  In this paper we investigate system identification for general quantum linear
systems. We consider the situation where the input field is prepared as
stationary (squeezed) quantum noise. In this regime the output field is
characterised by the power spectrum, which encodes covariance of the output
state. We address the following two questions: (1) Which parameters can be
identified from the power spectrum? (2) How to construct a system realisation
from the power spectrum? The power spectrum depends on the system parameters
via the transfer function. We show that the transfer function can be uniquely
recovered from the power spectrum, so that equivalent systems are related by a
symplectic transformation.
