Entanglement and quantum transport in integrable systems

Understanding the entanglement structure of out-of-equilibrium many-body systems is a challenging yet revealing task. Here, we investigate the entanglement dynamics after a quench from a piecewise homogeneous initial state in integrable systems. This is the prototypical setup for studying quantum transport, and it consists in the sudden junction of two macroscopically different and homogeneous states. By exploiting the recently developed integrable hydrodynamic approach and the quasiparticle picture for the entanglement dynamics, we conjecture a formula for the entanglement production rate after joining two semi-infinite reservoirs, as well as for the steady-state entanglement entropy of a finite subregion. We show that both quantities are determined by the quasiparticles created in the nonequilibrium steady state (NESS) appearing at large times at the interface between the two reservoirs. Specifically, the steady-state entropy coincides with the thermodynamic entropy of the NESS, whereas the entropy production rate reflects its spreading into the bulk of the two reservoirs. Our results are numerically corroborated using time-dependent density matrix renormalization group (tDMRG) simulations in the paradigmatic XXZ spin-1/2 chain.