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arXiv · 2606.15062

Discontinuous strong-to-weak symmetry breaking transition from thermal pure states

Taiki Haga·Masaya Kunimi·Masaya Nakagawa
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Abstract

We investigate the nonequilibrium dynamics of strong-to-weak spontaneous symmetry breaking in many-body quantum systems undergoing decoherence from thermal pure states. For generic initial pure states with volume-law entanglement entropy, we show that the system undergoes a discontinuous dynamical phase transition at a critical time. This transition is accompanied by a singularity in the entropy of the system, which saturates to its maximum value at the same critical time. Through numerical simulations of the dephasing Ising and hard-core boson models, we establish the universality of this transition across different symmetries. Our results reveal that the dynamical emergence of a decohered mixed state from a highly entangled state is not a gradual asymptotic relaxation, but rather a sharp phase transition driven by a sudden collapse of global coherence.

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Taiki Haga, Masaya Kunimi, Masaya Nakagawa. 2026-06-13. Discontinuous strong-to-weak symmetry breaking transition from thermal pure states. https://arxiv.org/abs/2606.15062

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