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Estimation d'état et de paramètres pour les systèmes quantiques ouverts

Abstract : In recent years, the scientifical community has succeeded in experimentally building simple quantum systems on which series of measurements are successively acquired along quantum trajectories, without any reinitialization of their state (density operator) by the physicist. The subject of this thesis is to adapt the quantum tomography techniques (state and parameters estimation) to this frame, in order to take into account the feedback of the measurement on the state, the decoherence and experimental imperfections.During the measurement process, the evolution of the quantum state is then governed by a hidden-state Markov process (Belavkin quantum filters). Concerning continuous-time measurements, we begin by showing how to discretize the stochastic master equation, while preserving the positivity and the trace of the quantum state, and so reducing to discrete-time quantum filters. Then, we develop,starting from trajectories of discrete-time measurements, some maximum-likelihood estimation techniques for initial state and parameters. This estimation is coupled with its confidence interval. When it concerns the value of parameters (quantum process tomography), we provide a result of robustness using the formalism of particular filters, and we propose a maximization technique based on the calculus of gradient by adjoint method, which is well adapted to the multi-parametric case. When the estimation concerns the initial state (quantum state tomography), we give an explicit formulation of the likelihood function thanks to the adjoint states, show that its logarithm is a concave function of the initial state and build an intrinsic expression of the variance, obtained from asymptotic developments of Bayesian means, lying on the geometry of the space of density operators.These estimation techniques have been applied and experimentally validated for two types of quantum measurements: discrete-time non-destructive measurements of photons in the group of cavity quantum electro-dynamics of LKB at Collège de France, diffusive measurements of the fluorescence of a supra-conducting qubit in the quantum electronics group of LPA at ENS Paris.
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Pierre Six. Estimation d'état et de paramètres pour les systèmes quantiques ouverts. Physique mathématique [math-ph]. Université Paris sciences et lettres, 2016. Français. ⟨NNT : 2016PSLEM019⟩. ⟨tel-01511698⟩



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