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Physical Review A

Kinematical bounds on evolution and optimization of mixed quantum states

Marvin D. Girardeau, M. Ina, S. G. Schirmer, T. Gulsrud

Published Mar 1, 1997• 22 citations
Publisher

Abstract

Upper and lower bounds are established for time-dependent ensemble averages of observables of driven quantum systems in mixed states. They limit controllability of observables independently of the control fields. Narrower bounds are established when the observable is a projector onto a pure quantum state or subspace. They are optimal in the sense of being kinematically achievable. Calculations on nonlinear optimal control of a four-level model indicate that these kinematical bounds are dynamically achievable asymptotically with increasing control pulse strength.

DOI
10.1103/physreva.55.r1565

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Upper and lower bounds are established for time-dependent ensemble averages of observables of driven quantum systems in mixed states. They limit controllability of observables independently of the control fields. Narrower bounds are established when the observable is a projector onto a pure quantum state or subspace. They are optimal in the sense of being kinematically achievable. Calculations on nonlinear optimal control of a four-level model indicate that these kinematical bounds are dynamically achievable asymptotically with increasing control pulse strength.
In this paper we investigate the limits of control for mixed-state quantum systems. The constraint of unitary evolution for nondissipative quantum systems imposes kinematical bounds on the optimization of arbitrary observables. We summarize our previous results on kinematical bounds and show that these bounds are dynamically realizable for completely controllable systems. Moreover, we establish improved bounds for certain partially controllable systems. Finally, the question of dynamical realizability of the bounds for arbitrary partially controllable systems is shown to depend on the accessible sets of the associated control system on the unitary group U(N)U(N)U(N) and the results of a few control computations are discussed briefly.
Girardeau et al. (1998) derived kinematical bounds on the optimization of observables for mixed-state quantum systems and showed that they are dynamically realizable if the system is completely controllable. In this paper the problem of finding dynamically realizable bounds for systems that are not completely controllable is addressed. We derive such bounds for systems whose dynamics can be decomposed into subspace dynamics. We also study systems that are not decomposable yet fail to be completely controllable. For these systems, the question of dynamical realizability of the kinematical bounds depends on the accessibility of the target states for which the expectation value of the observable assumes its kinematical maximum.
In quantum optimal control theory, kinematic bounds are the minimum and maximum values of the control objective achievable for any physically realizable system dynamics. For a given initial state of the system, these bounds depend on the nature and state of the controller. We consider a general situation where the controlled quantum system is coupled to both an external classical field (referred to as a classical controller) and an auxiliary quantum system (referred to as a quantum controller). In this general situation, the kinematic bound is between the classical kinematic bound (CKB), corresponding to the case where only the classical controller is available, and the quantum kinematic bound (QKB), corresponding to the ultimate physical limit of the objective's value. Specifically, when the control objective is the expectation value of a quantum observable (a Hermitian operator on the system's Hilbert space), the QKBs are the minimum and maximum eigenvalues of this operator. We present, both qualitatively and quantitatively, the necessary and sufficient conditions for surpassing the CKB and reaching the QKB, through the use of a quantum controller. The general conditions are illustrated by examples in which the system and controller are initially in thermal states. The obtained results provide a basis for the design of quantum controllers capable of maximizing the control yield and reaching the ultimate physical limit.
Existing quantum speed limits for controlled open quantum systems depend on the specified trajectory. For example, lower bounds on quantum annealing times in the presence of dissipation depend explicitly on the chosen annealing schedule. Recently, schedule-independent speed limits have been derived for annealing in the closed quantum system setting (SciPost Phys. 18, 159 (2025)). In this work, we generalize these results to open quantum systems, deriving schedule-independent lower bounds for quantum annealing times in systems described by a Lindblad master equation. We analyze the interplay between coherent control and dissipation in single- and two-qubit examples, demonstrating that the derived lower bounds capture key scaling behavior with respect to the strength of the dissipator. Finally, we apply the bound to thermal state preparation and show that the bound matches the expected asymptotic behavior for an Ising model in the high temperature limit.