Środowiskowe Seminarium z Informacji i Technologii Kwantowych

Multiphoton indistinguishability is a resource for photonic quantum technologies, but its characterisation can be resource-intensive. Here we present an efficient interferometric protocol certifying closeness (in Ulhmann fidelity) of an unknown N-photon state to the state of perfectly indistinguishable photons. The protocol uses a single Fourier interferometer followed by particle-number measurements and a simple classical post-processing. The sample complexity of certifying fidelity to the perfectly indistinguishable case has optimal sampling complexity and is independent of the number of photons N, unlike previous approaches which exhibit exponential scaling. Our efficient method, based on a generalisation of the Hong-Ou-Mandel test, brings the rigorous and operationally grounded certification of multiphoton indistinguishability within reach of current-day photonic technologies.