The Jussieu extension and the tunable laser revolution

From the early 1950s, it became clear that the Faculty of Science could no longer accommodate the growing number of post-war students within the old Sorbonne. The first solution adopted was to build two large concrete blocks on stilts—above the internal corridors of the Halle aux Vins—overlooking the Quai Saint-Bernard and Rue Cuvier. However, this quickly proved to be insufficient. A decision was then made to evict the merchants and wine stocks in order to construct on the entire quadrilateral. The new Halle aux Sciences was built, featuring a metallic structure with a central tower of 24 stories. This construction, initiated by Dean Zamansky, lasted several years. The first teaching facilities were available by 1966, followed by the research laboratories in 1967. Professors Kastler and Brossel were allocated approximately one thousand square meters of space, all in one piece, comparable to the total of the scattered offices and classrooms they had occupied on Rue Lhomond.

The relocation to Jussieu helped relieve congestion on Rue Lhomond, moving to the first floor of Tower 12. Among the equipment was the cumbersome setup of Robert Romestain with the large Varian magnet inherited from Jean Margerie. This was supplemented by the equipment for making tubes for electronic collisions, inherited from Jean-Claude Pebay-Péroula and used by Jean-Pierre Descoubes. Three new teams also moved in: those of Jean-Pierre Descoubes and Alain Omont (who were just finishing their doctoral theses). They hired Michèle Glass and Lucile Julien on one hand, and Jean-Claude Gay on the other. Then came Bernard Cagnac, returning from Orsay with his pumping bench and glassblower Guy Flory. He alleviated some of Jean Brossel’s workload, although Brossel continued to handle the more delicate cell fillings. Shortly after, the new team of Jean-Claude Lehmann joined. He returned from his cooperation service with the project of pumping molecules by using those among their numerous spectral lines that coincided with existing laser lines. He would hire Gérard Gouédard, Michel Broyer, and Jacques Vigué.

The student uprising of 1968 and the occupation of the Jussieu premises day and night did not simplify life, but it was ultimately short-lived (about two months). The year 1969, on the other hand, was occupied with the reconstruction of new universities and the separation of Jussieu’s facilities and equipment between the two distinct universities, Paris VI and Paris VII. Things then returned to normal. The unity of the laboratory manifested itself numerous times in the following years through personnel or equipment exchanges between the two sites. Notably, Elisabeth Giacobino, Jean-Pierre Faroux, Michel Pinard, and Claude Fabre arrived at Jussieu. Later on, Nicolas Billy, Serge Reynaud, Antoine Heidmann, and Jean-Michel Courty would join as well. Conversely, in 1978-79, the entire optical pumping team of molecules (Lehmann, Broyer, Vigué, Gouédard) moved back from the banks of the Seine to Rue Lhomond, atop the Sainte-Geneviève hill.

This gain of space was not insignificant for accommodating the growth of the group. However, what had a far more important consequence, profoundly changing the laboratory’s research themes, was the arrival in 1970 of the first tunable wavelength lasers. It probably took some time for the range of available wavelengths to expand and cover nearly the entire spectrum. There was undoubtedly significant technical work within the various teams to develop or even fully construct these new devices (cf. François Biraben) and learn to control their wavelengths. Yet, within a few years, the laboratory underwent profound renewal. Paradoxically, it was at the very moment when the pioneers of lasers in the laboratory (Decomps, Dumont, Ducloy) began to branch out to the University of Paris-Nord in Saint-Denis (then to Villetaneuse, where they would later bring J.P. Descoubes) that all the other teams started to get involved. This transformation was largely complete by the mid-seventies, yielding the following results:

The Cohen-Tannoudji team (with Jacques Dupont-Roc and Serge Reynaud), after having “dressed” atoms in all their aspects with intense radiofrequency waves, began to “dress” them with intense laser waves, soon allowing them to unravel the problems of the fluorescence triplet; eventually focusing on the slowing and trapping of atoms with Jean Dalibard, Antoine Heidmann, and Alain Aspect (who temporarily left the Institut d’Optique after his thesis on quantum correlations of photons in 1983). Around the same time, Claude Cohen’s appointment to the Collège de France (1974) enabled him to expand his essential role in the theoretical understanding of interactions between photons and atoms outside the laboratory (leading to the two volumes “Photons et Atomes,” co-written with J. Dupont-Roc and G. Grynberg).

Marie-Anne Bouchiat completed the study of Van der Waals molecules in alkali relaxation and prepared her experiment on parity violation through the laser excitation of highly forbidden lines, bringing along Lionel Pottier, and later Jocelyne Guéna and Philippe Jacquier. This experiment led to the measurement in 1982 of the effects of weak interactions in atoms.

The new Haroche team (after returning from the USA: with Claude Fabre and Michel Gross; and later Jean-Michel Raimond) used two powerful pulsed lasers to create, in step excitation, Rydberg states that emit quantum beats. Later, clever transfers of angular momentum in appropriate fields enabled the creation of circular Rydberg states with relatively long lifetimes, used to generate single photons in microwave cavities, which they passed through one by one, and to realize quantum entanglement phenomena.

The Laloë – Leduc team (with Michel Pinard, Valérie Lefèvre; and later Pierre-Jean Nacher) began using a laser to orient certain nuclei of rare gases and was preparing to develop the 1.08 µm laser that would soon enable substantial progress with helium and advance the study of quantum fluids. The quality of signals provided by highly polarized helium would also lead to medical applications for pulmonary diagnostics, in collaboration with Otten’s group from Mainz.

The Cagnac – Grynberg – Biraben team (joined later by Elisabeth Giacobino; and more recently Lucile Julien) utilized the enhancement of the field of a continuous laser inside a Fabry-Pérot cavity to observe the very fine lines of two-photon transitions without Doppler broadening, which would soon be used in hydrogen to measure the Rydberg constant with eleven significant digits.

The Omont – Gay team used a laser to extend the study of Holtsmark collisions to sodium, building on previous research conducted on mercury (but Alain Omont himself would later branch out to Grenoble in 1977 to focus on astrophysics).

We also recall for reference the three teams that had already started working with lasers when they were not yet tunable:

– The molecular pumping team of J.C. Lehmann, with Broyer, Vigué, and Gouédard.
– Two distinct light scattering teams:
– In compressed gases and liquids, with Pierre Lallemand and Catherine Allain-Demoulin.
– On interfaces, with Jacques Meunier, Dominique Langevin, and Anne-Marie Cazabat (whom Marie-Anne would leave to focus on parity violation).