# Collectively induced exceptional points of quantum emitters coupled to nanoparticle surface plasmons

## TL;DR

This paper theoretically demonstrates the existence of exceptional points in quantum emitters coupled to metal nanoparticles, showing their potential for enhanced quantum sensing through collective effects and spectral analysis.

## Contribution

It introduces a new quantum system exhibiting exceptional points via strong coupling and collective effects, advancing the understanding of non-Hermitian degeneracies in plasmonic systems.

## Key findings

- Exceptional points arise from strong coupling effects.
- Collective coupling relaxes the strong-coupling requirement.
- Power spectra reveal exceptional points clearly.

## Abstract

Exceptional points, resulting from non-Hermitian degeneracies, have the potential to enhance the capabilities of quantum sensing. Thus, finding exceptional points in different quantum systems is vital for developing such future sensing devices. Taking advantage of the enhanced light-matter interactions in a confined volume on a metal nanoparticle surface, here we theoretically demonstrate the existence of exceptional points in a system consisting of quantum emitters coupled to a metal nanoparticle of subwavelength scale. By using an analytical quantum electrodynamics approach, exceptional points are manifested as a result of a strong coupling effect and observable in a drastic splitting of originally coalescent eigenenergies. Furthermore, we show that exceptional points can also occur when a number of quantum emitters is collectively coupled to the dipole mode of localized surface plasmons. Such a quantum collective effect not only relaxes the strong-coupling requirement for an individual emitter, but also results in a more stable generation of the exceptional points. Furthermore, we point out that the exceptional points can be explicitly revealed in the power spectra. A generalized signal-to-noise ratio, accounting for both the frequency splitting in the power spectrum and the system's dissipation, shows clearly that a collection of quantum emitters coupled to a nanoparticle provides a better performance of detecting exceptional points, compared to that of a single quantum emitter.