Entanglement transitions and quantum bifurcations under continuous long-range monitoring

Abstract

We study the asymptotic bipartite entanglement entropy of the quantum trajectories of a free-fermionic system, when subject to a continuous nonlocal monitoring. The measurements are described by Gaussian-preserving two-point operators, whose strength decays as a power law with exponent $\ensuremath{\alpha}$. Different behaviors of the entanglement entropy with the system size emerge: for $\ensuremath{\alpha}$ below a given threshold value a volume-law behavior sets in, while for larger $\ensuremath{\alpha}$ we observe a transition from subvolume to area law, whose exact location depends on the measurements rate and on the presence of a Hamiltonian dynamics. We also consider the expectation probability distribution of the measurement operators, and find that this distribution features a transition from a unimodal to a bimodal shape. We discuss the possible connections between this qualitative change of the distribution and the entanglement transition points.