Quantum instanton approach to metastable collective spins

• URL: http://arxiv.org/abs/2604.15091v1
• Date: Thu, 16 Apr 2026 14:50:53 GMT
• Title: Quantum instanton approach to metastable collective spins
• Abstract summary: Collective spin systems play an important role in atomic and solid-state physics.<n>Their nonlinearity gives rise to multiple long-lived metastable states that relax to a unique most probable state.<n>We show that these features are not accurately described by the previously applied semiclassical Wigner approach.
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• License: http://creativecommons.org/licenses/by/4.0/
• 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.

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