Time delay is frequently encountered in practical quantum feedback control systems with long transmission lines and measurement process. This paper is concerned with measurement-based feedback H 1 control for quantum systems with time delays appearing in the feedback loops. A physical model is presented for the quantum time-delay system described by complex quantum stochastic differential equations. Quantum versions of some fundamental properties, such as dissipativity and stability, are discussed for this model. A numerical procedure is proposed for H 1 controller synthesis, which can deal with a non-convex optimization problem arising in the design processes.ROBUST H 1 CONTROLLER DESIGN FOR LINEAR QUANTUM SYSTEMS WITH TIME DELAY 381 controller synthesis for a class of linear quantum stochastic systems has been formulated and solved. Reference [24] presented an experimental realization of a coherent quantum feedback control system using H 1 control theory. In Reference [25], an H 1 control problem has been considered for a class of linear quantum systems with time delays described by position and momentum quadratures for the quantum system variables.In this paper, H 1 control of a class of quantum time-delay systems is investigated, where the systems are described by complex quantum stochastic differential equations (QSDEs) in terms of annihilation and creation operators. The representation of complex QSDEs is a natural description for a class of widely used quantum optical systems [12,26,27]. In this paper, we call the quantum system described by complex QSDEs a complex quantum system [12,26]. The system under consideration is a mixed quantum-classical system in the sense that the plant is quantum while the controller is classical. Relations between the complex QSDEs under consideration and the corresponding real QSDEs defined in terms of quadrature variables are presented. The performance properties of the proposed model such as stability and dissipativity are also derived and characterized in complex algebraic terms, which leads to a complex quantum version of the bounded real lemma. It is found that controller synthesis suffers from several limitations because of the nonlinear and non-convex constraints and many decision variables involved in design procedures. To conquer these limitations, a numerical procedure is developed for H 1 controller design.The rest of the paper is organized as follows. Section 2 gives a brief overview of quantum systems with real quadrature representation and annihilation-creation operator representation, respectively, and presents a connection between them. Section 3 presents the setup of the closed loop system with time delay. Section 4 investigates performance characteristics such as dissipativity and stability for the model given in Section 3. Section 5 presents the H 1 controller synthesis for the proposed model, which is verified by a numerical example. Section 6 concludes this paper.