Using a circuit QED device, we present a theoretical study of real-time quantum state estimation via quantum Bayesian approach. Suitable conditions under which the Bayesian approachcan accurately update the density matrix of the qubit are analyzed. We also consider the correlation between some basic and physically meaningful parameters of the circuit QED and the performance of the Bayesian approach. Our results advance the understanding of quantum Bayesian approach and pave the way to study quantum feedback control and adaptive control.
We consider the feedback stabilization of Rabi oscillations in a superconducting qubit which is coupled to a microwave readout cavity. The signal is readout by homodyne detection ofthe in-phase quadrature amplitude of the weak measurement output. By multiplying the time-delayed Rabi reference, one can extract the signal, with maximum signal-to-noise ratio, from the noise. We further track and stabilize the Rabi oscillations by using Lyapunov feedback control to properly adjust the input Rabi drives. Theoretical and simulation results illustrate the effectiveness of the proposed control law.
We propose how to realize high-fidelity quantum storage using a hybrid
quantum architecture including two coupled flux qubits and a nitrogen-vacancy
center ensemble (NVE). One of theflux qubits is considered as the quantum
computing processor and the NVE serves as the quantum memory. By separating the
computing and memory units, the influence of the quantum computing process on
the quantum memory can be effectively eliminated, and hence the quantum storage
of an arbitrary quantum state of the computing qubit could be achieved with
high fidelity. Furthermore the present proposal is robust with respect to
fluctuations of the system parameters, and it is experimentally feasibile with
currently available technology.