I was born on April 1, 1933 in
Constantine, Algeria, which was then part of France. My family,
originally from Tangier, settled in Tunisia and then in Algeria
in the 16th century after having fled Spain during the
Inquisition. In fact, our name, Cohen-Tannoudji, means simply the
Cohen family from Tangiers. The Algerian Jews obtained the French
citizenship in 1870 after Algeria became a French colony in
1830.
My parents lived a modest life and their main concern was the
education of their children. My father was a self-taught man but
had a great intellectual curiosity, not only for biblical and
talmudic texts, but also for philosophy, psychoanalysis and
history. He passed on to me his taste for studies, for
discussion, for debate, and he taught me what I regard as being
the fundamental features of the Jewish tradition - studying,
learning and sharing knowledge with others.
As a child, I was very lucky to escape the tragic events which
marked this century. The arrival of the Americans in Algeria, in
November of 1942, saved us from the nazi persecutions that were
spreading throughout Europe at the time. I completed my primary
and secondary school education in Algiers. And I was also lucky
enough to finish high school in very good conditions and to leave
Algiers for Paris, in 1953, before the war in Algeria and the
stormy period that preceded the independence.
I came to Paris because I was admitted to the Ecole Normale
Supérieure. This French "grande école", founded
during the French Revolution about 200 years ago, selects the top
high school students who do well in the selective final
examination. The four years at this school, from 1953 to 1957,
were indeed a unique experience for me. During the first year, I
attended a series of fascinating lectures in mathematics given by
Henri Cartan and Laurent Schwartz, in physics by Alfred Kastler. Initially, I was more
interested in mathematics but Kastler's lectures were so
stimulating, and his personality so attractive, that I ended up
changing to physics.
In 1955, when I joined Kastler's group to do my "diploma" work,
the group was very small. One of Kastler's first students, Jean
Brossel, who had returned four years before from M.I.T. where he had
done research work with Francis Bitter, was supervising the
thesis work of Jacques Emile Blamont and Jacques Michel
Winter.
We were a small group, but the enthusiasm for research was
exceptional and we worked hard. Brossel and Kastler were in the
lab nearly day and night, even on weekends. We had endless
discussions on how to interpret our experimental results. At the
time, the equipment was rather poor and we did what we could
without computers, recorders and signal averagers. We measured
resonance curves point by point with a galvanometer, each curve
five times, and then averaged by hand. We were, somehow, able to
get nice curves and exciting results. I think that what I learned
during that period was essential for my subsequent research work
and key personalities such as Alfred Kastler and Jean Brossel
certainly had a significant role in it.
We were going together, once a week, to attend the new lectures
given in Saclay by Albert Messiah on quantum mechanics, by
Anatole Abragam on NMR and by Claude Bloch on nuclear physics. I
can still remember the stimulating atmosphere of these
lectures.
During the summer of 1955, I also spent two months at the famous
Les Houches summer school in the Alps. This school
has contributed largely to the development of theoretical physics
in France. At that time, the school offered an intense training
in modern physics with about six lectures a day, for two months,
and the lecturers were J.
Schwinger, N. Ramsey, G.
Uhlenbeck, W. Pauli, A. Abragam,
A. Messiah, C. Bloch... to mention a few.
After finishing my "diploma" studies, I still had to get through
the final examination "Agrégation" before leaving Ecole
Normale as a student. The "Agrégation" is a competitive
examination for teaching posts in high schools. The preparation
consists of theoretical and experimental courses as well as some
pedagogical training. You give a lecture attended by other
students and a professor and after, there is a moment of general
debate and constructive criticism in view of perfecting your
lecture. Kastler, I remember, participated in the pedagogical
training and he taught us how to organize and present our
lecture.
Well, about this time I met Jacqueline who became my wife in
1958. She has shared with me all the difficult and happy times of
life. She has been able to pursue her own career as a high school
physics and chemistry teacher, to raise our three children Alain,
Joëlle and Michel, to be part of the daily life of a
researcher which can sometimes be very difficult and demanding.
We have had, as many, our share of family tragedy and losing our
oldest son Alain was a great misfortune to us all. Alain died in
1993, of a long illness, at the age of 34.
After the "Agrégation", I left the Ecole Normale and did my
military service which was very long (28 months) because of the
Algeria war. I was, though, assigned part of the time to a
scientific department supervised by Jacques Emile Blamont. We
were studying the upper atmosphere with rockets releasing sodium
clouds at the sunset. By looking at the fluorescence light
reemitted by the sodium atoms excited by the sunlight, it was
possible to measure the variations with the altitude of various
parameters such as the wind velocity or the temperature.
Then, in the beginning of 1960, I came back to the laboratory to
do a Ph.D. under the supervision of Alfred Kastler and Jean
Brossel with a research post at the CNRS (French
National Center for Scientific Research). The lab had by then
been expanded. Bernard Cagnac was finishing his thesis on the
optical pumping of the odd isotopes of mercury and I was trying,
with Jean-Pierre Barrat, to derive a master equation for the
optical pumping cycle and to understand the physics of the
off-diagonal elements of the density matrix (the so-called atomic
"coherences"). Our calculations predicted the existence of "light
shifts" for the various Zeeman sublevels, a curious phenomenon we
did not expect at all. I decided to try to see this effect.
Cagnac left me his experimental set up during Christmas vacations
and I remember getting the first experimental evidence on
Christmas Eve of 1960. I was very excited and both Kastler and
Brossel were very happy indeed. Kastler called the effect the
"Lamp shift", since it is produced by the light coming from a
discharge lamp. Nowadays, it is called light shift or a.c. Stark
shift. I built a new experimental set up to check in detail
several other predictions of our calculations, especially the
conservation of Zeeman coherences during the optical pumping
cycle. I submitted my Ph.D. in December of 1962. The members of
the committee were Jean Brossel, Pierre Jacquinot, Alfred Kastler
and Jacques Yvon.
Shortly after my Ph.D. Alfred Kastler urged me to accept a
teaching position at the University of Paris. I followed his
advice and started to teach at the undergraduate level. At about
this time, there was a new reform in the University system: the
so-called "troisième cycle" that consisted of teaching a
graduate level with a flexible program. Jean Brossel asked me to
teach quantum mechanics. He was teaching atomic physics, Alfred
Kastler and Jacques Yvon statistical physics, Pierre Aigrain and
Pierre-Gilles de Gennes solid state physics.
We had the best students of the Ecole Normale attending these
lectures, so I set up a small group where every year a new
student would join in and do a post-graduate thesis or a Ph.D. In
1967, I was asked to teach quantum mechanics at a lower level
(second cycle). The book "Quantum Mechanics" originated from this
teaching experience and was done in collaboration with Franck
Laloë and Bernard Diu.
Understanding atom-photon interactions in the high intensity
limit where perturbative treatments are no longer valid was one
of the main goals of our research group. This led us to develop a
new approach to these problems where one considers the "atom +
photons" system as a global isolated system described by a
time-independent Hamiltonian having true energy levels. We called
such a system the "dressed atom". Although the quantum
description of the electromagnetic field used in such an approach
is not essential to interpret most physical effects encountered
in atomic physics, it turned out that the dressed atom approach
was very useful in providing new physical insights into
atom-photon interactions. New physical effects, which were
difficult to predict by standard semiclassical methods, were
appearing clearly in the energy diagram of the dressed atom when
examining how this energy diagram changes when the number of
photons increases. We first introduced the dressed atom approach
in the radio-frequency range while Nicole Polonsky, Serge
Haroche, Jacques Dupont-Roc, Claire Landré, Gilbert
Grynberg, Maryvonne Ledourneuf, Claude Fabre were working on
their thesis. One of the new effects which were predicted and
observed was the modification, and even the cancellation of the
Landé factor of an atomic level by interaction with an
intense, high frequency radio-frequency field. This effect
presents some analogy with the g-2 anomaly of the electron
spin except that it has the opposite sign: theg-factor of
the atomic level is reduced by virtual absorption and reemission
of RF photons whereas the factor of the electron spin is enhanced
by radiative corrections.
We devoted a lot of efforts to the interpretation of this change
of sign and this led us, years later (with Jacques Dupont-Roc and
Jean Dalibard), to propose new physical pictures involving the
respective contributions of vacuum fluctuations and radiation
reaction. And while this was going on, we had some very
stimulating discussions with Victor Weisskopf who has always been
interested in the physical interpretation of the g-2
anomaly.
The dressed atom approach has also been very useful in the
optical domain. Spontaneous emission plays an important role as a
damping mechanism and as a source of fluorescence photons. Serge
Reynaud and I applied this approach to the interpretation of
resonance fluorescence in intense resonant laser beams. New
physical pictures were given for the Mollow triplet and for the
absorption spectrum of a weak probe beam, with the prediction and
the observation of new Doppler free lines resulting from a
compensation of the Doppler effect by velocity dependent light
shifts. The picture of the dressed atom radiative cascade also
provided new insights into photon correlations and photon
antibunching. New types of time correlations between the photons
emitted in the two sidebands of the Mollow triplet were predicted
in this way and observed experimentally at the Institut d'Optique
in Orsay, in collaboration with Alain Aspect.
An important event in my scientific life has been my appointment
as a Professor at the Collège de France in 1973. The
Collège de France is a very special institution created in
1530, by King François I, to counterbalance the influence of
the Sorbonne which was, at that time, too scholastic and where
only latin and theology were taught. The first appointed by the
King were 3 lecturers in Hebrew, 2 in Greek and 1 in Mathematics.
This institution survived all revolutions and remains, to this
day, reputed for its flexibility. Today there are 52 professors
in all subjects, and lectures are open to all, for there is no
registration and no degrees given. We professors are free to
choose the topics of our lectures. The only rule is that these
lectures must change and deal with different topics every year,
which is very difficult and demanding. It is, however, very
stimulating because this urges one to broaden one's knowledge, to
explore new fields and to challenge oneself. No doubt that
without such an effort I would not have started many of the
research lines that have been explored by my research group. I am
very grateful to Anatole Abragam who is at the origin of my
appointment at the Collège de France. Part of this teaching
experience incited the two books on quantum electrodynamics and
quantum optics written with Jacques Dupont-Roc and Gilbert
Grynberg.
In the early 1980s, I chose to lecture on radiative forces, a
field which was very new at that time. I was also trying with
Serge Reynaud, Christian Tanguy and Jean Dalibard to apply the
dressed atom approach to the interpretation of atomic motion in a
laser wave. New ideas were emerging from such an analysis related
to, in particular, the interpretation of the mean value, the
flucalations and the velocity dependence of dipole forces in
terms of spatial gradients of dressed state energies and of
spontaneous transitions between these dressed states.
When in 1984 I was given the possibility to appoint someone to
the position of Associate Director for my laboratory, at the
Collège de France, I offered the post to Alain Aspect and
then invited him to join me in forming, with Jean Dalibard, a new
experimental group on laser cooling and trapping. A year later,
Christophe Salomon who came back from a postdoctoral stay in JILA
with Jan Hall, decided to join our group. This was a new very
exciting scientific period for us. We began to investigate a new
cooling mechanism suggested by the dressed atom approach and that
resulted from correlations between the spatial modulations of the
dressed state energies in a high intensity laser standing wave
and the spatial modulations of the spontaneous rates between the
dressed states. As a result of these correlations, the moving
atom is running up potential hills more frequently than down. We
first called such a scheme "stimulated blue molasses" because it
appears for a blue detuning of the cooling lasers, contrary to
what happens for Doppler molasses which require a red detuning.
In fact, this new scheme was the first high intensity version of
what is called now "Sisyphus cooling", a denomination that we
introduced in 1986. We also observed, shortly after, the
channeling of atoms at the nodes or antinodes of a standing wave.
This was the first demonstration of laser confinement of neutral
atoms in optical-wavelength-size regions.
A few years later, in 1988, when sub-Doppler temperatures were
observed by Bill Phillips, who had been collaborating with us, we
were prepared with our background in optical pumping, light
shifts and dressed atoms, to find the explanation of such
anomalous low temperatures. In fact, they were resulting from yet
another (low intensity) version of Sisyphus cooling. Similar
conclusions were reached by Steve Chu and his
colleagues. At the same time, we were exploring, with Alain
Aspect and Ennio Arimondo, the possibility of applying coherent
population trapping to laser cooling. By making such a quantum
interference effect velocity selective, we were able to
demonstrate a new cooling scheme with no lower limit, which can
notably cool atoms below the recoil limit corresponding to the
recoil kinetic energy of an atom absorbing or emitting a single
photon. These exciting developments opened the microKelvin and
even the nanoKelvin range to laser cooling, and they allowed
several new applications to be explored with success.
These applications will not be described here since they are the
subject of the Nobel Lecture which follows this presentation. The
purpose here was merely to give an idea of my scientific
itinerary and to express my gratitude to all those who have
helped me live such a great adventure: my family, my teachers, my
students and my fellow colleagues all over the world.
I dedicate my Nobel Lecture to the memory of my son Alain.
From Les Prix Nobel. The Nobel Prizes 1997, Editor Tore Frängsmyr, [Nobel Foundation], Stockholm, 1998
This autobiography/biography was written at the time of the award and later published in the book series Les Prix Nobel/ Nobel Lectures/The Nobel Prizes. The information is sometimes updated with an addendum submitted by the Laureate.
Copyright © The Nobel Foundation 1997
Claude Cohen-Tannoudji
