Thermal-fluctuator driven decoherence of an oscillator resonantly coupled to a two-level system

TL;DR

This paper investigates how interactions between a resonantly coupled oscillator and two-level systems, including thermally activated fluctuators, cause decoherence and phase decay, with implications for quantum device performance.

Contribution

It provides a theoretical framework analyzing the effects of TLS and TLF interactions on oscillator coherence, including regimes of non-exponential decay and the impact of a few versus many TLFs.

Findings

Single TLF can cause coherence oscillations or suppression of Rabi oscillations.

Bath-driven TLF transitions lead to irreversible coherence decay sensitive to coupling and transition rates.

Different regimes of non-exponential decay are characterized for ensembles of TLFs.

Abstract

Recent experiments on a range of engineered quantum systems have highlighted the important role of interacting two-level systems (TLSs) in modifying device properties and generating fluctuations. Focusing on the case of an oscillator coupled to a single near-resonant TLS, we explore how interactions between the TLS and lower-frequency thermally activated two-level fluctuators (TLFs) degrade the oscillator's coherence. Depending on the strength of the couplings, a single TLF can give rise to coherence oscillations that appear alongside, or supplant, Rabi oscillations of the oscillator-TLS system. Bath-driven transitions in the TLF cause irreversible coherence decay at a rate that is highly sensitive to both the couplings and the transition rate. For an ensemble of TLFs, we identify and characterise the different regimes of non-exponential phase-averaging-driven coherence decay that the oscillator can display. Using numerical calculations, we examine the extent to which systems with just a few TLFs differ from the limit of a large (continuum) TLF ensemble. Our work provides a theoretical framework for understanding the interplay of coherent TLS interactions and TLF-induced dephasing in quantum devices such as superconducting and phononic resonators.