### Early work (1965-1979)

Serge Haroche's early work includes the elaboration of the “dressed atom” formalism and the experimental study of dressed atom properties in optical pumping experiments (thesis work from 1965 to 1971), the development of laser induced quantum beat spectroscopy (1973-76) and the study of superradiance of laser-excited atomic ensembles (1976-78). His research has then focused on the study of the radiative properties of Rydberg atoms, starting with the millimiter wave high resolution spectroscopy of these atoms and the first realization of Rydberg atom masers (1979).

### First Cavity QED studies (1979-1992)

Serge Haroche's experiments with Rydberg atoms and microwaves stored in cavities led him, in the early 1980's, to become one of the main actors in the then emerging field of Cavity Quantum Electrodynamics (Cavity QED). This is the domain of quantum optics which studies the radiative behavior of atoms confined by reflecting boundaries in a limited region of space. The modification of the spectrum of the vacuum field surrounding the atom results in a change of the spontaneous emission rate of the atom, which can be either enhanced or inhibited. In a 1983 experiment, Serge Haroche and his team have observed the enhancement of the emission rate of a single Rydberg atom in a resonant cavity, an effect which had been predicted by Purcell forty years earlier. They have also observed, at about the same time, the fully symmetrical superradiance of an atomic ensemble in a cavity, an effect which had been theoretically predicted long before by R.Dicke but never observed under such simple conditions. Serge Haroche and coworkers have operated in 1987 the first two-photon maser oscillator, stimulating Rydberg atoms to emit photons by pairs in a very high Q superconducting cavity. Such a two-photon quantum oscillator has threshold and statistical properties very different from usual lasers operating on single photon atomic transitions. It had been proposed since the early days of quantum electronics, but its actual realization has been made possible only by the progresses of cavity QED physics. In the 1980's and early 1990's, during his stay at Yale University, Serge Haroche, working with E.Hinds and D.Meschede, has performed the first experiments on the inhibition of atomic spontaneous emission in a microcavity at optical frequencies and observed in a text book context the van der Waals shifts induced on the energy levels of atoms by their images in mirrors.

### Cavity QED , Quantum Information and decoherence studies (1992-2005)

Exploiting cavity QED methods, Serge Haroche and his ENS team - including colleagues Jean-Michel Raimond and Michel Brune - have started in the early 1990's to investigate the interaction between single atoms and one or more photons stored in a very high Q supercondcting cavity (strong coupling regime of Cavity QED). This has led to unprecedented tests of quantum mechanics of relevance to quantum information and decoherence studies. Between 1996 and 2005, Haroche and his team have demonstrated various schemes of entanglement between atoms and photons, realized quantum gates and simple quantum logic operations. They have in 1996 observed the Rabi oscillation of an atom in a very small field made of a few photons and shown that the Fourier analysis of this oscillation directly reveals the photon graininess of the quantum field. The same year, they have observed the size-dependent progressive decoherence of a quantum system in an experiment exploring the quantum-classical boundary. They have performed in 1999 the first quantum non-destructive measurements of a single photon. Starting in 2001, they have developed and demonstrated new methods to directly measure with atoms the phase space distributions of non-classical fields stored in a cavity (the so called Q and Wigner functions). In later experiments (2003-2005), Serge Haroche and his group have shown that the Rabi oscillation of an atom in a coherent field made of many photons results in an atom-field entanglement offering a direct investigation of quantum superposition and entanglement in mesoscopic systems of still larger size.

### Quantum non-destructive observation of trapped photons, detection of field quantum jumps, reconstruction of non-classical field states, direct monitoring of decoherence and quantum feedback demonstrations (2006-2011)

Recently, Haroche and his team have developed super-high finesse cavities which hold microwave photons over more than a tenth of a second. This has led to experiments in which trapped photon fields are manipulated and detected with unprecedented sensitivity and accuracy. The stored photons are continuously detected in a non-destructive way (Quantum Non-Demolition or QND method) by having them interact dispersively with atoms crossing the cavity one by one. The progressive projection of the field into Fock states corresponding to definite photon numbers has been observed and the quantum jumps of a light field have been monitored in this way. For the first time, single photons are continuously observed in a box and the random times at which they are created or annihilated are directly recorded. The method, first applied to a single photon field, has been extended to fields made up of several photons, which are non-destructively counted in the cavity as if they were marbles in a box. The quantum Zeno effect has been observed by the ENS team on this system. Haroche and coworkers have shown that the coherent growth of the field when the cavity is coupled to a microwave source is frozen if the field is repeatedly watched by non-absorbing atoms. Combining QND photon counting with a homodyne mixing method, Haroche and his team have recently reconstructed the full quantum state of Fock and Schrödinger cat states of light whose classical components differ by up to 12 photons. By taking snapshots of these states at successive times, they have realized actual movies of the decoherence process in progress, which clearly illustrate the transition from quantum to classical in a microscopic system coupled to an environment. Extension of these experiments to two cavities coupled by an atomic beam are in progress. They will make it possible to study the non-local properties of mesosocopic systems made of tens of photons or atoms. The non-destructive QND method has very recently been used to implement a quantum feedback procedure steering the cavity field towards a predetermined target Fock state and subsequently reversing the effects of quantum jumps out of this state. The method relies on repeated weak measurements of the field allowing a fast computer to estimate in real time the state of the field and to compute the field to be injected into the cavity in order to bring its field closer to the target. The procedure, operating in loops has been used to stabilize Fock states with up to four photons.

In another set of experiments, Haroche and his group are developing novel kinds of atom chips in which cold atoms are trapped by the magnetic field produced by superconducting wires. These devices open perspectives for building new atomic sources of cold atoms for Cavity QED studies and for fundamental tests involving the coupling of atomic systems with mesoscopic circuits.