We report on a universal method to measure the genuine indistinguishability of n-photons - a crucial parameter that determines the accuracy of optical quantum computing. Our approach relies on a low-depth cyclic multiport interferometer with N = 2n modes, leading to a quantum interference fringe whose visibility is a direct measurement of the genuine n-photon indistinguishability. We experimentally demonstrate this technique for a 8-mode integrated interferometer fabricated using femtosecond laser micromachining and four photons from a quantum dot single-photon source. We measure a four-photon indistinguishability up to 0.81±0.03. This value decreases as we intentionally alter the photon pairwise indistinguishability. The low-depth and low-loss multiport interferometer design provides an efficient and scalable path to evaluate the genuine indistinguishability of resource states of increasing photon number.
@article{arxiv.2201.13333,
title = {Quantifying n-photon indistinguishability with a cyclic integrated interferometer},
author = {Mathias Pont and Riccardo Albiero and Sarah E. Thomas and Nicolò Spagnolo and Francesco Ceccarelli and Giacomo Corrielli and Alexandre Brieussel and Niccolo Somaschi and Hêlio Huet and Abdelmounaim Harouri and Aristide Lemaître and Isabelle Sagnes and Nadia Belabas and Fabio Sciarrino and Roberto Osellame and Pascale Senellart and Andrea Crespi},
journal= {arXiv preprint arXiv:2201.13333},
year = {2022}
}