Unraveling superradiance: Entanglement and mutual information in collective decay

TL;DR

This paper investigates the collective decay of two-level emitters, revealing that superradiant bursts can be understood through classical models despite initial quantum entanglement, and introduces a classical theory for squeezed superradiance.

Contribution

It demonstrates that superradiant decay features are primarily classical, providing a new classical framework for understanding superradiance in various scenarios.

Findings

Superradiant burst is well described by factorizable states.

A classical theory accurately models squeezed superradiance.

Numerical methods perform well even with subradiant states.

Abstract

We study the collective decay of an initially inverted ensemble of two-level emitters in two distinct scenarios: when coupled to a squeezed photonic reservoir and when interacting with a one-dimensional waveguide. Using a quantum-state diffusion approach to unravel the emission process, we investigate entanglement and classical correlations along individual quantum trajectories over time. This numerical analysis shows that despite an initial build-up of entanglement and a significant amount of entanglement due to either spin squeezing or dark states at late times, the essential features of the superradiant burst are well described by averages over fully factorizable states. We explain this observation in terms of an almost complete factorization of all 2-local observables, which we identify as a generic property of superradiant decay. Based on this insight, we provide a purely classical theory for the burst in squeezed superradiance, which is both intuitive and exact for a large number of emitters. Moreover, we find that our numerical approach also performs well in the presence of subradiant states, which dominate the slow residual decay of non-uniform ensembles at late times.