Exact asymptotics of long-range quantum correlations in a nonequilibrium steady state

TL;DR

This paper analytically derives the exact asymptotic behavior of quantum correlations, including mutual information and negativity, in a nonequilibrium steady state of free fermions with an impurity, revealing universal logarithmic corrections.

Contribution

It provides the first exact formulas for subleading logarithmic corrections to quantum correlation measures in a nonequilibrium steady state with an impurity, using a novel hybrid Toeplitz determinant approach.

Findings

Logarithmic corrections depend on subsystem sizes and impurity scattering probabilities.

The derived formulas match numerical calculations with high precision.

The method can be applied to other inhomogeneous quantum systems.

Abstract

Out-of-equilibrium states of many-body systems tend to evade a description by standard statistical mechanics, and their uniqueness is epitomized by the possibility of certain long-range correlations that cannot occur in equilibrium. In quantum many-body systems, coherent correlations of this sort may lead to the emergence of remarkable entanglement structures. In this work, we analytically study the asymptotic scaling of quantum correlation measures -- the mutual information and the fermionic negativity -- within the zero-temperature steady state of voltage-biased free fermions on a one-dimensional lattice containing a noninteracting impurity. Previously, we have shown that two subsystems on opposite sides of the impurity exhibit volume-law entanglement, which is independent of the absolute distances of the subsystems from the impurity. Here we go beyond that result and derive the exact form of the subleading logarithmic corrections to the extensive terms of correlation measures, in excellent agreement with numerical calculations. In particular, the logarithmic term of the mutual information asymptotics can be encapsulated in a concise formula, depending only on simple four-point ratios of subsystem length-scales and on the impurity scattering probabilities at the Fermi energies. This echoes the case of equilibrium states, where such logarithmic terms may convey universal information about the physical system. To compute these exact results, we devise a hybrid method that relies on Toeplitz determinant asymptotics for correlation matrices in both real space and momentum space, successfully circumventing the inhomogeneity of the system. This method can potentially find wider use for analytical calculations of entanglement measures in similar scenarios.