Hybrid completely positive Markovian quantum-classical dynamics

TL;DR

This paper presents a clear derivation of hybrid quantum-classical dynamics using Markovian master equations, clarifying its foundations, stochastic interpretations, and relation to quantum measurement theory.

Contribution

It provides a simplified, self-contained derivation of hybrid quantum-classical dynamics, revising and completing previous results, and clarifies its conceptual and technical aspects.

Findings

Hybrid dynamics is a special case of composite quantum dynamics with commuting observables.

Derivation of jump and diffusive hybrid master equations with stochastic interpretations.

Establishment of the equivalence between hybrid formalism and standard quantum measurement theory.

Abstract

A concise and self-contained derivation of hybrid quantum-classical dynamics is given in terms of Markovian master equations. Many previously known results are re-derived, revised, some of them completed or corrected. Using as simple method as possible, our goal is a brief introduction to state-of-the-art of hybrid dynamics, with a limited discussion of the implications for foundations. and without discussion of further relevance in quantum-gravity, or chemistry, numeric methods, etc. Hybrid dynamics is defined as special case of composite quantum dynamics where the observables of one of the two subsystems are restricted for the commuting set of diagonal operators in a fixed basis. With this restriction, the derivation of hybrid dynamical equations is clear conceptually and simple technically. Jump and diffusive dynamics follow in the form of hybrid master equations. Their stochastic interpretation (called unravellings) is derived. We discuss gauge-type ambiguities, problems of uniqueness, and covariance of the diffusive master equation. Also conditions of minimum noise and of monitoring the quantum trajectory are derived. We conclude that hybrid formalism is equivalent with standard Markovian theory of time-continuous quantum measurement (monitoring) on one hand, and is a motivating alternative formalism on the other hand.