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arXiv:0709.1339 · IEEE Transactions on Automatic Control

Control of Quantum Systems Despite Feedback Delay

Kenji Kashima, Naoki Yamamoto

Published Mar 30, 2009• 28 citations
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Abstract

Feedback control (based on the quantum continuous measurement) of quantum systems inevitably suffers from estimation delays. In this technical note we give a delay-dependent stability criterion for a wide class of nonlinear stochastic systems including quantum spin systems. We utilize a semi-algebraic problem approach to incorporate the structure ofdensitymatrices. To show the effectiveness of the result, we derive a globally stabilizing control law for a quantum spin-1/2 systems in the face of feedback delays.

DOI
10.1109/tac.2008.2010969

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Feedback control (based on the quantum continuous measurement) of quantum systems inevitably suffers from estimation delays. In this technical note we give a delay-dependent stability criterion for a wide class of nonlinear stochastic systems including quantum spin systems. We utilize a semi-algebraic problem approach to incorporate the structure ofdensitymatrices. To show the effectiveness of the result, we derive a globally stabilizing control law for a quantum spin-1/2 systems in the face of feedback delays.
In this paper, feedback control of quantum spin systems is investigated. The quantum control scenario follows the formalism of continuous quantum measurement and quantum state estimation using the so-called stochastic master equation. In quantum feedback control implementations the state estimation is delayed, which is known to degrade the resulting control performance. As yet there are no theoretical means to take such estimation delays into consideration. In this paper we derive a globally stabilizing control law for two-dimensional quantum spin systems taking estimation delays into explicit account. From a control theoretic viewpoint, the obtained result is a delay-dependent stability criterion for a nonlinear stochastic switching system.
Accurate control of quantum spin is necessary in order to utilize quantum spin as the physical realization of the quantum bit. Feedback control (based on continuous quantum measurement) of quantum spin systems inevitably suffers from estimation delays. In this paper, we state that a control system which takes into account these delays can be described by using stochastic delay differential systems. Moreover, we can restrict our analysis to the class of systems which have polynomial coefficients and evolve in a bounded semi-algebraic set. Then, the problem of stabilizing such systems is shown to be solvable effectively via reduction to a semi-algebraic problem.
In this paper, feedback control of quantum spin systems is investigated. The quantum control scenario follows the formalism of continuous quantum measurement and quantum state estimation using the so-called stochastic master equation. In quantum feedback control implementations the state estimation is delayed, which is known to degrade the resulting control performance. As yet there are no theoretical means to take such estimation delays into consideration. In this paper we derive a stabilizing control law for two-dimensional quantum spin systems taking estimation delays into explicit account. From a control theoretic viewpoint, the obtained result is a delay-dependent stability criterion for a nonlinear stochastic switching system.
This paper addresses the problem of measurement-based feedback control for quantum systems, in which the time to compute the filter-based control input is taken into account by considering the input delay. It starts with a new Lyapunov–LaSalle-like theorem for delay-dependent stochastic stability of a class of time delay stochastic nonlinear systems. Nonsmooth time delay control is then constructed to compensate for the control-computation time, which is known but arbitrarily long, while globally stabilizing the quantum filters almost surely. The nonsmooth property enables the control to deal with the symmetric topology of filter state space. The effectiveness of the proposed control is illustrated through the global stabilization of the spin-1/21/21/2 systems. Simulation results are presented and discussed to show the effectiveness of the proposed control.
In this paper, a stabilization problem of quantum spin systems in general dimension under continuous measurement is considered and it is shown that the global stabilization at an eigenstate by continuous inputs is possible. Quantum states of spin systems under continuous measurement by mutual interference with laser beams can be continuously estimated by quantum filtering and with its information, the intension of magnetic field, which is applied to atoms, can be controlled. Because of the structural symmetry of the nonlinear dynamics, this stabilization problem by continuous feedback has been considered to be hard and recently a switching rule was introduced which attains the global stability. Our proposing control rule is the sum of two terms: a term which attracts the quantum states to a target and the other term which draw apart from the other equilibrium points. The proof is done by the strict analysis on the sample paths of solutions of a stochastic differential equation. We also show numerical examples to demonstrate the efficiency of the proposing control rule.
In this chapter, we introduce our recent results on feedback control of quantum spin systems. The quantum systems are composed of atoms held in a cavity under continuous measurement by photons and a magnetic field that is applied to atoms for controlling the quantum states. The quantum states can be estimated by quan- tum filtering through the continuous measurement, and the intension of magnetic field is controlled depending on the estimations. We show that our proposing control input attains the global stability on the assigned eigenstates.