Serge Haroche main research activities have been in quantum optics and quantum information science. He has made important contributions to Cavity Quantum Electrodynamics (Cavity QED), the domain of quantum optics which studies the behaviour of atoms interacting strongly with the field confined in a high-Q cavity. An atom-photon system isolated from the outside world by highly reflecting metallic walls realizes a very simple experimental model which Serge Haroche has used to test fundamental aspects of quantum physics such as state superposition, entanglement, complementarity and decoherence. Some of these experiments are actual realizations in the laboratory of the "thought experiments" imagined by the founding fathers of quantum mechanics. Serge Haroche's main achievements in cavity QED include the observation of single atom spontaneous emission enhancement in a cavity (1983), the direct monitoring of the decoherence of mesoscopic superpositions of states (so-called Schrödinger cat states) (1996) and the quantum-non-demolition measurement of a single photon (1999). By manipulating atoms and photons in high-Q cavities, he has also demonstrated many steps of quantum information procedure such as the generation of atomatom and atom-photon entanglement (1997), the realization of a photonic memory (1997) and the operation of quantum logic gates involving photons and atoms as "quantum bits" (1999).
In 2006, Serge Haroche and his ENS team have developed a super-high-Q cavity able to store photons between mirrors for times longer than a tenth of a second. Trapping light quanta in this cavity has allowed the ENS team to detect repeatedly and non-destructively the same field, to project it into states with definite photon numbers (so called Fock states) and to observe the quantum jumps of light due to the loss or gain of a single photon in the cavity (2007). This constitutes a completely new way to look at light. Whereas photons are usually destroyed upon measurement, they can now be counted and counted again in the cavity as one would do with marbles in a box. This non-destructive detection method has led Serge Haroche and his team to develop novel ways to generate and reconstruct non-classical states of radiation trapped in a cavity and to investigate in details their decoherence, the phenomenon essential to explain the transition from quantum to classical (2008). The ENS team has recently pushed these experiments further by demonstrating a quantum feedback procedure achieving the preparation of predetermined non-classical state of a field trapped in a cavity and counteracting the effects of decoherence on these states (2011).
Many of the ideas developed by S.Haroche and his research team in microwave cavity QED experiments have been exploited in other contexts to build new devices playing an increasing role in opto-electronics and optical communication science. Manipulating the emission properties of quantum dots embedded in solid state micro-cavities has become a widely exploited method to build solid state sources and generate non classical light of various sorts. Strong coupling of light emitters with micro-cavity structures is being developed to achieve operations useful for quantum communication and quantum information processing purposes. By coupling artificial atoms made of superconducting junctions with strip-line microwave cavities, many groups word-wide are now developing a new field of physics dubbed "Circuit QED" which borrows many of its concepts from microwave cavity QED experiments. These examples show the impact of fundamental Cavity QED work on areas of research which could lead to promising applications for technology.