Every year since 1954, the CNRS has awarded its highest distinction, the Gold Medal, to outstanding members of the French scientific community whose work has contributed in an exceptional manner to the development and influence of French research. The medal is a crowning reward to scientists whose work and career have earned them international acclaim.

The physicist Claude Cohen-Tannoudji was awarded the 1996 CNRS Gold Medal for his work on quantum optics and the interactions between atoms and electromagnetic waves.
Understanding these interactions is of crucial importance to basic physics and technology. His work and that of his collaborators has led to the discovery of a new field of nuclear physics and opened new prospects for scientific research in terms both of theoretical knowledge and practical applications. In a context of intense international competition, his research and teaching have greatly contributed to the development of French nuclear physics, thanks to which, in this field of science, France now plays a pioneering role in the world.

Claude Cohen-Tannoudji is a member of the French Academy of Sciences, a graduate of the Ecole Normale Supérieure and a former student of Alfred Kastler (Nobel Prize in physics) and Jean Brossel, who supervised his doctoral dissertation. His entire scientific career was spent at the Ecole Normale Supérieure, working in the laboratory which now bears the name of these two scientists. After passing the physics agrégation in 1957, he entered the CNRS in 1960. From 1962 on, he trained numerous French and foreign researchers, supervising doctoral dissertations and teaching graduate courses at the University of Paris VI. In 1973, he became professor and chairman of the nuclear and molecular physics department of the Collège de France. He teaches courses on different subjects each year, and each year his lectures are duplicated and circulated among students.

With Bernard Diu and Franck Laloë, Claude Cohen-Tannoudji has published a two-volume opus on quantum mechanics, which has been translated into German and English and serves as reference to students and reseachers worldwide. He has also written two books with two former students, Jacques Dupont-Roc and Gilbert Grynberg, on quantic electrodynamics and on matter/radiation interactions. Both works have been translated into English.

His research has fostered the development of a top-level French scientific research center, the Kastler-Brossel laboratory at the Ecole Normale Supérieure. This laboratory is famous worldwide and iwelcomes each year many foreign researchers wishing to spend a "sabbatical" year in France or carry out postdoctoral research. Claude Cohen-Tannoudji is now supervising the work of a team studying matter/radiation interactions and the cooling and trapping of atoms by laser beams.

Claude Cohen-Tannoudji has taught as a visiting professor in many foreign universities: Harvard, Toronto, Pisa, New-York, Leiden, etc.; he is an associate member of several foreign Academies: American Academy of Arts and Sciences, National Academy of Science (USA), Académie Royale des Sciences, des Lettres et des Beaux-Arts de Belgique, Academia dei Lincei in Italy, etc.

He also presided the CNRS Nuclear and Molecular Physics Commission for 4 years.

Claude Cohen-Tannoudji's Scientific Works

Claude Cohen-Tannoudji has predicted and observed several new physical phenomena such as the energy level variations of atoms lighted by a non-resonant light wave and the changes occurring in the physical properties of atoms when they are surrounded by a cloud of photons which is constantly absorbed and reemitted. Together with his students, he has founded a new theoretical approach aiming to pinpoint the effects of interactions between atoms and highly intense electromagnetic radiations. The image of the atom "lit up" by photons (light) has been extremely useful and effective in the analysis of the light absorbed and emitted by atoms and to understand the forces which influence atoms. The "dressed (lit up) atom" approach is now adopted by researchers worldwide.

The atom/radiation interactions can be considered from two angles:

• first, they can help obtain information on the structure of atoms. This is the principle of spectroscopy, which consists in analyzing the properties of the light absorbed or emitted by an atom, in order to determine its energy level.

• secondly, it has been recently established that these interactions can be used to influence atoms, so as to control not only their internal state, but also their movements, which makes it possible to manipulate them. Claude Cohen-Tannoudji has specialized in the latter field. His team's recent research work is devoted to the study of the impact of laser beams on atoms and to experimenting whether this force can be used to manipulate atoms, by slowing them down, cooling or "trapping" them, in other words by confining them to a small area and maintaining them in cavities. With his team, he has discovered new and very effective cooling mechanisms such as the "Sysiphus cooling system", which consists in forcing an atom to climb "hills" of potential, or the "black state cooling system", thanks to which the speed of an atom can be reduced to a level previously considered impossible to reach. By improving the understanding of the physical mechanisms involved, this research has made it possible to control more effectively the movement of atoms inside a laser beam and to lower their temperature down to microkelvin level, or even nanokelvin level, in other words a temperature level one hundred billion times lower than normal room temperature.

The international scientific context

The study of the interaction between atoms and an electromagnetic radiation is an ideal testing ground for quantic mechanics as they are known today and can help us understand:
- quantum interference effects,
- the nature of quantum jumps,
- the measuring process,
- the transition from quantum world to ordinary world.

This explains the extraordinary vitality of nuclear physics today: pioneering research is being conducted on a variety of topics, from quantum electrodynamics in cavities, non-linear optics, reducing quantum noise, or "quantum non-demolition". This basic research will probably lead to numerous practical applications. Laser beams are an example of the unforeseen consequences of research in basic nuclear physics.

What are the potential applications of the ultracold atoms studied by Claude Cohen-Tannoudji's team of researchers and by other, increasingly numerous, teams around the world? Here are a few possibilities:

• Since ultracold atoms move very slowly, they can be observed at leisure; it is thus possible to set up atomic clocks which are locked on much narrower atomic lines, and build clocks which are from one hundred to one thousand times more accurate than present nuclear clocks. Such clocks could be used in satellites and would considerably improve the accuracy of navigating systems and the Global Positioning System (GPS).

• Slowing down atoms has another consequence: the "de Broglie" wavelength - the quantum nature of atoms means that each atom is linked to a quantum wave, the "de Broglie" wave increases considerably. The slowness increasez the undulatory movements; thus, with the "de Broglie" waves, one can carry out the optics and interferometry experiments usually conducted with light waves. Thanks to "de Broglie" optics, it will be possible to produce extremely accurate measuring instruments and develop new technologies.

• The recent discovery in the USA of a Bose-Einstein condensation of ultracold atoms raises hopes that it will be someday possible to create a "nuclear laser", an equivalent of the ordinary laser, except that in the former, photons are replaced by atoms obeying Bose statistics.

Contact at the department of physical and mathematical sciences of the CNRS:
Frédérique Laubenheimer - Phone: 33 1 44 96 42 63