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This paper provides a stabilizing method for preparation of quantum Gaussian states by utilizing continuous measurement. We first present a necessary and sufficient condition for the Riccati equation of the covariance matrix of the Gaussian states to have a unique stabilizing steady solution. Then general parametric conditions of how to prepare a Gaussian state with desired covariance matrix are provided. These conditions are much weaker than those required when preparing Gaussian states via dissipation-induced approach. Thus, our method provides more degrees of freedom to choose the system Hamiltonian and the system–environment coupling operators. These results may benefit practical experimental implementations in preparing quantum Gaussian states. [ABSTRACT FROM AUTHOR]

Abstract: As a matter of course, designed control laws are to be acted on actual systems. However, most designed control laws are proven to be effective when performed on ideal models of the systems as opposed to the actual systems themselves. One fundamental problem is how to ensure the satisfactory performance of the designed control laws when they are performed on actual systems. Focusing on the state stabilization of quantum systems, a two-step strategy is proposed to solve this problem. A feedback strategy and the open-loop control technique of dynamical decoupling are combined therein to deal with the inevitable differences between an actual system and its model, by taking advantage of the distinct quantum characteristics: the measurement-induced-state-transfer and the tensor product structure, with the structure characterizing the coupling between a quantum system and its environment. Specifically, in the first step, a measurement-based feedback control strategy is selected according to a model of the actual system. In the second step, by identifying the differences between the actual system and the established model as decoherence noise, a specific control procedure is designed through dynamical decoupling. This procedure allows the realization of the chosen control strategy so as to deal with the differences. [Copyright &y& Elsevier]

Abstract The identification of an unknown quantum gate is a significant issue in quantum technology. In this paper, we propose a quantum gate identification method within the framework of quantum process tomography. In this method, a series of pure states are applied to the gate and then a fast state tomography on the output states is performed and the data are used to reconstruct the quantum gate. The algorithm has computational complexity O (d 3) with the system dimension d. The identification approach is compared with the maximum likelihood estimation method for the running time, which shows an efficiency advantage of our method. An error upper bound is established for the identification algorithm and the robustness of the algorithm against impurities in the input states is also tested. We perform a quantum optical experiment on a single-qubit Hadamard gate to verify the effectiveness of the identification algorithm. [ABSTRACT FROM AUTHOR]