arXiv:2408.15928 · Nature Communications

Observing time-dependent energy level renormalisation in an ultrastrongly coupled open system

Alessandra Colla, Florian Hasse, Deviprasath Palani, Tobias Schaetz, Heinz‐Peter Breuer, Ulrich Warring

Published Mar 13, 2025• 10 citations

Abstract

Understanding how strong coupling and memory effects influence energy levels in open quantum systems is a fundamental challenge. Here, we experimentally probe these effects in a two-level open system coupled to a single-mode quantum environment, using Ramsey interferometry in a trapped ion. Operating in the strong coupling regime, we observe both dissipative effects and time-dependent energy shifts of up to 15% of the bare system frequency, with the total system effectively isolated from external environments. These dynamic shifts, likely ubiquitous across quantum platforms, arise solely from ultra-strong system-mode interactions and correlation build-up and are accurately predicted by the minimal-dissipation Ansatz. Our approach identifies these as generalised Lamb shifts, matching conventional predictions on time-average. We provide experimental fingerprints supporting the Ansatz of minimal-dissipation, thereby suggesting it as a testable quantum thermodynamics framework and establishing a foundation for future benchmarks in strong-coupling quantum thermodynamics and related technologies.

DOI: 10.1038/s41467-025-57840-4

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