# Quantum interference and exceptional points

## TL;DR

This paper investigates how quantum interference with entangled photons affects the signatures of exceptional points in non-Hermitian optical systems, revealing that quantum effects can obscure classical EP signatures.

## Contribution

It demonstrates that quantum interference can mask exceptional point signatures in optical systems when using entangled photon states, especially as particle statistics shift from bosonic to fermionic.

## Key findings

- Quantum interference can hide EP signatures with entangled photons.
- EP phase transition signatures are smoothed out with non-classical light.
- Changing particle statistics from bosonic to fermionic affects EP visibility.

## Abstract

Exceptional points (EPs), i.e. branch point singularities of non-Hermitian Hamiltonians, are ubiquitous in optics. So far, the signatures of EPs have been mostly studied assuming classical light. In the passive parity-time ($\mathcal{PT}$) optical coupler, a fingerprint of EPs resulting from the coalescence of two resonance modes is a qualitative change of the photon decay law, from damped Rabi-like oscillations to transparency, as the EP is crossed by increasing the loss rate. However, when probed by non-classical states of light, quantum interference can hide EPs. Here it is shown that, under excitation with polarization-entangled two-photon states, EP phase transition is smoothed until to disappear as the effective particle statistics is changed from bosonic to fermionic.