Theoretical and experimental study of quantum coherent superpositions and of non-Gaussian entangled states of the light

摘要

In this thesis, we develop new methods to generate and analyse non-classical states of the light for quantum information processing. We show that the tools of discrete variables, where light is described in terms of photons, can be combined with a continuous approach, where one considers the quadratures of the electromagnetic field, to efficiently create, transform and analyze complex quantum states. Using ultrashort light pulses, we prepared optical "Schrodinger cats", defined as quantum superpositions of coherent states. In this case the Wigner function, describing the quantum statistics of the electromagnetic field, differs from a classical probability distribution and takes negative values. Time-resolved homodyne tomography allowed us to realize the first experimental observation of this negativity for small free-propagating "Schrodinger kittens". Furthermore, we developped and experimentally demonstrated a protocol to prepare arbitrarily large "Schrodinger cat" states, opening a way towards numerous quantum information processing protocols. We have shown experimentally that contitional photon subtraction allows one to increase the entanglement of gaussian states. We used this method to entangle two initially independent distant light pulses, using a low-transmission quantum channel. This approach allows one to prepare strongly entangled states with negative Wigner functions between distant sites, as required for entanglement distillation and long-distance quantum communications.