# Experimental quantification of genuine four-photon indistinguishability

## TL;DR

This paper presents an efficient four-photon experiment in a linear-optical setup that accurately quantifies genuine four-photon indistinguishability, advancing tools for multiparticle quantum resource characterization.

## Contribution

It introduces a heralding-free interferometer design and applies a new theoretical framework to measure and bound multiphoton indistinguishability in a resource-efficient manner.

## Key findings

- High agreement between experimental results and theory
- Effective bounds on two-photon overlaps
- Novel technique for multiphoton interference characterization

## Abstract

Photon indistinguishability plays a fundamental role in information processing, with applications such as linear-optical quantum computation and metrology. It is then necessary to develop appropriate tools to quantify the amount of this resource in a multiparticle scenario. Here we report a four-photon experiment in a linear-optical interferometer designed to simultaneously estimate the degree of indistinguishability between three pairs of photons. The interferometer design dispenses with the need of heralding for parametric down-conversion sources, resulting in an efficient and reliable optical scheme. We then use a recently proposed theoretical framework to quantify genuine four-photon indistinguishability, as well as to obtain bounds on three unmeasured two-photon overlaps. Our findings are in high agreement with the theory, and represent a new resource-effective technique for the characterization of multiphoton interference.