  This paper gives an overview of parameter estimation and system
identification for quantum input-output systems by continuous observation of
the output field. We present recent results on the quantum Fisher information
of the output with respect to unknown dynamical parameters. We discuss the
structure of continuous-time measurements as solutions of the quantum Zakai
equation, and their relationship to parameter estimation methods. Proceeding
beyond parameter estimation, the paper also gives an overview of the emerging
topic of quantum system identification for black-box modeling of quantum
systems by continuous observation of a traveling wave probe, for the case of
ergodic quantum input-output systems and linear quantum systems. Empirical
methods for such black-box modeling are also discussed.
