Model reduction of linear quantum stochastic systems by subsystem truncation

The purpose of the paper is to develop new theoretical results on model reduction of linear quantum stochastic systems by truncation of subsystems of such systems. It is shown that a truncated system is guaranteed to be physically realizable (i.e., it represents the dynamics of a valid open Markov quantum system). In addition, it is also shown that the class of completely passive linear quantum stochastic systems is invariant under such subsystem truncation.