A Single Spin Switches the Steady-State Phase of an Open Quantum System

Abstract:Changing a many-body system by one constituent is normally expected to produce only a vanishing correction to an intensive observable. Here we show that, above a critical dissipation imbalance, adding one spin at fixed intensive controls switches the steady state of a dissipative collective spin between a phase-averaged ring and a south-polar fixed point. Unlike previously proposed one-spin sensing through dark-state interference, parity here determines whether the Dicke ladder samples a zero of the state-dependent nonlinear-loss amplitude. The integer-spin ladder samples this zero, making Dicke states with m<0 transient; the half-integer ladder misses it and remains a single recurrent class. Exact finite-size steady states show that exponential competition between stationary weights amplifies this microscopic connectivity difference, yielding a first-order dissipative phase transition confined to the odd-N sequence. One-spin loading, with all populated spin sectors retained, confirms that rare quantum fluctuations induce the switch without postselection. Our results establish jump-operator zeros as a route for single-constituent control of steady-state phases.

arXiv:2608.28390 [quant-ph]

(or arXiv:2608.28390v1 [quant-ph] for this version)

https://doi.org/10.48550/arXiv.2608.28390 Focus to learn more arXiv-issued DOI via DataCite (pending registration)

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