Serge Haroche

He subsequently prepared a state thesis under the supervision of Claude Cohen-Tannoudji, as a research associate at the CNRS, which he defended in 1971. He participated in the creation and development, both experimentally and theoretically, of the concept of the “dressed atom”, which is still one of the essential tools of atomic physics and quantum optics.

After a post-doctoral stay in the USA, he set up his own research group at the Laboratoire de Spectroscopie Hertzienne in 1973, with the support of Jean Brossel, then director of the laboratory. This group, which is still active today, has welcomed over 80 trainees, PhD students and post-docs who have spent at least a year there. Many of them now occupy distinguished academic positions. All have benefited from Serge Haroche’s talents as a physicist, of course, but also from his remarkable culture, his unfailing sense of humor and his attentive kindness.

He quickly climbed the academic ladder: appointed chargé de recherches at the CNRS in 1971, maître de recherches in 1973 on his return from post-doctorate, he became professor at the Université Pierre et Marie Curie (Paris 6) in 1975, where he taught at the same time as at the ENS. He was also a lecturer at École Polytechnique (1973-1984), a part-time professor at Yale University (1984-1993) and a member of the first “senior” class of the Institut Universitaire de France (1991-2001). He headed the Physics Department at the ENS from 1994 to 2000. He was appointed Professor at the Collège de France in 2001, where he held the Chair of Quantum Physics, and became Administrator of the institution from 2012 until his emeritus in 2015.

From his early days teaching undergraduate medical students and in the renowned “Brossel DEA,” a graduate program that trained a whole generation of researchers in atomic physics, to his lectures at the Collège de France, which drew a remarkable audience, and the many talks he gives around the world, Serge Haroche is recognized as an outstanding teacher and science communicator.

With Jean-Michel Raimond, he combined these two fields by observing, in 1979, the millimetric superradiance of an ensemble of Rydberg atoms. This collective emission can be facilitated by a high-finesse resonant cavity (200) surrounding the atomic medium. The collective emission threshold is then only a few hundred atoms. In a visionary conclusion to the article presenting these results, Serge Haroche foresaw the possibility of further lowering this threshold and observing maser emission with just a few atoms, thus opening up a completely new field for experiments.

In 1983, by using superconducting mirrors to increase the cavity finesse to a few million, the group was able to observe the acceleration by the resonant cavity of the emission of a single atom, an effect predicted by Purcell as early as 1946 but never observed before.

This experiment shortly preceded the observation in 1985 by Daniel Kleppner (MIT, Cambridge) of the opposite effect, the inhibition of Rydberg atom emission, and the realization of the first “micromaser” by Herbert Walther (Max Planck Institut für Quantenoptik, Munich). This micromaser produces continuous millimetric radiation with an average of much less than one Rydberg atom in its superconducting cavity.

These experiments marked the beginnings of Cavity Quantum Electrodynamics (CQED), the study of one or a few atoms coupled to a single mode of radiation containing just a few photons, illustrating matter-radiation interaction in its simplest form. Most of the scientific work of Serge Haroche and his colleagues has been devoted to this exploration.

His team at Yale (where he was a part-time professor from 1984 to 1993) demonstrated the inhibition of spontaneous emission in the optical domain and studied the interactions between atoms and surfaces. At the same time, Valérie Lefèvre and Jean Hare’s experiments at the ENS focused on the study of very high-quality optical resonances in small silica spheres, and Michel Brune’s work in 1988 on the construction of a two-photon micromaser. This was the first oscillator to operate continuously on a transition between two Rydberg levels of equal parity, by simultaneous emission of two photons of equal frequency. Luis Davidovich and Nicim Zagury (Rio de Janeiro) began a long-term collaboration with the ENS group, exploring with them the remarkable quantum properties of this oscillator.

Experiments at the Laboratoire Kastler Brossel took a new turn in the early 1990s with the construction of a new CQED setup, motivated by the then distant prospect of using the extraordinary radiation sensitivity of Rydberg atoms to detect the presence of a few photons in the cavity without destroying them.

This experiment uses “circular” Rydberg atoms, already studied in the group by Michel Gross and Jean Hare, following the seminal work of Daniel Kleppner. Having a maximum angular momentum (in classical terms, the electron orbit is a circle, that of the Bohr model), these levels combine a huge coupling to radiation and a very long lifetime of several tens of milliseconds in a cryogenic environment at a fraction of an absolute degree. Combined with superconducting cavities of record-breaking finesse (from a few million to over a billion), they enable the realization of an ideal CQED system, in which atom-field coupling dominates by far all parasitic processes. The experimental results are then a direct illustration of the most fundamental postulates of quantum physics, realizing, so to speak, some of the thought experiments of the founding fathers.

With this new device, significant results followed in rapid succession, in step with progress on cavity quality. In 1996, the group observed the quantum Rabi oscillation of the atom in a coherent field of just a few photons, providing direct and intuitive evidence of the quantization of the field in the cavity. Almost simultaneously, the first quantum superposition of different classical states of the field was obtained. This is a field with two distinct phases, an elementary “Schrödinger cat”, both dead and alive. The group observes its decoherence, i.e. the transition from a quantum superposition to a simple probabilistic alternative. The more different the superposed states, the faster this process occurs. Its observation is the first direct illustration of a mechanism fundamental to any quantum measurement. The group is also developing, with Gilles Nogues, some of the very first prototypes of quantum logic operations, which are at the heart of quantum information processing and one of the most active topics at present.

A major milestone was reached in 2007. Using a cavity capable of storing a microwave photon for more than a tenth of a second, Sébastien Gleyzes achieved the first Quantum Non Demolition (QND) detection of a single photon. The group shows that it is possible to detect this photon several hundred times without absorbing or losing it! After 17 years, the group has thus realized its initial dream, illustrating the long timescales that fundamental research often requires to carry out such complex and delicate experiments.

Shortly afterwards, the method was extended to count up to 7 photons and observe quantum jumps in light intensity as the photons disappeared one by one from the cavity. These experiments directly illustrate the theory of quantum measurement, and are now included in teaching textbooks. Thanks to this non-destructive measurement of photon numbers, Igor Dotsenko, Clément Sayrin and the group succeeded in stabilizing the number of photons in the cavity despite the losses, thus achieving the first quantum feedback. They also succeed in fully reconstructing the state of the field in the cavity, or even a non-local state shared by two cavities.

The development of CQED has had a major impact, far beyond the experiments mentioned here. The ability to slow down or accelerate spontaneous emission at will, to the point of making it reversible, is at the heart of many of today’s optical and optronic devices. The concepts of atomic CQED also paved the way for the development, from 2004 onwards, of Circuit Quantum Electrodynamics, which uses superconducting circuits as cavities and Josephson junctions as atoms. The performance of these circuits is now better than that of atoms, and the field is expanding rapidly. Many of the devices at the heart of prototype quantum computers are therefore based on CQED concepts.