Universal Entanglement Transitions of Free Fermions with Long-range Non-unitary Dynamics

Quantum 6, 723 (2022).

https://doi.org/10.22331/q-2022-05-27-723

Non-unitary evolution can give rise to novel steady states classified by their entanglement properties. In this work, we aim to understand the effect of long-range hopping that decays with $r^{-alpha}$ in non-Hermitian free-fermion systems. We first study two solvable Brownian models with long-range non-unitary dynamics: a large-$N$ SYK$_2$ chain and a single-flavor fermion chain and we show that they share the same phase diagram. When $alpha>0.5$, we observe two critical phases with subvolume entanglement scaling: (i) $alpha>1.5$, a logarithmic phase with dynamical exponent $z=1$ and logarithmic subsystem entanglement, and (ii) $0.5 < alpha < 1.5$, a fractal phase with $z=frac{2alpha-1}{2}$ and subsystem entanglement $S_Apropto L_A^{1-z}$, where $L_A$ is the length of the subsystem $A$. These two phases cannot be distinguished by the purification dynamics, in which the entropy always decays as $L/T$. We then confirm that the results are also valid for the static SYK$_2$ chain, indicating the phase diagram is universal for general free-fermion systems. We also discuss phase diagrams in higher dimensions and the implication in measurement-induced phase transitions.