Renormalized Lindblad Driving: A Numerically-Exact Nonequilibrium Quantum Impurity Solver

Abstract

The accurate characterization of nonequilibrium strongly-correlated quantum systems has been a longstanding challenge in many-body physics. Notable among them are quantum impurity models, which appear in various nanoelectronic and quantum computing applications. Despite their seeming simplicity, they feature correlated phenomena, including emergent energy scales and non-Fermi-liquid physics, requiring renormalization group treatment. This has typically been at odds with the description of their nonequilibrium steady-state under finite bias, which exposes their nature as open quantum systems. We present a novel numerically-exact method for obtaining the nonequilibrium state of a general quantum impurity coupled to metallic leads at arbitrary voltage or temperature bias, which we call "RL-NESS" (Renormalized Lindblad-driven NonEquilibrium Steady-State). It is based on coherently coupling the impurity to discretized leads which are treated exactly. These leads are furthermore weakly coupled to reservoirs described by Lindblad dynamics which impose voltage or temperature bias. Going beyond previous attempts, we exploit a hybrid discretization scheme for the leads together with Wilson's numerical renormalization group, in order to probe exponentially small energy scales. The steady-state is then found by evolving a matrix-product density operator via real-time Lindblad dynamics, employing a dissipative generalization of the time-dependent density matrix renormalization group. In the long-time limit, this procedure converges to the steady-state at finite bond dimension due to the introduced dissipation, which bounds the growth of entanglement. We test the method against the exact solution of the noninteracting resonant level model. We demonstrate its power using an interacting two-level model, for which it correctly reproduces the known limits, and gives the full $I$-$V$ curve between them.