The Rotating-Wave Approximation: Consistency and Applicability from an Open Quantum System Analysis

TL;DR

This paper critically examines the validity of the rotating-wave approximation (RWA) in open quantum systems, highlighting its limitations and the importance of proper timing in its application for accurate modeling.

Contribution

It provides a detailed analysis of when the RWA is valid in open quantum systems and emphasizes the need for careful application, especially in coupled systems, to avoid inaccuracies.

Findings

RWA after tracing reproduces system timescales but not quantum states

RWA before tracing leads to incorrect frequency shifts and non-Markovian behavior

Naive combination of RWA-applied systems yields inaccurate dynamics

Abstract

We provide an in-depth and thorough treatment of the validity of the rotating-wave approximation (RWA) in an open quantum system. We find that when it is introduced after tracing out the environment, all timescales of the open system are correctly reproduced, but the details of the quantum state may not be. The RWA made before the trace is more problematic: it results in incorrect values for environmentally-induced shifts to system frequencies, and the resulting theory has no Markovian limit. We point out that great care must be taken when coupling two open systems together under the RWA. Though the RWA can yield a master equation of Lindblad form similar to what one might get in the Markovian limit with white noise, the master equation for the two coupled systems is not a simple combination of the master equation for each system, as is possible in the Markovian limit. Such a naive combination yields inaccurate dynamics. To obtain the correct master equation for the composite system a proper consideration of the non-Markovian dynamics is required.