Markovian master equations for quantum-classical hybrid systems

TL;DR

This paper develops a framework for modeling quantum-classical hybrid systems using Markovian dynamics, introducing hybrid dynamical semigroups and analyzing their structure, including quasi-free generators and information flow.

Contribution

It formalizes the construction of quantum-classical hybrid dynamics via Markovian semigroups, connecting to quantum measurement theory and analyzing quasi-free generators with interaction terms.

Findings

Hybrid dynamics can be modeled with positive operator valued measures.

Interactions require dissipative terms for quantum-to-classical information flow.

Quasi-free semigroups include both Gaussian and jump-type processes.

Abstract

The problem of constructing a consistent quantum-classical hybrid dynamics is afforded in the case of a quantum component in a separable Hilbert space and a continuous, finite-dimensional classical component. In the Markovian case, the problem is formalized by the notion of hybrid dynamical semigroup. A classical component can be observed without perturbing the system and information on the quantum component can be extracted, thanks to the quantum-classical interaction. This point is formalized by showing how to introduce positive operator valued measures and operations compatible with the hybrid dynamical semigroup; in this way the notion of hybrid dynamics is connected to quantum measurements in continuous time. Then, the case of the most general quasi-free generator is presented and the various quantum-classical interaction terms are discussed. To bee quasi-free means to send, in the Heisenberg description, hybrid Weyl operators into multiples of Weyl operators; the results on the structure of quasi-free semigroups were proved in the article arXiv:2307.02611. Even in the pure quantum case, a quasi-free semigroup is not restricted to have only a Gaussian structure, but also jump-type terms are allowed. An important result is that, to have interactions producing a flow of information from the quantum component to the classical one, suitable dissipative terms must be present in the generator. Finally, some possibilities are discussed to go beyond the quasi-free case.