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The Scientific World Journal

Volume 2013 (2013), Article ID 814080, 7 pages

http://dx.doi.org/10.1155/2013/814080

## Lyapunov Control of Quantum Systems with Impulsive Control Fields

Department of Mathematics, Tongji University, Shanghai 200092, China

Received 10 April 2013; Accepted 26 April 2013

Academic Editors: C. Chen, D. Dong, M. Jiang, and L.-C. Wang

Copyright © 2013 Wei Yang and Jitao Sun. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We investigate the Lyapunov control of finite-dimensional quantum systems with impulsive control fields, where the studied quantum systems are governed by the Schrödinger equation. By three different Lyapunov functions and the invariant principle of impulsive systems, we study the convergence of quantum systems with impulsive control fields and propose new results for the mentioned quantum systems in the form of sufficient conditions. Two numerical simulations are presented to illustrate the effectiveness of the proposed control method.

#### 1. Introduction

In the last few years, because of a wide variety of applications of the quantum control theory, such as quantum chemistry, quantum information processing, and quantum electronics, considerable attention has been focused on quantum control theory, and the growing interest in this subject has been attributed to both theoretical and experimental breakthroughs ([1–9] and references therein); it indicates that quantum control has become an important area of research.

Controllability is one important part in the quantum control theory. Different definitions of controllability have been studied in [10–12], and sufficient conditions which are based on the Lie algebra of system Hamiltonian are also given. Referring to the control method, Lyapunov-based techniques are good approaches, such as implicit Lyapunov control [9, 13], Lyapunov functions based on state distance [5], average value of an imaginary mechanical quantity [14, 15], and state error [15, 16].

In [17], Turinici and Rabitz considered the wavefunction controllability method based on graph theory. When a quantum system is not wavefunction controllable with one control field, Dong and Petersen introduced the switching control method to drive the system by using two controllers to arbitrary target state based on graph theory [18]. In [19], Zhao et al. considered another switching control method of closed quantum systems, which was via the Lyapunov method.

Inspired by the switching control method, we developed the impulsive control method to drive a quantum system to a given target state. As we know, impulsive dynamical systems are a special class of dynamical systems, which exhibit continuous evolution typically described by ordinary differential equations and instantaneous state jumps or impulses. Nowadays, there has been increasing interest in the analysis and synthesis of impulsive systems, or impulsive control systems, due to their significance in both theory and applications; see [20–23] and the references therein.

By adding an impulsive control field besides the continuous control field, we apply the impulsive control method to control quantum systems. From the result in [24], when one control field contributes with given frequency, quantum systems governed by the Schrödinger equation can be described as impulsive dynamical systems.

In this paper, based on the Lyapunov method and invariant principle of impulsive systems [25], our attention is focused on the Lyapunov control of quantum systems with impulsive control fields. In Section 2, we present the quantum systems with impulsive control fields and introduce the invariant principle of impulsive systems. In Section 3, we give different control fields to drive quantum systems based on three Lyapunov functions and analyze the asymptotic stability of quantum systems with impulsive control fields. We justify the effectiveness of the proposed control fields in two simulation experiments in Section 4.

#### 2. Notations and Definitions

Consider the impulsive dynamical system described by the following: where denotes the system state, is a continuous function from to , the set is an unbounded, closed, and discrete subset of which denotes the set of times when jumps occur, and denotes the incremental change of the state at the time . In the -dimensional complex space , we choose the most common norm , where is represented as a column vector and denotes its conjugate transpose. Denote by the space of complex matrices with an inner product , and the norm .

Consider the following -level quantum system with two control fields, and set the Plank constant : where the ket represents the state vector of quantum systems, which is right continuous, and the state vector evolves on or in a sphere with radius one, and we denote the set of quantum states by , and is the Dirac impulse. Physically, two states and that differ by a phase , that is, , describe the same physical state in or on the sphere of . We denote the bra associated with the ket with . When the quantum system evolves freely under its own internal dynamics, that is, there is no external field implemented on the system, just the free Hamiltonian is introduced. represent the interaction energy between the system and the external classical control fields and are called interaction Hamiltonian. are all self-adjoint operators in the -dimensional Hilbert space and assumed to be time independent. In this paper, we set that the first control function is a continuous function, the other one only takes effect to quantum systems at the impulsive points .

Multiplying both sides of (3) by , we have where , skew-Hermitian matrices.

In quantum control, the target state is usually an eigenstate of the free Hamiltonian, and we set the target state satisfies: where is the eigenvalue of corresponding to .

By the same method in [24], we obtain that quantum systems (3) with impulsive control fields can be described as

When taking nontrivial geometry about states, we add a second control corresponding to into consideration [16]: then investigate the following quantum systems where is the identity matrix. If the control field only takes effect at the impulsive points , the quantum systems with impulsive control fields are

Subject to quantum systems (3) or (7), we focus on finding control fields and , such that the quantum systems with impulsive control field (6) or (8) are driven to target states. Firstly, we introduce the invariant principle of impulsive systems.

Lemma 1 (see [25]). *Consider the impulsive dynamical system (1), assume that is a compact positively invariant set with respect to (1), and assume that there exists a function such that *(1)*;
*(2)*. **
Let , and let denote the largest invariant set contained in . If , then as . *

#### 3. Main Results

Theorem 2. *For quantum system (6), if is nondegenerate, set control fields and where constants and the image of function passes the origin of plane monotonically and lies in quadrant I or III, then quantum systems with impulses (6) converge to the largest invariant set where . If all the states in are equivalent to the target state , then the systems will converge asymptotically to the target state . *

*Proof. *Choose the Lyapunov function based on the state distance

When ,
as discussed in [15], by the control field
we have

When ,
by the control field
and , we have
where can be chosen properly to adjust the control amplitude. If , or , we set , or .

By the definition of invariant set and properties of limit point, if we choose the control field (11) which is the same as that in [15], the largest invariant set of quantum systems with impulses (6) is , where . From the invariant principle Lemma 1, quantum systems with impulsive control fields (6) will converge to .

Thus, we complete the proof.

When the phase is considered, we choose the Lyapunov function based on the state error [15, 16].

Theorem 3. *For quantum system (8), if is nondegenerate, set the control fields , , and , where constants and the image of function passes the origin of plane monotonically and lies in quadrant I or III, then quantum systems with impulses (8) converge to the largest invariant set , where . If all the states in are equivalent to the target state , then the systems will converge asymptotically to the target state . *

*Proof. *Choose the Lyapunov function based on the state error

When ,
and the simple control field
we have

When ,
since , by the control field , we have

Using the control field (18), the largest invariant set of quantum systems with impulsive control fields (8) is [15, 16], where . From the invariant principle Lemma 1, the quantum systems with impulsive control fields (8) will converge to .

Thus, we complete the proof.

Set that the eigenvalues of are , , and the corresponding eigenstates are , .

Theorem 4. *For quantum systems with impulsive control field (6), if is nondegenerate, set , and , where constant , is a positive definite Hermitian matrix and satisfies , then quantum systems with impulsive control field (6) converge to the largest invariant set , where , . If all the states in are equivalent to the target state , then the systems will converge asymptotically to the target state . *

*Proof. *Choose another Lyapunov function based on the average value of an imaginary mechanical quantity
Considering the case , one has
and since there is no relation between and the control component, we can set for convenience. If we choose simple and effective control field
where , then .

Since the state is right continuous at the impulsive points, we have

For Hermitian matrices and , and are real numbers, and

By the control function , we have

Using the control field (24), the largest invariant set of quantum systems with impulsive control field (6) is [15], where , . From the invariant principle Lemma 1, the quantum systems controlled by (6) will converge to .

Thus we complete the proof.

#### 4. Illustrative Examples

In this section, in order to illustrate the effectiveness of the proposed method in this paper, two numerical simulations have been presented for two five-level quantum systems and the fourth-order Runge-Kutta method is used to solve with time steps size 0.06.

*Example 1. *Consider the five-level quantum system with internal Hamiltonian, the first control Hamiltonian [15], and the second control Hamiltonian given as follows:
Let the initial state and the target state be and , respectively. The parameters are chosen as , . Seting the state , by the control fields
we have the simulation result shown in Figure 1. It demonstrates the control performance with impulsive control field , and the final transition probability attains about 0.94149, which excels the one (about 0.93785) in [15].

*Example 2. *Consider the five-level quantum system with internal Hamiltonian and the control Hamiltonians given as follows:
Let the initial state and the target state also be and , respectively. The parameters are chosen as , . Seting the state , by the same control fields in Example 1, we have the simulation results shown in Figure 2. In Figure 2(a), the population of the system with impulsive control field is shown, and the result shown in Figure 2(b) demonstrates the control performance without impulsive control field. The quantum system whose Hamiltonians are (30) is driven to the target state , and the final transition probability attains about 0.99942 in Figure 2(a), which is better than the one (about 0.99581) in Figure 2(b), and significantly, the control method with one impulsive control field can prevent the evolution from decaying.

#### 5. Conclusion

In this paper, we have introduced the Lyapunov control method to quantum systems with impulsive control fields and given three kinds of control fields based on different Lyapunov functions. The theoretical results have been verified by numerical simulations to illustrate the effectiveness and advantages of the proposed method compared with existing results.

#### Acknowledgment

This work is supported by the National Natural Science Foundation of China under Grant 61174039 and the Leading Academic Discipline Project of Shanghai Dianji University under Grant no. 10XKJ01.

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