Nonadiabatic elimination of auxiliary modes in continuous quantum measurements

In quantum measurement or control processes, there are often auxiliary modes coupling to the quantum system that we are interested in—they together form a bath or an environment for the system. The bath can have finite memory (non-Markovian), and simply ignoring its dynamics (i.e., adiabatically eliminating it) will prevent us from predicting the true quantum behavior of the system. We generalize the technique introduced by Strunz et al. [Phys. Rev. Lett. 82 1801 (1999)], and develop a formalism that allows us to eliminate the bath nonadiabatically in continuous quantum measurements, and obtain a non-Markovian stochastic master equation for the system that we focus on. This formalism also illuminates how to design the bath—acting as a quantum filter—to effectively probe interesting system observables (e.g., the quantum-nondemolition observable).