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arXiv:2111.03500 · Physical review. B./Physical review. B

Entanglement transitions from stochastic resetting of non-Hermitian quasiparticles

Xhek Turkeshi, Marcello Dalmonte, Rosario Fazio, Marco Schirò

Published Jun 28, 2022• 147 citations
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Abstract

Entanglement is a key tool to classify quantum phases of matter in and out of equilibrium. Here, the authors introduce a phenomenological quasiparticle picture for entanglement dynamics in monitored systems evolving under the effect of quantum jumps. Within this picture, entanglement is carried by non-Hermitian quasiparticles, associated to the no-click limit of the measurement protocol, that propagate ballistically and are randomly reset with a rate given by their inverse lifetime. The authors show how this picture qualitatively reproduces the entanglement transition in the monitored Ising chain.

DOI
10.1103/physrevb.105.l241114

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Entanglement is a key tool to classify quantum phases of matter in and out of equilibrium. Here, the authors introduce a phenomenological quasiparticle picture for entanglement dynamics in monitored systems evolving under the effect of quantum jumps. Within this picture, entanglement is carried by non-Hermitian quasiparticles, associated to the no-click limit of the measurement protocol, that propagate ballistically and are randomly reset with a rate given by their inverse lifetime. The authors show how this picture qualitatively reproduces the entanglement transition in the monitored Ising chain.
Monitored quantum many-body systems display a rich pattern of entanglement dynamics, which is unique to this nonunitary setting. This work studies the effect of quantum jumps on the entanglement dynamics beyond the no-click limit corresponding to a deterministic non-Hermitian evolution. To this aim, we introduce a new tool that looks at the statistics of entanglement-entropy gain and loss after and in between quantum jumps. This insight allows us to build a simple stochastic model of a random walk with partial resetting, which reproduces the entanglement dynamics, and to dissect the mutual role of jumps and non-Hermitian evolution on the entanglement scaling. We apply these ideas to the study of measurement-induced transitions in monitored fermions. We demonstrate that significant deviations from the no-click limit arise whenever quantum jumps strongly renormalize the non-Hermitian dynamics, as in the case of models with U ( 1 ) symmetry at weak monitoring. On the other hand, we show that the weak-monitoring phase of the Ising chain leads to a robust subvolume logarithmic phase due to weakly renormalized non-Hermitian dynamics. Published by the American Physical Society 2024
Recent years have witnessed major progress in the understanding of monitored quantum dynamics, an open quantum setting where the environment is a measurement apparatus that probes the system locally. For the resulting quantum trajectory, unitary evolution and local measurements have competing tendencies in the building of quantum correlations: while the coherent interactions in the former increase the entanglement, the resolving nature of the local measurement collapses the state of the system. Here, the authors study the interplay of monitoring and system dynamics by focusing on the monitored quantum Ising chain as the measurement strength is varied, going from the regime of weak detection, corresponding to the limit of infinite small jumps, to the case of post-selected no-jump case. A sharp transition is found between a logarithmic law phase to an area law phase for the entanglement entropy, which occurs at the same value of the measurement rate in the two limiting situations.
We study dynamical phase transitions occurring in the stationary state of the dynamics of integrable many-body non-hermitian Hamiltonians, which can be either realized as a no-click limit of a stochastic Schrödinger equation or using spacetime duality of quantum circuits. In two specific models, the Transverse Field Ising Chain and the Long Range Kitaev Chain, we observe that the entanglement phase transitions occurring in the stationary state have the same nature as that occurring in the vacuum of the non-hermitian Hamiltonian: bounded entanglement entropy when the imaginary part of the quasi-particle spectrum is gapped and a logarithmic growth for gapless imaginary spectrum. This observation suggests the possibility to generalize the area-law theorem to non-Hermitian Hamiltonians.
Recently, there has been interest in the dynamics of monitored quantum systems using linear jump operators related to the creation or annihilation of particles. Here, we study the dynamics of the entanglement entropy under quantum jumps that induce local particle losses in a model of free fermions with hopping and Z2{\mathbb{Z}}_{2}Z2​ pairing. We solve the nonunitary dynamics using the recently developed Faber polynomial method and explore the different steady-state entanglement regimes by tuning the pairing strength, thus interpolating between monitored free fermions coherently driven by a particle-number-conserving Hamiltonian and a parity-conserving one. In the absence of pairing, all quantum trajectories approach the vacuum at long times, with the entanglement entropy showing nonmonotonic behavior over time that we capture with a phenomenological quasiparticle ansatz. In this regime, quantum jumps play a key role, and we highlight this by exactly computing their waiting-time distribution. On the other hand, the interplay between losses and pairing gives rise to quantum trajectories with entangled steady states. We show that by tuning the system parameters, a measurement-induced entanglement transition occurs, where the entanglement entropy scaling changes from logarithmic to area law. We compare this transition with the one derived in the no-click limit and observe qualitative agreement in most of the phase diagram. Furthermore, the statistics of entanglement gain and loss are analyzed to better understand the impact of the linear jump operators.
We numerically investigate the robustness against various perturbations of measurement-induced phase transition in monitored quantum Ising models in the no-click limit, where the dynamics is described by a non-Hermitian Hamiltonian. We study perturbations that break the integrability and/or the symmetry of the model, as well as modifications in the measurement protocol, characterizing the resulting chaos and lack of integrability through the dissipative spectral form factor. We show that while the measurement-induced phase transition and its properties appear to be broadly insensitive to lack of integrability and breaking of the Z2{\mathbb{Z}}_{2}Z2​ symmetry, a modification of the measurement basis from the transverse to the longitudinal direction makes the phase transition disappear altogether.
We investigate out-of-equilibrium entanglement dynamics in a generalization of the so-called Q S S E P model, which is a free-fermion chain with stochastic in space and time hopping amplitudes. In our setup, the noisy amplitudes are spatially modulated satisfying a ν -site translation invariance but retaining their randomness in time. For each noise realization, the dynamics preserves Gaussianity, which allows to obtain noise-averaged entanglement-related quantities. The statistics of the steady-state correlators satisfy nontrivial relationships that are of topological nature. They reflect the Haar invariance under multiplication with structured momentum-dependent random S U ( ν ) matrices. We discuss in detail the case with ν = 1 and ν = 2 . For ν = 1 , i.e., spatially homogeneous noise, we show that the entanglement dynamics is describable by a stochastic generalization of the quasiparticle picture. Precisely, entanglement is propagated by pairs of quasiparticles. The entanglement content of the pairs is the same as for the deterministic chain. However, the trajectories of the quasiparticles are random walks, giving rise to diffusive entanglement growth.