Kondo breakdown as an entanglement transition driven by continuous measurement

TL;DR

This paper investigates how continuous measurement via a local magnetic field causes a transition in the entanglement structure of a Kondo system, leading to a breakdown of screening and emergence of a non-Fermi liquid phase.

Contribution

It introduces a non-perturbative RG approach to analyze measurement-driven entanglement transitions in Kondo systems, revealing a phase diagram and critical behavior.

Findings

Identification of a measurement-driven entanglement transition separating Kondo-screened and local-moment phases.

Derivation of a field-dependent RG phase diagram with fixed points for each phase.

Characterization of the transition through impurity spectral functions and thermalization signatures.

Abstract

We study the breakdown of Kondo screening by a local magnetic field from the perspective of a measurement-driven entanglement transition in a monitored quantum system. Here, the Kondo coupling leads to the growth in entanglement of an impurity spin with it's fermionic environment, while the local field plays the role of a continuous observer. Using a non-perturbative Unitary Renormalization Group (URG) approach, we derive coupled renormalization-group flow equations for the Kondo exchange and the local field, and obtain a field-dependent RG phase diagram. The RG flows separate a low-energy Kondo-screened phase, where the impurity is absorbed into the Fermi sea and forms an entangled singlet with the conduction bath, from a polarized local-moment phase in which screening is frustrated and impurity-bath entanglement is suppressed. We identify the fixed-point Hamiltonians governing the two phases and the critical regime, and relate the transition to the emergence of a novel non-Fermi liquid. Various impurity signatures such as the spectral function and thermalisation of impurity observables are used to characterise this entanglement transition. These results offer insight into the interplay of decoherence and measurement in governing the dynamics of a prototypical quantum system.