# Observation of critical phenomena in parity-time-symmetric quantum dynamics

## TL;DR

This paper experimentally investigates critical phenomena in PT-symmetric non-Hermitian quantum dynamics, revealing oscillations, information flow, and power-law behavior near the exceptional point, with implications for quantum information retrieval.

## Contribution

First experimental observation and characterization of critical phenomena in PT-symmetric non-unitary quantum dynamics, including information flow and entanglement effects.

## Key findings

- Oscillations of quantum-state distinguishability observed in PT-unbroken regime
- Power-law behavior near the exceptional point in key quantities
- Information retrieval linked to finite-dimensional entanglement

## Abstract

We experimentally simulate non-unitary quantum dynamics using a single-photon interferometric network and study the information flow between a parity-time (PT)-symmetric non-Hermitian system and its environment. We observe oscillations of quantum-state distinguishability and complete information retrieval in the PT-symmetry-unbroken regime. We then characterize in detail critical phenomena of the information flow near the exceptional point separating the PT-unbroken and -broken regimes, and demonstrate power-law behavior in key quantities such as the distinguishability and the recurrence time. We also reveal how the critical phenomena are affected by symmetry and initial conditions. Finally, introducing an ancilla as an environment and probing quantum entanglement between the system and the environment, we confirm that the observed information retrieval is induced by a finite-dimensional entanglement partner in the environment. Our work constitutes the first experimental characterization of critical phenomena in PT-symmetric non-unitary quantum dynamics.