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Optimal Control Applications and Methods

Quantum state transfer control based on the optimal measurement

Sajede Harraz, Jingbei Yang, Kezhi Li, Shuang Cong

Published Oct 27, 2016• 9 citations
Publisher

Abstract

Summary This paper explores the optimal control of quantum state transfer in a two‐dimensional quantum system by a sequence of non‐selective projection measurements. We show that for a given initial state, one can always find the corresponding projection operator that can effectively drive the given initial state to any arbitrary target pure state. An external control field is proposed to compensate the effect of the free evolution of system. Numerical simulations and characteristics analysis are given in three cases: without considering free evolution, considering free evolution, and with the action of external control field. The simulating experimental results show that the optimal measurement control is more effective by using proposed external control field. Copyright © 2016 John Wiley & Sons, Ltd.

DOI
10.1002/oca.2287

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Summary This paper explores the optimal control of quantum state transfer in a two‐dimensional quantum system by a sequence of non‐selective projection measurements. We show that for a given initial state, one can always find the corresponding projection operator that can effectively drive the given initial state to any arbitrary target pure state. An external control field is proposed to compensate the effect of the free evolution of system. Numerical simulations and characteristics analysis are given in three cases: without considering free evolution, considering free evolution, and with the action of external control field. The simulating experimental results show that the optimal measurement control is more effective by using proposed external control field. Copyright © 2016 John Wiley & Sons, Ltd.
This paper explores the manipulation between eigenstates in a two-level system by a sequence of instantaneous projective measurements. Three cases of the manipulations are studied: the manipulation of optimal measurement-based control; the optimal measurement-based manipulation with the effect of free evolution of system; and the external control fields being used to compensate for the effect caused by the free evolution. Numerical simulations are conducted to verify the results obtained from the theoretically analytical solutions. The optimal parameters for each manipulation case are obtained. The experimental results indicate that the external control fields can make the optimal measurement-based control more effective.
This paper considers the optimal control of quantum systems. The controlled quantum systems are described by the probability-density-matrix-based Liouville–von Neumann equation. Using projection operators, the states of the quantum system are decomposed into two sub-spaces, namely the 'main state' space and the 'remaining state' space. Since the control energy is limited, a solution for optimizing the external control force is proposed in which the main state is brought to the desired main state at a certain target time, while the population of the remaining state is simultaneously suppressed in order to diminish its effects on the final population of the main state. The optimization problem is formulated by maximizing a general cost functional of states and control force. An efficient algorithm is developed to solve the optimization problem. Finally, using the hydrogen fluoride (HF) molecular population transfer problem as an illustrative example, the effectiveness of the proposed scheme for a quantum system initially in a mixed state or in a pure state is investigated through numerical simulations.
Dynamical model of open quantum systems in a matrix form is transformed into the model in a vector form using Liouville super-operator.Given a dynamical model of a dissipative quantum system in the presence of a time-dependent external control field,we achieve an improved optimal control law with the squared error between density matrices of system state and target state as the performance index.The states transfer experiment on a spin 1/2 particle system is carried out.Under the proposed optimal control,eigenstates,pure states,and mixed states are taken as initial and target states of the system,respectively.Detailed comparative analyses are given.
A general scheme is presented for controlling quantum systems using evolution driven by nonselective von Neumann measurements, with or without an additional tailored electromagnetic field. As an example, a two-level quantum system controlled by nonselective quantum measurements is considered. The control goal is to find optimal system observables such that consecutive nonselective measurement of these observables transforms the system from a given initial state into a state which maximizes the expected value of a target operator (the objective). A complete analytical solution is found including explicit expressions for the optimal measured observables and for the maximal objective value given any target operator, any initial system density matrix, and any number of measurements. As an illustration, upper bounds on measurement-induced population transfer between the ground and the excited states for any number of measurements are found. The anti-Zeno effect is recovered in the limit of an infinite number of measurements. In this limit the system becomes completely controllable. The results establish the degree of control attainable by a finite number of measurements.
We investigate the control of a three-level quantum system by laser fields assisted by von Neumann measurements. We consider a system which is not completely controllable by unitary evolution but which becomes controllable if particular measurements are used. The optimal control is defined from a cost functional which takes into account the measurements. The cost corresponds either to the minimization of the duration of the control or to the minimization of the energy of the laser field. Using the Pontryagin maximum principle, we determine the optimal control which steers the system from a given initial state toward a desired target state. This allows one to determine which observable has to be chosen for the measurement and the time at which the measurement has to be performed.
We discuss the achievability of optimal control of the evolution of an unbounded quantum system by the action of external fields. Previous work has established that the evolution of a quantum system with a nondegenerate discrete and bounded spectrum of states can, in principle, be fully controlled, i.e., that the system wave function can be guided by external fields to approach arbitrarily closely a selected target state wave function. The optimal control of the evolution of a quantum system with a discrete and bounded spectrum of states has also been studied, using a method of analysis that depends on the localized character of bound state wave functions and the fact that the spectrum of states is bounded. In this paper, we examine whether it is possible to control, partially or fully, the evolution of an unbounded quantum system. We show that optimal control of the evolution of an unbounded quantum system is possible, in the sense that it is possible to minimize the difference between the product function formed at time t0 from a localizing function and a continuum wave function and a similarly defined target function at time tf. We have not been able to establish that such optimal control is equivalent to full control, i.e., that the difference between the initial and target functions can be made arbitrarily small, which would require showing that the set of control functions is complete with respect to the function space of the system. Our analysis establishes the existence of external fields that can optimally guide the unbounded system evolution in the absence of other constraints than the Schrödinger equation of motion, but does not provide an algorithm for the construction of such fields. The relationship between optimal control of the evolution of a quantum system and the existence of chaotic dynamics in the continuum domain is briefly discussed. We argue that the optimally controlled system cannot have chaotic dynamics even if the bare system does. As illustrations of the results obtained, we discuss briefly the optimal control of evolution in the subset of discrete states from a spectrum with both discrete and continuum states, the optimal control of evolution in resonant states, and the inversion of the optimally controlled localized product continuum target function to yield the corresponding system wave function.