Robust Stability of Quantum Systems with a Nonlinear Coupling Operator
Abstract
This paper considers the problem of robust stability for a class of uncertain quantum systems subject to unknown perturbations in the system coupling operator. A general stability result is given for a class of perturbations to the system coupling operator. Then, the special case of a nominal linear quantum system is considered with nonlinear perturbations to the system coupling operator. In this case, a robust stability condition is given in terms of a scaled strict bounded real condition.
I Introduction
An important concept in modern control theory is the notion of robust or absolute stability for uncertain nonlinear systems in the form of a Lur’e system with an uncertain nonlinear block which satisfies a sector bound condition; e.g., see [1]. This enables a frequency domain condition for robust stability to be given. This characterization of robust stability enables robust feedback controller synthesis to be carried out using control theory; e.g., see [2]. In a recent paper [3], classical results on robust stability were extended to the case of nonlinear quantum systems with nonquadratic perturbations to the system Hamiltonian. The aim of this paper is to extend classical results on robust stability to the case of nonlinear quantum systems with nonlinear perturbations to the system coupling operator.
In recent years, a number of papers have considered the feedback control of systems whose dynamics are governed by the laws of quantum mechanics rather than classical mechanics; e.g., see [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. In particular, the papers [13, 17] consider a framework of quantum systems defined in terms of a triple where is a scattering matrix, is a vector of coupling operators and is a Hamiltonian operator. The paper [17] then introduces notions of dissipativity and stability for this class of quantum systems. In this paper, we build on the results of [17] to obtain robust stability results for uncertain quantum systems in which the quantum system coupling operator is decomposed as where is a known nominal coupling operator and is a perturbation coupling operator, which is contained in a specified set of coupling operators .
For this general class of uncertain quantum systems, a general stability result is obtained. The paper then considers the case in which the nominal quantum system is a linear quantum system in which the Hamiltonian is a quadratic function of annihilation and creation operators and the coupling operator is a linear function of annihilation and creation operators; e.g., see [7, 8, 10, 11, 16]. In this special case, a robust stability result is obtained in terms of a scaled frequency domain condition.
The remainder of the paper proceeds as follows. In Section II, we define the general class of uncertain quantum systems under consideration. In Section III, we consider a special class of nonlinear perturbation coupling operators. In Section IV, we specialize to the case of a linear nominal quantum systems and obtain a robust stability result for this case in which the stability condition is given in terms of a strict bounded real condition dependent on three scaling parameters. In Section V, we present some conclusions.
Ii Quantum Systems
We consider open quantum systems defined by parameters where ; e.g., see [13, 17]. The corresponding generator for this quantum system is given by
(1) 
where . Here, denotes the commutator between two operators and the notation denotes the adjoint of an operator. Also, is a selfadjoint operator on the underlying Hilbert space referred to as the system Hamiltonian. is the nominal system coupling operator and is referred to as the perturbation coupling operator. Also, is a unitary matrix referred to as the scattering matrix. Throughout this paper, we will assume that . The triple , along with the corresponding generator define the Heisenberg evolution of an operator according to a quantum stochastic differential equation
e.g., see [17]. Here, in the case of operators, the notation denotes the adjoint transpose of a vector or matrix of operators; see also [17] for a definition of the quantities , , and which will not be further considered in this paper. Also, in the case of standard matrices, the notation refers to the complex conjugate transpose of a matrix.
The problem under consideration involves establishing robust stability properties for an uncertain open quantum system for the case in which the perturbation coupling operator is contained in a given set . The main robust stability results presented in this paper will build on the following result from [17].
Lemma 1 (See Lemma 3.4 of [17].)
Consider an open quantum system defined by and suppose there exists a nonnegative selfadjoint operator on the underlying Hilbert space such that
(2) 
where and are real numbers. Then for any plant state, we have
Here denotes the Heisenberg evolution of the operator and denotes quantum expectation; e.g., see [17].
Iia Commutator Decomposition
Given a set of nonnegative selfadjoint operators and real parameters , , , , we now define a particular set of perturbation coupling operators . This set is defined in terms of the commutator decomposition
(3) 
for where , and are given scalar operators. We say if the following sector bound condition holds:
(4) 
and
(5) 
(6) 
Here, we use the convention that for operator inequalities, terms consisting of real constants are interpreted as that constant multiplying the identity operator.
Then, we define
(7) 
Using this definition, we obtain the following theorem.
Theorem 1
Consider a set of nonnegative selfadjoint operators and an open quantum system where and defined in (7). If there exists a and real constants , , such that is a constant and
then
for all .
Iii Nonlinear Perturbation Coupling Operators
In this section, we define a set of nonlinear perturbation coupling operators denoted . For a given set of nonnegative selfadjoint operators and real parameters , , , , consider perturbation coupling operators defined in terms of the following power series (which is assumed to converge in some suitable sense)
(16) 
Here is a scalar operator on the underlying Hilbert space and .
Also, we let
(17) 
(18) 
and consider the sector bound condition
(19) 
and the conditions
(20) 
(21) 
Then we define the set as follows:
(22) 
In this section, the set of nonnegative selfadjoint operators will be assumed to satisfy the following assumption:
Assumption 1
Given any , the quantity
is a constant.
Lemma 2
Suppose the set of selfadjoint operators satisfies Assumption 1. Then
Iv The Case of a Linear Nominal System
We now consider the case in which the nominal quantum system corresponds to a linear quantum system; e.g., see [7, 8, 10, 11, 16]. In this case, we assume that is of the form
(28) 
where is a Hermitian matrix of the form
and , . Here is a vector of annihilation operators on the underlying Hilbert space and is the corresponding vector of creation operators. In the case vectors of operators, the notation refers to the vector of adjoint operators and in the case of complex matrices, this notation refers to the complex conjugate matrix.
The annihilation and creation operators are assumed to satisfy the canonical commutation relations:
(29)  
We also assume is of the form
(30) 
where and . Also, we write
In addition we assume that is of the form
(31) 
where is a positivedefinite Hermitian matrix of the form
(32) 
Hence, we consider the set of nonnegative selfadjoint operators defined as
(33) 
In the linear case, we will consider a specific notion of robust mean square stability.
Definition 1
In order to address the issue of robust mean square stability for the uncertain linear quantum systems under consideration, we first require some algebraic identities.
Proof: The proof of these identities follows via straightforward but tedious calculations using (29).
We now specialize the results of Section II to the case of a linear nominal system with where is defined as in Section III. In this case, we define
(35)  
where is assumed to be a scalar operator. Then, we show that a sufficient condition for robust mean square stability is the existence of constants , , and such that the following scaled strict bounded real condition is satisfied:

The matrix
(36) 
(37) where
(38) and
(39)
This leads to the following theorem.
Theorem 2
Consider an uncertain open quantum system defined by such that where is of the form (30), is of the form (28) and . Furthermore, assume that there exists constants , , and such that the strict bounded real condition (36), (37) is satisfied. Then the uncertain quantum system is robustly mean square stable.
In order to prove this theorem, we require the following lemma.
Lemma 4
Proof: The proof of this result follows via a straightforward but tedious calculation using (29).
Proof of Theorem 2. If the conditions of the theorem are satisfied, then (37) implies
Hence, it follows from the strict bounded real lemma that the matrix inequality
will have a solution of the form (32); e.g., see [2, 11]. This matrix defines a corresponding operator as in (31). Then using (38) and (39), it follows that we can write.
Hence, we can choose and sufficiently small so that
(40)  