Beyond the Quantum Regression Theorem in Variational Polaron Master Equations with Low-Dimensional Baths

Quantum Physics [Submitted on 15 Apr 2026] Beyond the Quantum Regression Theorem in Variational Polaron Master Equations with Low-Dimensional Baths Matias Bundgaard-Nielsen, Jake Iles-Smith While the quantum regression theorem (QRT) is the standard tool for computing multi-time correlation functions in open quantum systems, it relies on system-bath separability and an environment that remains in equilibrium, assumptions that are violated once dynamical correlations develop. Using the projection operator formalism, we derive an extension to the QRT that explicitly incorporates these correlation-induced corrections. We apply this framework to the variational polaron master equation for the spin-boson model in ohmic and super-ohmic regimes, where the polaron transformation mixes system-bath degrees of freedom to produce a non-thermal effective environment. Benchmarking against numerically exact tensor-network simulations demonstrates quantitative agreement for single- and two-time observables, including linear-response spectra, even at strong coupling. Our approach broadens the reach of analytic master equations to strong-coupling regimes, enabling treatment of multi-time observables where environmental memory effects and system-bath correlations are crucial. Comments: 21 Pages, 6 Figures Subjects: Quantum Physics (quant-ph) Cite as: arXiv:2604.13541 [quant-ph]   (or arXiv:2604.13541v1 [quant-ph] for this version)   https://doi.org/10.48550/arXiv.2604.13541 Focus to learn more Submission history From: Matias Bundgaard-Nielsen [view email] [v1] Wed, 15 Apr 2026 06:44:37 UTC (223 KB) Access Paper: HTML (experimental) view license Current browse context: quant-ph < prev   |   next > new | recent | 2026-04 References & Citations INSPIRE HEP NASA ADS Google Scholar Semantic Scholar Export BibTeX Citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer (What is the Explorer?) Connected Papers Toggle Connected Papers (What is Connected Papers?) Litmaps Toggle Litmaps (What is Litmaps?) scite.ai Toggle scite Smart Citations (What are Smart Citations?) Code, Data, Media Demos Related Papers About arXivLabs Which authors of this paper are endorsers? | Disable MathJax (What is MathJax?)