Entanglement based observables for quantum impurities

TL;DR

This paper introduces an entanglement-based observable, the impurity's purity, to characterize strong correlations in quantum impurity systems, demonstrated through exact solutions and a proposed experimental measurement protocol.

Contribution

It proposes using impurity purity as a novel entanglement witness for quantum impurities and demonstrates its effectiveness in the open Kondo box model.

Findings

Characterized the metal-insulator transition in the Kondo box model.

Showed how impurity entanglement relates to phase transitions.

Proposed an experimentally feasible purity measurement protocol.

Abstract

Quantum impurities exhibit fascinating many-body phenomena when the small interacting impurity changes the physics of a large noninteracting environment. The characterisation of such strongly correlated non-perturbative effects is particularly challenging due to the infinite size of the environment, and the inability of local correlators to capture the build-up of long-ranged entanglement in the system. Here, we harness an entanglement-based observable - the purity of the impurity - as a witness for the formation of strong correlations. We showcase the utility of our scheme by exactly solving the open Kondo box model in the small box limit, and thus describe all-electronic dot-cavity devices. Specifically, we conclusively characterise the metal-to-insulator phase transition in the system and identify how the (conducting) dot-lead Kondo singlet is quenched by an (insulating) intra-impurity singlet formation. Furthermore, we propose an experimentally feasible tomography protocol for the measurement of the purity, which motivates the observation of impurity physics through their entanglement build-up.