Rare measurements on a 1D spinful s-wave BCS chain dynamically project soft modes onto an SO(R) NLSM whose R→1 weak-anti-localization flow yields steady-state entanglement S(L) ~ ln² L without a WZW term.
Measurement-enhanced entanglement in a monitored superconducting chain
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abstract
A common view in monitored quantum dynamics is that local measurements suppress entanglement growth. We show that this intuition can fail in a one-dimensional spinful fermionic chain governed by a BCS Hamiltonian with pairing strength $\Delta$ and subject to continuous, on-site, spin-resolved charge measurements at rate $\gamma$. Using free-fermion simulations and quasiparticle analysis, we show that pairing suppresses entanglement growth, while measurements suppress pairing. Their competition yields measurement-enhanced entanglement: for $\Delta>0$, the steady-state entanglement $S_s$ increases with $\gamma$ over a finite interval $0<\gamma<\gamma_{\rm peak}$. This occurs because stronger measurements suppress pairing correlations, which would otherwise suppress entanglement growth. Using a nonlinear sigma-model calculation and free-fermion simulations, we provide evidence that for $\Delta>0$ and small but finite $\gamma$, the steady-state entanglement scales as $S_s\sim \ln^2 L$. This implies that, in this setting, measurement-enhanced entanglement does not persist in the thermodynamic limit.
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2026 1verdicts
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Super-Logarithmic Entanglement Scaling in a Monitored Superconducting Chain
Rare measurements on a 1D spinful s-wave BCS chain dynamically project soft modes onto an SO(R) NLSM whose R→1 weak-anti-localization flow yields steady-state entanglement S(L) ~ ln² L without a WZW term.