Open quantum systems with non-commuting coupling operators: An analytic approach

TL;DR

This paper introduces an analytic method for open quantum systems with strong, noncommuting couplings to multiple environments, revealing cooperative effects and enabling the engineering of quantum interactions.

Contribution

It generalizes the reaction-coordinate polaron transform to handle noncommuting couplings, providing a theoretical foundation for phenomena observed in simulations.

Findings

Suppression of relaxation by decoherence in a spin impurity.

Prediction of cooperative effects of multiple baths.

Engineering of Kitaev XY spin chain interactions.

Abstract

We present an analytic approach to treat open quantum systems strongly coupled to multiple environments via noncommuting system operators, a prime example is a qubit concurrently coupled to both decoherring and dissipative baths. Our approach, which accommodates strong system-bath couplings, generalizes the recently developed reaction-coordinate polaron transform method [PRX Quantum ${\bf 4}$, 020307 (2023)] to handle couplings to baths via noncommuting system operators. This approach creates an effective Hamiltonian that reveals the cooperative effect of the baths on the system. For a spin impurity coupled to both dissipative and decoherring environments, the effective Hamiltonian predicts the suppression of relaxation by decoherence -- a phenomenon previously observed in simulations but lacking a theoretical foundation. We also apply the method to an ensemble of spins coupled to local baths through noncommuting operators, demonstrating the engineering of the Kitaev XY spin chain interaction. Noncommutativity is a feature of quantum systems; future prospects of our approach include the study of thermal machines that leverage such genuine quantum effects.