Quantum instanton approach to metastable collective spins

TL;DR

This paper introduces a quantum instanton method to analyze metastable states and phase transitions in collective spin systems, highlighting the importance of non-Gaussian fluctuations for accurate modeling.

Contribution

The authors develop a real-time instanton approach based on quantum quasiprobability dynamics for large-spin systems, improving upon semiclassical methods.

Findings

The approach accurately captures stationary states and relaxation rates.

Non-Gaussian fluctuations are crucial for correct description of metastable states.

Semiclassical Wigner approach fails to account for key quantum effects.

Abstract

Collective spin systems -- spin ensembles coupled to a common reservoir and effectively described by a single macrospin -- play an important role in both atomic and solid-state physics. Their intrinsic nonlinearity gives rise to multiple long-lived metastable states that ultimately relax to a unique most probable state. This dominant state can change with a control parameter, leading to first-order phase transitions. We develop a real-time instanton approach based on quantum quasiprobability dynamics that captures the stationary state in the large-spin limit and the asymptotic scaling of relaxation rates. We further show that these features are not accurately described by the previously applied semiclassical Wigner approach due to its neglect of non-Gaussian fluctuations.