# Quantum PT-Phase Diagram in a Non-Hermitian Photonic Structure

## TL;DR

This paper analytically explores the quantum PT-phase diagram in a non-Hermitian bi-photonic structure, revealing how quantum states behave under PT-symmetry and broken phases, with implications for quantum state engineering.

## Contribution

It provides the first analytical quantum PT-phase diagram for a bi-photonic system with loss and gain, advancing understanding of quantum properties in non-Hermitian photonics.

## Key findings

- Quantum PT-phase diagram characterized analytically.
- Photon concentration in dominant waveguide in PT-broken phase.
- Implications for quantum state engineering and quantum information processing.

## Abstract

Photonic structures have an inherent advantage to realize PT-phase transition through modulating the refractive index or gain-loss. However, quantum PT properties of these photonic systems have not been comprehensively studied yet. Here, in a bi-photonic structure with loss and gain simultaneously existing, we analytically obtained the quantum PT-phase diagram under the steady state condition. To characterize the PT-symmetry or -broken phase, we define an Hermitian exchange operator expressing the exchange between quadrature variables of two modes. If inputting several-photon Fock states into a PT-broken bi-waveguide splitting system, most photons will concentrate in the dominant waveguide with some state distributions. Quantum PT-phase diagram paves the way to the quantum state engineering, quantum interferences, and logic operations in non-Hermitian photonic systems.