Parity-time symmetry and coherent perfect absorption in a cooperative atom response

TL;DR

This paper demonstrates how a single-layer atomic array can exhibit effective parity-time symmetry and coherent perfect absorption through cooperative interactions, without requiring gain, by balancing scattering and loss in collective modes.

Contribution

It introduces a novel passive system where effective $ ext{PT}$ symmetry arises from cooperative atomic interactions, enabling control over light absorption and reflection.

Findings

Effective $ ext{PT}$ symmetry achieved in atomic arrays

Coherent perfect absorption observed at exceptional points

Control of reflection and absorption via mode polarization balance

Abstract

Parity-Time ($\mathcal{PT}$) symmetry has become an important concept in the design of synthetic optical materials, with exotic functionalities such as unidirectional transport and non-reciprocal reflection. At exceptional points, this symmetry is spontaneously broken, and solutions transition from those with conserved intensity to exponential growth or decay. Here we analyze a quantum-photonic surface formed by a single layer of atoms in an array with light mediating strong cooperative many-body interactions. We show how delocalized collective excitation eigenmodes can exhibit an effective $\mathcal{PT}$ symmetry and non-exponential decay. This effective symmetry is achieved in a passive system without gain by balancing the scattering of a bright mode with the loss from a subradiant dark mode. These modes coalesce at exceptional points, evidenced by the emergence of coherent perfect absorption where coherent incoming light is perfectly absorbed and scattered only incoherently. We also show how $\mathcal{PT}$ symmetry can be generated in total reflection and by balancing scattering and loss between different polarizations of collective modes.