General criteria for quantum state smoothing with necessary and sufficient criteria for linear Gaussian quantum systems

Abstract

). However, this smoothing technique requires an observer (Alice, say) to know the nature of the measurement records that are unknown to her in order to characterize the possible true states for Bob’s (say) systems. If Alice makes an incorrect assumption about the set of true states for Bob’s system, she will obtain a smoothed state that is suboptimal, and, worse, may be unrealizable (not corresponding to a valid evolution for the true states) or even unphysical (not represented by a state matrix