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Proceedings of the ... American Control Conference/Proceedings of the American Control Conference

Feedback preparation of Bell states for two-qubit systems with time delay

Yanan Liu, Daoyi Dong, Ian R. Petersen, Sen Kuang, Hidehiro Yonezawa

Published Jul 1, 2019• 8 citations
Publisher

Abstract

Bell states are the maximally entangled states in two-qubit quantum systems, and play a significant role in quantum computation and quantum communication. Feedback control of stochastic quantum systems usually suffers from time-delay problems caused by the computation time for filter states and the control input. This paper addresses the preparation of Bell states in two-qubit systems with a constant delay time. A Lyapunov method is used to design a switching control law and a constant is introduced to compensate for the computation time of the estimated states and the feedback control input, thereby stabilizing the target Bell state globally. Numerical results show the effectiveness of the proposed control law.

DOI
10.23919/acc.2019.8814881

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Bell states are the maximally entangled states in two-qubit quantum systems, and play a significant role in quantum computation and quantum communication. Feedback control of stochastic quantum systems usually suffers from time-delay problems caused by the computation time for filter states and the control input. This paper addresses the preparation of Bell states in two-qubit systems with a constant delay time. A Lyapunov method is used to design a switching control law and a constant is introduced to compensate for the computation time of the estimated states and the feedback control input, thereby stabilizing the target Bell state globally. Numerical results show the effectiveness of the proposed control law.
Bell states play a central role in the field of quantum information. For two‐qubit systems, this study proposes a new non‐smooth control strategy within the theoretical framework of quantum continuous measurement, and achieves global feedback stabilisation in any target Bell state. For a target Bell state, the authors select one measured observable and two control channels, where the control law associated with one channel is kept constant. They design a non‐smooth control law for the other control channel, and prove the stability of the whole closed‐loop system. Some simulation experiments show that the control strategy in this paper has a good control effect.
For two-qubit systems with a degenerate measurement operator, this paper presents a control approach to achieve rapid stabilization to a target Bell state. This approach uses two control channels where the control laws associated with these two channels are designed as a constant and a switching control law, respectively. In the control scheme, the system state space is divided into two parts: the set containing the target state, and its complementary set. We design corresponding control laws in these two sets via different Lyapunov functions. In this scheme, the system trajectory switches only one or two times between the two sets and the convergence rate to the target state can be improved. We provide the conditions on the control Hamiltonians to stabilize the target state by analyzing the stability of the closed-loop system.
For stochastic quantum systems with measurement feedback, this article proposes a rapid switching control scheme based on state space partition and realizes the rapid stabilization of an eigenstate of an observable operator. Meanwhile, we apply the proposed scheme to the preparation of typical entangled states in multiqubit systems. In view of the convergence obstacle caused by the symmetric structure of the state space, especially in the case with degenerate observable operators, we first partition the state space into a subset containing the target state and its complement to distinguish the target state from its antipodal points, and then design the corresponding control laws in these two subsets, respectively, by using different Lyapunov functions. The interaction Hamiltonians are also constructed to drive the system state to the desired subset first, and further to the target state. In particular, the control law designed in the undesired subset guarantees the strictly monotonic descent of the corresponding Lyapunov function, which makes the system trajectory switch between the two subsets at most twice and has the potential to speed up the convergence process. We also prove the stability of the closed-loop system with the proposed switching control law based on the stochastic Lyapunov stability theory. By applying the proposed switching control scheme to a three-qubit system, we achieve the preparation of a GHZ state and a W state.
The exponential quantum projection filtering algorithm is applied in measurement-based quantum feedback control and a specific switching control law is designed to realize the rapid preparation of a target Bell state. First, we use an exponential projection filter to obtain an approximate estimate of the system state, which can reduce the calculation time spent in the measurement process. Then, the quantum state space is partitioned into two subsets to separate the target state from other equilibrium states, two Lyapunov functions are constructed and the corresponding control laws are designed in the two subsets. It is shown that the switching control scheme in this paper ensures that the system trajectory crosses the boundary of the two subsets no more than twice from any initial state, which effectively reduces the time that the system converges to the target Bell state. Numerical simulation on a two-qubit system verifies the effectiveness of the proposed control scheme.
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.
Lyapunov-Based Feedback Preparation of GHZ Entanglement of NNN -Qubit Systems
The Greenberger-Horne-Zeilinger (GHZ) entangled states are a typical class of entangled states in multiparticle systems and play an important role in the applications of quantum communication and quantum computation. For a general quantum system of qubits, degenerate measurement operators are often met, which cause the convergence obstacle in the state preparation or stabilization problem. This paper first generalizes the traditional quantum state continuous reduction theory to the case of a degenerate measurement operator and chooses a measurement operator for an arbitrarily given target GHZ entangled state, then presents a state stabilization control strategy based on the Lyapunov method and achieves the feedback preparation of the target GHZ state. In our stabilization strategy, we separate the target GHZ state and all the other GHZ states that often form the equilibrium points of the closed-loop system by dividing the state space into several different regions; and formally design a switching control law between the regions, which contains the control Hamiltonians to be constructed. By analyzing the stability of the closed-loop system in the different regions, we propose a systematic method for constructing the control Hamiltonians and solve the convergence problem caused by the degenerate measurement operator. The global stability of the whole closed-loop stochastic system is strictly proved. Also, we perform some simulation experiments on a three-qubit system and prepare a three-qubit GHZ entangled state. At the same time, the simulation results show the effectiveness of the switching control law and the construction method for the control Hamiltonians proposed in this paper.