Thomas Martin Müller

arXiv2026

Quantum Mpemba effect in chaotic systems with conservation laws

Closed chaotic quantum systems relax after a quench into a Gibbs ensemble. At late times, the relaxation speed is determined by their conservation laws and hydrodynamics. As a result, there exist pairs of initial states which thermalize to the same ensemble, yet exhibit drastically different hydrodynamic relaxation. We show in two chaotic spin chains how this enables a simple and robust realization of the quantum Mpemba effect: a system initially closer to equilibrium relaxes slower than one that starts farther away, despite both approaching the same final state.

quant-ph↗

arXiv2025

Monitored interacting Dirac fermions

We analytically study interacting Dirac fermions, described by the Thirring model, under weak local particle number measurements with monitoring rate $γ$. This system maps to a bosonic replica field theory, analyzed via the renormalization group. For a nonzero attractive interaction, a phase transition occurs at a critical measurement strength $γ_c$. When $γ>γ_c$, the system enters a localized phase characterized by exponentially decaying density-density correlations beyond a finite correlation length; for $γ<γ_c$, the correlations decay algebraically. The transition is of BKT-type, reflected by a characteristic scaling of the correlation length. In the non-interacting limit, $γ_c\to0$ shifts to zero, reducing the algebraic phase to a single point in parameter space. This identifies weak measurements in the free case as an implicit double fine-tuning to the critical endpoint of the BKT phase transition. Along the non-interacting line, we compute the entanglement entropy from density-density correlation functions and find no entanglement transition at nonzero measurement strength in the thermodynamic limit.

cond-mat.stat-mech↗