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Chinese Control Conference (CCC)

The characteristics of limit points of any trajectory of mixed-state quantum systems in the Lyapunov-based control design

Sen Kuang, Shuang Cong

Published Jul 29, 2010• 0 citations
OpenAlex

Abstract

For mixed-state quantum systems without degenerate transition, this paper chooses the expectation value of a mechanical quantity as a Lyapunov function, designs the control laws that guarantee the monotonic decreasing of the Lyapunov function, and mainly studies the characteristics of limit points of any system trajectory under the action of control fields. Research results show that for a fully connected system, the limit point of any trajectory is one of its equilibrium states while for an only connected system, the limit points of any trajectory is not isolated. In the latter case, this paper also gives the strict expression of the limit points via the Bloch vector framework of density operators.

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For mixed-state quantum systems without degenerate transition, this paper chooses the expectation value of a mechanical quantity as a Lyapunov function, designs the control laws that guarantee the monotonic decreasing of the Lyapunov function, and mainly studies the characteristics of limit points of any system trajectory under the action of control fields. Research results show that for a fully connected system, the limit point of any trajectory is one of its equilibrium states while for an only connected system, the limit points of any trajectory is not isolated. In the latter case, this paper also gives the strict expression of the limit points via the Bloch vector framework of density operators.
This paper studies the population control problem associated with the equilibrium states of mixed-state quantum systems by using a Lyapunov function with degrees of freedom. The control laws are designed by ensuring the monotonicity of the Lyapunov function; main results on the largest invariant set in the sense of LaSalle are given; and the strict expression of any state in the largest invariant set is normally deduced in the framework of Bloch vectors. By analyzing the obtained largest invariant set and the Lyapunov function itself, this paper also discusses the determination problem of the degrees of freedom. Numerical simulation experiments on a three-level system show the validity of research results.
A Lyapunov-based approach of trajectory tracking in Liouville equation for quantum systems is proposed. By selecting density matrix of controlled system as the system state, and defining the error between controlled state and target one as an error state, the paper regards this error as a new controlled state of system. In such a way the quantum state trajectory tracking problem is changed into the state regulation problem by driving the error state to zero. The control law is designed according to the Lyapunov stability theorem, which can be applied to track different kinds of quantum states including pure state, superposition state and mixed-state. Finally, numerical simulation experiments and results analysis are given.
The natural trajectory tracking problem is studied for generic quantum states represented by density operators. A control design based on the Hilbert-Schmidt distance as a Lyapunov function is considered. The control dynamics is redefined on an extended space where the LaSalle invariance principle can be correctly applied even for non-stationary target states. LaSalle's invariance principle is used to derive a general characterization of the invariant set, which is shown to always contain the critical points of the Lyapunov function. Critical point analysis of the latter is used to show that, for generic states, it is a Morse function with n! isolated critical points, including one global minimum, one global maximum and n!-2 saddles. It is also shown, however, that the actual dynamics of the system is not a gradient flow, and therefore a full eigenvalue analysis of the linearized dynamics about the critical points of the dynamical system is necessary to ascertain stability of the critical points. This analysis shows that a generic target state is locally asymptotically stable if the linearized system is controllable and the invariant set is regular, and in fact convergence to the target state (trajectory) in this case is almost global in that the stable manifolds of all other critical points form a subset of measure zero of the state space. On the other hand, if either of these sufficient conditions is not satisfied, the target state ceases to be asymptotically stable, a center manifold emerges around the target state, and the control design ceases to be effective.
A Lyapunov-based method is presented for stabilizing and controlling of closed quantum systems. The proposed method is constructed upon a novel quantum Lyapunov function of the system state trajectory tracking error. A positive-definite operator in the Lyapunov function provides additional degrees of freedom for the designer. The stabilization process is analyzed regarding two distinct cases for this operator in terms of its vanishing or non-vanishing commutation with the Hamiltonian operator of the undriven quantum system. To cope with the global phase invariance of quantum states as a result of the quantum projective measurement postulate, equivalence classes of quantum states are defined and used in the proposed Lyapunov-based analysis and design. Results show significant improvement in both the set of stabilizable quantum systems and their invariant sets of state trajectories generated by designed control signals. The proposed method can potentially be applied for high-fidelity quantum control purposes in quantum computing frameworks.
Under the ideal conditions that the controlled systems are non-degenerate,transitionally non-degenerate,and fully connected,the stabilization problem of mixed-state quantum systems is studied by using a Lyapunov function with free degrees. Based on the LaSalle's principle,the largest invariant set of the closed-loop systems and the convergent state set associated with any initial state are deduced,and the construction principles of free degrees that ensure the system can be asymptotically stabilized in any equilibrium state contained in the largest invariant set are given. A numerical simulation experiment on a two-level system shows the rationality of the obtained theoretical results.
Abstract State transition control of open quantum systems is a research hotspot and difficulty. The control method in the light of the Lyapunov stability theory is a common method. Current research shows that if the open quantum systems meet the constraints that the free Hamiltonian is strong regular and the interaction Hamiltonian is fully connected, by devising appropriate control fields in the light of the Lyapunov stability theory, the open quantum system can move to the desired final state. In view of the fact that most practical open quantum systems do not meet these two constraints, this paper propose a control method. The implicit function perturbation terms are introduced into the control fields to resolve the problem that the open quantum systems do not meet these two convergence constraints. And the main control fields are devised in the light of the Lyapunov stability theory to make the state transition of the open quantum systems converging to the desired final state.