Recovering dark states by non-Hermiticity

TL;DR

This paper demonstrates that non-Hermiticity can be used to restore and protect dark states in quantum systems, compensating for perturbations and enabling new quantum phases.

Contribution

It introduces a novel approach using non-Hermiticity to maintain dark states despite system perturbations, applicable to many-body systems and complex quantum phases.

Findings

Non-Hermiticity produces unidirectional couplings that preserve dark states.

Losses and gains can recover flat bands and edge states.

Potential for new quantum phases in non-Hermitian systems.

Abstract

Dark states, which are incapable of absorbing and emitting light, have been widely applied in multiple disciplines of physics. However, the existence of dark states relies on certain strict constraints on the system. For instance, in the fundamental {\Lambda} system, a perturbation breaking the degeneracy between two energy levels may destroy the destructive interference and demolish the dark state. Here, we show that non-Hermiticity can be exploited as a constructive means to restore a dark state. By compensating for the undesired perturbations, non-Hermiticity produces unidirectional couplings such that the dark state remains decoupled from the rest of the system. Implementing this scheme in many-body systems, flat bands and edge states can be recovered by losses and gains. Further taking into account interactions, a range of novel quantum phases could arise in such non-Hermitian systems.