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arXiv:0709.1675

Indirect control of the asymptotic states of a quantum dynamical semigroup

Romano, Raffaele

Published Sep 11, 2007• 1 citation
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Abstract

In the dynamics of open quantum systems, the interaction with the external environment usually leads to a contraction of the set of reachable states for the system as time increases, eventually shrinking to a single stationary point. In this contribution we describe to what extent it is possible to modify this asymptotic state by means of indirect control, that is by using an auxiliary system coupled to the target system in order to affect its dynamics, when there is a purely dissipative coupling between the two systems. We prove that, also in this restrictive case, it is possible to modify the asymptotic state of the relevant system, give necessary conditions for that and provide physical examples. Therefore, in indirect control schemes, the environmental action has not only a negative impact on the dynamics of a system, it is rather possible to make use of it for control purposes.

DOI
10.48550/arxiv.0709.1675

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In the dynamics of open quantum systems, the interaction with the external environment usually leads to a contraction of the set of reachable states for the system as time increases, eventually shrinking to a single stationary point. In this contribution we describe to what extent it is possible to modify this asymptotic state by means of indirect control, that is by using an auxiliary system coupled to the target system in order to affect its dynamics, when there is a purely dissipative coupling between the two systems. We prove that, also in this restrictive case, it is possible to modify the asymptotic state of the relevant system, give necessary conditions for that and provide physical examples. Therefore, in indirect control schemes, the environmental action has not only a negative impact on the dynamics of a system, it is rather possible to make use of it for control purposes.
We prove that it is possible to modify the stationary states of a quantum dynamical semigroup, describing the irreversible evolution of a two-level system, by means of an auxiliary two-level system, a quantum probe that can be suitably prepared. The target system and the probe can be initially entangled or uncorrelated. We find that this indirect control of the stationary states is possible, even if there are no initial correlations, under suitable conditions on the dynamical parameters characterizing the evolution of the joint system.
In this thesis, we study the controllability and accessibility properties of quantum systems. First, we provide a new proof of the fact that, in general, open quantum systems are not controllable. By controllability we mean that for every pair of initial and target states of the system there exists a finite time T>0 and a control u such that by adding u into the system’s evolution the target state can be reached from the initial state in time T. We then consider a bipartite open system A+B consisting of two interacting qubits A and B under the assumption that only A is directly affected by control u and only A interacts with the environment. We investigate the controllability properties of B. We show that target pure states of B are not reachable in finite time from initial mixed states of B when the initial state of A+B lies in the interior of the set of states. We then examine whether it is possible to maintain the state of B pure. For three specific couplings between A and B we find that the only admissible trajectories are either unaffected by the control u or constant. These results constitute a first negative answer regarding the indirect manipulation of B. Finally, we study the accessibility of B, that is, whether for every initial state of B the attainable set contains an open subset of the set of states of B. For this property, we provide a sufficient condition that can also be applied to more general settings, such as those arising in geometric control theory.
In many experiments, a target quantum mechanical system is controlled and-or measured indirectly using an auxiliary quantum system. In the case of control, this means that the control action only affects the auxiliary system while the evolution of the target system is affected indirectly through the interaction with the auxiliary system. In the case of measurement, only the state of the auxiliary system can be measured, providing information on the initial state of the target system. Indirect controllability is the property of the target system of being driven between two arbitrary states while indirect observability refers to the possibility of extracting all the information on the state of the target system by measuring the auxiliary system. This paper has three main goals. First, we summarize recent notions and results introduced by us on the study of indirect controllability. Then, we present some new technical results on indirect controllability. In particular, we present a counterexample to the converse of a criterion for indirect controllability, which shows that the Lie algebraic condition introduced in this criterion is necessary but not sufficient for indirect controllability. This gives an open problem which is how to strengthen this condition to obtain a necessary and sufficient condition for indirect controllability. Lastly, we present a parallel treatment of indirect observability, give the relevant notions and definitions and relate the properties of indirect controllability and observability. Our final result says that strong notions of indirect controllability and observability are equivalent properties to complete controllability of the system. We discuss examples of physical systems which are controlled and-or measured indirectly. The research is motivated by experimental schemes of current interest
A quantum mechanical system S is indirectly controlled when the control affects an ancillary system A and the evolution of S is modified through the interaction with A only. A study of indirect controllability gives a description of the set of states that can be obtained for S with this scheme. In this paper, we study the indirect controllability of quantum systems in the finite dimensional case. After discussing the relevant definitions, we give a general necessary condition for controllability in Lie algebraic terms. We present a detailed treatment of the case where both systems, S and A, are two-dimensional (qubits). In particular, we characterize the dynamical Lie algebra associated with S+A, extending previous results, and prove that complete controllability of S+A and an appropriate notion of indirect controllability are equivalent properties for this system. We also prove several further indirect controllability properties for the system of two qubits, and illustrate the role of the Lie algebraic analysis in the study of reachable states.
A quantum mechanical system S is indirectly controlled when the control affects an ancillary system A and the evolution of S is modified through the interaction with A only. A study of indirect controllability gives a description of the set of states that can be obtained for S with this scheme. In this paper, we study the indirect controllability of quantum systems in the finite dimensional case. After discussing the relevant definitions, we give a general necessary condition for controllability in Lie algebraic terms. We present a detailed treatment of the case where both systems, S and A, are two-dimensional (qubits). In particular, we characterize the dynamical Lie algebra associated with S + A, extending previous results, and prove that complete controllability of S + A and an appropriate notion of indirect controllability are equivalent properties for this system. We also prove several further indirect controllability properties for the system of two qubits, and illustrate the role of the Lie algebraic analysis in the study of reachable states.
In several quantum control schemes, a target quantum system SSS is put in contact with an auxiliary system AAA and the coherent control can directly affect only AAA. The system SSS is controlled indirectly through the interaction with AAA. The system SSS is said to be indirectly controllable if every unitary transformation can be performed on the state of SSS with this scheme. The indirect controllability of SSS will depend on the dynamical Lie algebra L{\cal L}L characterizing the dynamics of the total system S+AS+AS+A and on the initial state of the auxiliary system AAA. In this paper, we describe this characterization exactly. A natural assumption is that the auxiliary system AAA is minimal, which means that there is no part of AAA that is not coupled to SSS, and we denote by nAn_AnA​ the dimension of such a minimal AAA, which we assume to be fully controllable. We show that if nAn_AnA​ is greater than or equal to 333, indirect controllability of SSS is verified if and only if complete controllability of the total system S+AS+AS+A is verified, i.e., L=su(nSnA){\cal L}=su(n_Sn_A)L=su(nS​nA​) or L=u(nSnA){\cal L}=u(n_Sn_A)L=u(nS​nA​), where nSn_SnS​ denotes the dimension of the system SSS. If nA=2n_A=2nA​=2, it is possible to have indirect controllability without having complete controllability. The exact condition for that to happen is given in terms of a Lie algebra LS{\cal L}_SLS​ which describes the evolution of the system SSS only. We prove that indirect controllability is verified if and only if LS=u(nS){\cal L}_S=u(n_S)LS​=u(nS​) and the initial state of the auxiliary system AAA is pure.