  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 dialled. 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.
