# Generation and applications of an ultrahigh-fidelity four-photon Greenberger-Horne-Zeilinger state

## TL;DR

This paper reports the first generation of a four-photon Greenberger-Horne-Zeilinger state with 98% fidelity, enabling advanced quantum information tasks and demonstrating genuine multipartite nonlocality.

## Contribution

It introduces a method to produce ultrahigh-fidelity four-photon entangled states, significantly improving multiphoton entanglement quality over previous approaches.

## Key findings

- Achieved 98% fidelity in four-photon GHZ state
- Demonstrated ultrahigh-fidelity entanglement swapping
- Verified genuine multipartite nonlocality via Hardy-like inequality

## Abstract

High-quality entangled photon pairs generated via spontaneous parametric down-conversion have made great contributions to the modern quantum information science and the fundamental tests of quantum mechanics. However, the quality of the entangled states decreases sharply when moving from biphoton to multiphoton experiments, mainly due to the lack of interactions between photons. Here, for the first time, we generate a four-photon Greenberger-Horne-Zeilinger state with a fidelity of $98\%$, which is even comparable to the best fidelity of biphoton entangled states. Thus, it enables us to demonstrate an ultrahigh-fidelity entanglement swapping---the key ingredient in various quantum information tasks. Our results push the fidelity of multiphoton entanglement generation to a new level and would be useful in some demanding tasks, e.g., we successfully demonstrate the genuine multipartite nonlocality of the observed state in the nonsignaling scenario by violating a novel Hardy-like inequality, which requires very high state-fidelity.