Crossing exceptional points in non-Hermitian quantum systems

TL;DR

This paper explores how two-photon quantum states behave in non-Hermitian systems at exceptional points, revealing unique quantum correlation changes and interference phenomena linked to parity-time symmetry breaking.

Contribution

It demonstrates the first experimental observation of quantum state behavior and interference changes across exceptional points in non-Hermitian photonic systems.

Findings

Quantum correlations change at the exceptional point.

Quantum interference switches from dips to peaks.

Behavior linked to parity-time symmetry breaking.

Abstract

Exceptional points facilitate peculiar dynamics in non-Hermitian systems. Yet, in photonics, they have mainly been studied in the classical realm. In this work, we reveal the behavior of two-photon quantum states in non-Hermitian systems across the exceptional point. We probe the lossy directional coupler with an indistinguishable two-photon input state and observe distinct changes of the quantum correlations at the output as the system undergoes spontaneous breaking of parity-time symmetry. Moreover, we demonstrate a switching in the quantum interference of photons directly at the exceptional point, where Hong-Ou-Mandel dips are transformed into peaks by a change of basis. These results show that quantum interference and exceptional points are linked in curious ways that can now be further explored.