274 NATURE | VOL 422 | 20 MARCH 2003 | www.nature.com/nature

Jean Brossel passed away on 4 February2003 in Périgueux, Dordogne, the city of his birth. He was one of the foundingfathers of the modern French school ofquantum optics and co-founder, withAlfred Kastler, of the Laboratoire KastlerBrossel at the Ecole Normale Supérieure,in Paris.

Brossel entered the Ecole Normale in1938, completing his studies in 1945 (aftera two-year interruption due to the SecondWorld War). Physics research in Francewas devastated by the war, and laggedbehind the scientific developments inBritain and the United States. To catch up, Kastler advised Brossel to begin hisresearch in the United Kingdom. Brosseltook his advice and joined SamuelTolansky’s group in Manchester, in thelaboratory headed by Patrick Blackett.There he learned how to applyinterferometry to the study of surfaces,and to use Fabry–Perot interferencetechniques for the measurement ofhyperfine structure in electronictransitions inside the atom. Throughouthis life, Brossel acknowledged the value of the expertise he acquired in Tolansky’slaboratory.

On Brossel’s return from England,Kastler recommended that he go to theUnited States to work for a PhD withFrancis Bitter at the MassachusettsInstitute of Technology. Bitter had justinformed Kastler about exciting new ideasfor Doppler-free spectroscopy (in whichDoppler broadening of optical emissionlines does not limit the resolution) thatmight extend radio-frequency studies ofatomic resonances to excited states. Bitter’sidea turned out to be incorrect, but itproved a fruitful starting point for Brosselthat later led to his invention of themethod of ‘double resonance’: in thisDoppler-free method, the atoms aresubjected to simultaneous irradiation with light and a radio-frequency field, and resonant effects are detected bymeasuring the polarization of thefluorescent light emitted by the atoms(equivalent to the angular momentum of the photons), instead of its frequency(the energy of the photon).

While Brossel was finishing his thesiswith Bitter, Kastler followed a similar line of work in Paris and devised ‘opticalpumping’, by which atoms in their lowest-energy (or ground) state acquireangular momentum through successivecycles of absorption of polarized opticalradiation, followed by spontaneousemission. The final result is a selective

population of magnetic sublevels insidethe ground state.

In 1951, Kastler and Brossel founded aresearch group together at the EcoleNormale — what became the LaboratoireKastler Brossel (LKB). Their primaryobjective was to demonstrate opticalpumping experimentally and to explorethe possibilities it created. Excellentstudents from the Ecole Normale soonjoined the group (Bernard Cagnac, JacquesWinter, Jacques Blamont and Jean-PierreBarrat, among many others). This field of research proved to be a treasure trove of new physics. Optical pumping wasdemonstrated in 1952, and was followed by the observation of multiphotonresonances, where several photons, insteadof a single one, are absorbed by the atomsat the same time — Brossel provided aphysical interpretation of the selectionrules for the polarization of the radiofrequency in terms of photon angularmomentum. Then, in work overseen by

Brossel and Kastler, came coherentradiation trapping and polarizationtransfer, the general distinction betweendiagonal and off-diagonal elements of the density matrix (populations andcoherences) and the discovery of lightshifts. After Kastler received the Nobelprize in 1966 for his work on opticalmethods for studying resonances of theatom, he often expressed his deep regretthat Brossel had not shared it with him.

Brossel encouraged young scientists topursue new directions of research in hislaboratory. This research included work on atomic masers and magnetometers,parity-violation in atomic physics, cavityelectrodynamics and Doppler-free two-photon spectroscopy. Perhaps the best-known application of optical pumping is in atomic clocks, and the work done at LKB, in particular on light shifts, was crucial to their development.Improvements in the optical pumping ofrubidium atoms led to the creation in 1969of a magnetometer that was sensitive tomagnetic fields at the level of 10�9 gauss. In 1997, Brossel’s colleague and formerstudent Claude Cohen-Tannoudji sharedthe Nobel prize for his work on ultra-coldatoms and laser cooling.

Although Brossel was interested in the applications of his research, he was

sceptical about application-driven researchin universities. He argued that technicalbreakthroughs are the fruit offundamental research driven by purescientific curiosity, and rarely ofapplication-oriented programmes. A goodexample is atomic clocks, which sprangfrom fundamental studies in atomicphysics rather than from a drive toimprove on mechanical or quartz clocks.Another example is optical pumping inhelium-3. First performed 20 years ago by King Walters and Laird Schearer in the United States, optical pumping in thisisotope was refined at LKB, motivated bythe study of quantum effects in opticallypumped gases. Now optical pumping in helium-3 forms the basis of a tool for the medical imaging of lung cavities inasthma patients.

Jean Brossel was for many years thehead of the physics department at the Ecole Normale, to which he attractedbrilliant and active colleagues. Teachingwas always important to him, in particular as a way of attacting the best students to physics. But even at thepeak of his administrative responsibilities,he never abandoned ‘hands-on’ work in the laboratory. For instance, hecontinued to prepare glass cells containing atomic gases, sometimes with elaborate coatings to prevent wallrelaxation — an art in which no one couldmatch his skill. He will be remembered by his colleagues and former students with great admiration. Franck Laloë

Franck Laloë is in the Laboratoire Kastler Brossel,Département de Physique de l’ENS, 24 rue Lhomond, 75005 Paris, France.e-mail: laloe@lkb.ens.fr

Obituary

Jean Brossel (1918–2003)

Physicist who openedup new perspectives in quantum optics

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