Hamiltonian and double-bracket flow formulations of quantum measurements

TL;DR

This paper presents a unified framework connecting quantum measurement, Hamiltonian, and double-bracket flows, offering new insights into wavefunction collapse and feedback control in quantum systems.

Contribution

It introduces explicit stochastic Hamiltonians and double-bracket gradient flows to interpret quantum measurements and feedback processes, unifying different dynamical descriptions.

Findings

Wavefunction collapse can be viewed as coarse-grained Hamiltonian or gradient flow dynamics.

New feedback protocols for state preparation and ground state targeting.

Reinterpretation of measurement dynamics facilitates feedback control design.

Abstract

We introduce a framework that unifies quantum measurement dynamics, Hamiltonian dynamics, and double-bracket gradient flows. We do so by providing explicit expressions for stochastic Hamiltonians that produce state dynamics identical to those that happen during continuous quantum measurements. When such dynamical processes are integrated over sufficiently long time intervals, they yield the same results and statistics as during wavefunction collapse. That is, wavefunction collapse can be interpreted as coarse-grained (stochastic) Hamiltonian dynamics. Alternatively, wavefunction collapse can be interpreted as double-bracket gradient flows determined by derivatives of (stochastic) potentials defined in terms of observables with direct physical interpretations. The gradient flows minimize the variance of the monitored observable. Our derivations hold for general monitoring described by non-Hermitian jump processes. We show that such reinterpretations of measurement dynamics facilitate the design of feedback processes. In particular, we introduce feedback processes that yield deterministic double-bracket flow equations, which prepare ground states of a target Hamiltonian, and feedback processes for state preparation. We conclude by re-interpreting feedback processes as gradient flows with tilted fixed points.