  Efficient and reliable measurements of photonic indistinguishability are
crucial to solidify claims of a quantum advantage in photonics. Existing
indistinguishability witnesses may be vulnerable to implementation loopholes,
showing the need for a measurement which depends on as few assumptions as
possible. Here, we introduce a semi-device-independent witness of photonic
indistinguishability and measure it on an integrated photonic processor,
certifying three-photon indistinguishability in a way that is insensitive to
implementation errors in our processor.
