Title: Wiener Filtering for Passive Linear Quantum Systems,
Abstract: This paper considers a version of the Wiener filtering problem for
equalization of passive quantum linear quantum systems. We demonstrate that
taking into consideration the quantum nature of the signals involved leads to
features typically not encountered in classical equalization problems. Most
significantly, finding a mean-square optimal quantum equalizing filter amounts
to solving a nonconvex constrained optimization problem. We discuss two
approaches to solving this problem, both involving a relaxation of the
constraint. In both cases, unlike classical equalization, there is a threshold
on the variance of the noise below which an improvement of the mean-square
error cannot be guaranteed.