Alberto J. B. Rosal, Patrick P. Potts, Gabriel T. Landi

Feedback control in open quantum dynamics is crucial for the advancement of various coherent platforms. For a proper theoretical description, however, most feedback schemes rely on stochastic trajectories, which require significant statistical sampling and lack analytical insight. Currently, only a handful of deterministic feedback master equations exist in the literature. In this letter we derive a set of deterministic equations for describing feedback schemes based on generic causal signals. Our formulation is phrased in terms of sequentially applied quantum instruments, and is therefore extremely general, recovering various known results in the literature as particular cases. We then specialize this result to the case of quantum jumps and derive a new deterministic equation that allows for feedback based on the channel of the last jump, as well as the time since the last jump occurred. The strength of this formalism is illustrated with a detailed study of population inversion of a qubit using coherent drive and feedback, where our formalism allows all calculations to be performed analytically.