Effects of noise in continuous variables quantum communication and measurement
Andrea Renato Rossi - Università degli Studi di Milano - 
In this thesis we thoroughly study the propagation of a twin beam (TWB) state of light in a generalized gaussian environment: this implies the presence of phase dependent, non classical fluctuations, together with thermal noise. We focus on the separability time as a function of the environment parameters and the signal power, finding that it is always shorter if non classical fluctuation are present. Nevertheless we find that there exist a particular class of squeezed signal states that maximize the teleportation fidelity. We therefore compare the fidelity of teleportation versus direct transmission of such states in the generalized gaussian noisy channel and find regimes in which teleportation is a better communication protocol. We than proceed analyzing, with numerically simulated experiments, a novel method to measure the photon statistics of a given quantum state by using only a number of on/off photodetectors with realistic and different quantum efficiencies. The collected data allow to retrieve the photon statistics making use of a maximum likelihood algorithm in the linear positive variant. We study the goodness of the reconstruction, parametrized by the fidelity, as a function of the experimental parameters and the properties of the state to be reconstructed. We than proceed requiring only few detectors with very low quantum efficiencies and find that it is possible to retrieve the photon statistics of singled peaked photon distribution by making use of a combined maximum likelihood - maximum entropy strategy. In both cases the fidelity of reconstruction achievable can be very close to one, provided that the experimental measurement are enough accurate and the photon statistics to be reconstructed are smooth enough.
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