glass walls. In a more theoretical domain, he contributed to the physical explanation of multi-photon transition spectra, with the detailed theory later developed by Winter, along with many other discoveries.

His atomic physics course for the DEA was very comprehensive and accurately traced the history of the discipline. He later published this material in a book in English (reference?). After their thesis defense, he encouraged his students to establish their own research groups at other universities, such as in Caen or Grenoble. Brossel’s national and international influence was substantial.

Jean Brossel (1918-2003) is regarded as one of the “re-founding fathers” of French physics. Kastler always deeply regretted that Brossel was not associated with his Nobel Prize in 1966. This may be because Brossel was not a co-author of Kastler’s original theoretical paper on optical pumping, despite having directly contributed to the first experimental demonstration of this general method. However, Brossel had the satisfaction of seeing one of his closest students, Claude Cohen-Tannoudji, awarded the prize in 1997.