Jinglei Zhang, Aarhus University
When a quantum system is monitored with a sequence of measurements, its evolution is given by a stochastic quantum trajectory. At any time the state, and therefore any prediction we can make about an observable, is dependent on previous measurement outcomes. Past quantum state, on the other hand, is a general theory that allows us to include the information collected about the system with later measurements. This allows for the retrodiction of observables and has been applied to various experimental systems, giving improvements in parameter estimation tasks.
In this talk I will apply past quantum state to continuous variable systems, and in particular focus on the case where they are continuously monitored and can be described by Gaussian states. Under the same conditions it is possible to develop a Gaussian formalism for the past quantum state that efficiently describes the system. I will discuss how the stochastic master equations that give the time evolution of the past quantum state can be translated into Gaussian formalism. Eventually, I will conclude by showing some examples of application of this new formalism to the retrodiction of quadratures, and show how hindsight can improve our ability to describe a system.