Shibei Xue

Quantum operation for a one-qubit system under a non-Markovian environment

This paper introduces a simple alternating-current (AC) control strategy to perform quantum state manipulations under non-Markovian noise. A genetic algorithm is adopted to optimize the parameters of the AC control, which can be further used to fulfil one-qubit quantum operations at a given final time. Theoretical analysis and simulations show that our method works almost equally well for 1/f noise, ohmic, sub-ohmic and super-ohmic noise, which demonstrates the robustness of our strategy for noise with various spectra. In comparison with the Markovian cases, our method is more suitable to be used to suppress non-Markovian noise.

Quantum filter for a class of non-Markovian quantum systems

In this paper we present a Markovian representation approach to constructing quantum filters for a class of non-Markovian quantum systems disturbed by Lorentzian noise. An ancillary system is introduced to convert white noise into Lorentzian noise which is injected into a principal system via a direct interaction. The resulting dynamics of the principal system are non-Markovian, which are driven by the Lorentzian noise. By probing the principal system, a quantum filter for the augmented system can be derived from standard theory, where the conditional state of the principal system can be obtained by tracing out the ancillary system.

Witnessing the boundary between Markovian and non-Markovian quantum dynamics: a Green's function approach

This paper presents a Green’s function-based root locus method to investigate the boundary between Markovian and non-Markovian open quantum systems in the frequency domain. A Langevin equation for the boson-boson coupling system is derived, where we show that the structure of the Green’s function dominates the system dynamics. In addition, by increasing the coupling between the system and its environment, the system dynamics are driven from Markovian to non-Markovian dynamics, which results from the redistribution in the modes of the Green’s function in the frequency domain. Both a critical transition and a critical point condition under Lorentzian noise are graphically presented using a root locus method. Related results are verified using an example of a boson-boson coupling system.

Modeling and Analysis of Non-Markovian Open Quantum Systems for Coherent Feedback

Abstract This paper presents a model for coherent feedback control of non-Markovian open quantum systems, where an integral-differential Langevin equation is derived to describe the controlled dynamics of the non-Markovian system. Moreover, a Green function based root locus method is presented to analyze the non-Markovian dynamics of the system under coherent feedback control in the frequency domain. Utilizing the quantum tunneling effect, our coherent feedback scheme is applied to a quantum dot system. Compared with the method in Ref. [Phys. Rev. A, 84, 052116, 2011], our coherent feedback can obtain a better performance of decoherence suppression for fermion systems.

Feedback Tracking Control of Non-Markovian Quantum Systems

In this paper, we present a feedback tracking control strategy to reject colored noise in linear non-Markovian quantum systems. This non-Markovian system is modeled as an augmented linear Markovian system, which includes a principal system to be controlled and multiancillary systems representing the effect of the internal modes of the non-Markovian environment. The colored noise generated by the multiancillary systems disturbs the principal system through a direct interaction. Based on this model, a whitening quantum filter can be obtained for estimating the dynamical variables of both the principal and multiancillary systems, which can be used for feedback control. To reject the colored noise, a linear quadratic Gaussian controller is designed to drive the dynamical variables of the principal system to track those of a reference system. In examples, we show that colored noise with both rational and nonrational spectra in linear non-Markovian quantum systems can be efficiently rejected using our tracking control strategy.

Realizing the dynamics of a non-Markovian quantum system by Markovian coupled oscillators: a Green's function-based root locus approach

In this paper, we develop a Green’s function-based root locus approach to realizing a Lorentzian-noise-disturbed non-Markovian quantum system by Markovian coupled oscillators in an extended Hilbert space. By using a Green’s function-based root locus method, we design an ancillary oscillator for Markovian coupled oscillators to be a Lorentzian noise generator. Thus a principal oscillator coupled to the ancillary oscillator via a direct interaction can capture the dynamics of a Lorentzian-noise-disturbed non-Markovian quantum system. By matching the root locus in the frequency domain, conditions for the realization are obtained and a critical transition in the non-Markovian quantum system can also be observed in the Markovian coupled oscillators.

Quantum filter for a non-Markovian single qubit system

In this paper, a quantum filter for estimating the states of a non-Markovian qubit system is presented in an augmented Markovian system framework including both the qubit system of interest and multi-ancillary systems for representing the internal modes of the non-Markovian environment. The colored noise generated by the multi-ancillary systems disturbs the qubit system via a direct interaction. The resulting non-Markovian dynamics of the qubit is determined by a memory kernel function arising from the dynamics of the ancillary system. In principle, colored noise with arbitrary power spectrum can be generated by a combination of Lorentzian noises. Hence, the quantum filter can be constructed for the qubit disturbed by arbitrary colored noise and the conditional state of the qubit system can be obtained by tracing out the multi-ancillary systems.

Feedback tracking control of a class of non-Markovian quantum systems

In this paper, we propose a feedback tracking control scheme to reject Lorentzian noise in a linear non-Markovian quantum system. This non-Markovian quantum system is modelled as an augmented linear Markovian quantum system where a principal system represents the plant and an ancillary system represents the internal mode of the non-Markovian environment for generating Lorentzian noise. Based on this augmented system model, a whitening quantum filter can be constructed for continuously monitoring the dynamical variables of the principal system. To reject Lorentzian noise, a linear quadratic Gaussian (LQG) controller, using the estimate of the whitening filter and the dynamical variables of a reference system, can be designed to drive the principal system to track the reference system.

LQG feedback control of a class of linear non-Markovian quantum systems

In this paper we present a linear quadratic Gaussian (LQG) feedback control strategy for a class of linear non-Markovian quantum systems disturbed by Lorentzian noise. The feedback control law is designed based on the estimated dynamical variables of a whitening quantum filter for an augmented Markovian model of the non-Markovian quantum system. In this augmented Markovian model, an ancillary system plays the role of internal modes of the environment converting white noise into Lorentzian noise and a principal system obeys non-Markovian dynamics due to the direct interaction with the ancillary system. The simulation results show the LQG controller with the whitening filter obtains a better control performance than that with a Markovian filter in the problem of cooling the principal system when the ancillary system is driven by thermal noise.

Identifying a damping rate function for a non-Markovian single qubit system

In this paper, we present a gradient algorithm to identify a damping rate function for a non-Markovian single qubit system. The dynamics of the single qubit system in a non-Markovian environment are assumed to obey a time convolutionless master equation, where all the non-Markovian effects of the environment are combined in the unknown damping rate function. To identify the damping rate function, we measure time trace observables of the qubit such that we can formulate the identification procedure as an optimization problem. Thus, we design a gradient algorithm to optimally reveal the damping rate function.