  This paper presents a model reduction method for the class of linear quantum
stochastic systems often encountered in quantum optics and their related
fields. The approach is proposed on the basis of an interpolatory projection
ensuring that specific input-output responses of the original and the
reduced-order systems are matched at multiple selected points (or frequencies).
Importantly, the physical realizability property of the original quantum system
imposed by the law of quantum mechanics is preserved under our tangential
interpolatory projection. An error bound is established for the proposed model
reduction method and an avenue to select interpolation points is proposed. A
passivity preserving model reduction method is also presented. Examples of both
active and passive systems are provided to illustrate the merits of our
proposed approach.
