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= 2 n 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 an eight-mode
integrated interferometer fabricated using femtosecond laser micromachining and four …

Quantum states of many indistinguishable photons are key resources in optical quantum
technologies. However, experimentally quantifying the multi-photon indistinguishability of a
state of many photons is not trivial. We introduce a universal method to measure the genuine
indistinguishability of n photons using a low-depth, cyclic interferometer with N= 2n modes.
We demonstrate this method using an 8-mode integrated interferometer fabricated using
femtosecond laser micromachining, and four photons from a quantum dot single-photon …