Lindbladian versus Postselected Non-Hermitian Topology

TL;DR

This paper investigates the robustness of non-Hermitian topological phenomena, like the skin effect, in open quantum systems described by Lindbladians, without relying on postselection, revealing new phase transitions and effects.

Contribution

It establishes a relationship between Lindbladian and non-Hermitian Hamiltonian topological invariants, showing how loss and gain influence the skin effect and phase transitions without postselection.

Findings

Lindbladian winding number equals the non-Hermitian one without gain or loss.

Presence of both loss and gain can change the sign of the Lindbladian winding number.

Removing postselection can induce a skin effect in trivial non-Hermitian systems.

Abstract

The recent topological classification of non-Hermitian `Hamiltonians' is usually interpreted in terms of pure quantum states that decay or grow with time. However, many-body systems with loss and gain are typically better described by mixed-state open quantum dynamics, which only correspond to pure-state non-Hermitian dynamics upon a postselection of measurement outcomes. Since postselection becomes exponentially costly with particle number, we here investigate to what extent the most important example of non-Hermitian topology can survive without it: the non-Hermitian skin effect and its relationship to a bulk winding number in one spatial dimension. After defining the winding number of the Lindbladian superoperator for a quadratic fermion system, we systematically relate it to the winding number of the associated postselected non-Hermitian Hamiltonian. We prove that the two winding numbers are equal (opposite) in the absence of gain (loss), and provide a physical explanation for this relationship. When both loss and gain are present, the Lindbladian winding number typically remains quantized and non-zero, though it can change sign at a phase transition separating the loss and gain-dominated regimes. This transition, which leads to a reversal of the Lindbladian skin effect localization, is rendered invisible by postselection. We also identify a case where removing postselection induces a skin effect from otherwise topologically trivial non-Hermitian dynamics.