  We investigate feedback control of linear quantum systems subject to
feedback-loop time delays. In particular, we examine the relation between the
potentially achievable control performance and the time delays, and provide
theoretical guidelines for the future experimental setup in two physical
systems, which are typical in this research field. The evaluation criterion for
the analysis is given by the optimal control performance formula, the
derivation of which is from the classical control theoretic results about the
input-output delay systems.
