Decoherence of a quantum two-level system by spectral diffusion

TL;DR

This paper investigates the dephasing mechanisms of high-frequency two-level systems in quantum materials, showing that spectral diffusion explains Ramsey dephasing but not echo dephasing, which may involve fast, strongly coupled TLSs.

Contribution

It introduces a model that accounts for spectral diffusion in TLS dephasing and proposes the existence of fast, strongly coupled TLSs to explain experimental discrepancies.

Findings

Spectral diffusion explains Ramsey dephasing dependence on strain.

Standard tunneling model underestimates echo dephasing rates.

Fast TLSs may cause white noise-like dephasing, matching experimental observations.

Abstract

We study the dephasing of an individual high-frequency tunneling two-level system (TLS) due to its interaction with an ensemble of low-frequency thermal TLSs which are described by the standard tunneling model (STM). We show that the dephasing by the bath of TLSs explains both the dependence of the Ramsey dephasing rate on an externally applied strain as well as its order of magnitude, as observed in a recent experiment [J. Lisenfeld et al.]. However, the theory based on the STM predicts the Hahn-echo protocol to be much more efficient, yielding too low echo dephasing rates, as compared to the experiment. Also the strain dependence of the echo dephasing rate predicted by the STM does not agree with the measured quadratic dependence, which would fit to a high-frequency white noise environment. We suggest that few fast TLSs which are coupled much more strongly to the strain fields than the usual TLSs of the STM give rise to such a white noise. This explains the magnitude and strong fluctuations of the echo dephasing rate observed in the experiment.