Semi-device-independent characterization of multiphoton indistinguishability

Multiphoton indistinguishability is a central resource for quantum enhancement in sensing and computation. Developing and certifying large-scale photonic devices requires reliable and accurate characterization of this resource, preferably using methods that are robust against experimental errors. Here, we propose a set of methods for the characterization of multiphoton indistinguishability, based on measurements of bunching and photon-number variance. Our methods are robust in a semi-device-independent way, in the sense of being effective even when the interferometers are incorrectly dialed. We demonstrate the effectiveness of this approach using an advanced photonic platform comprising a quantum dot single-photon source and a universal fully programmable integrated photonic processor. Our results show the practical usefulness of our methods, providing robust certification tools that can be scaled up to larger systems.