Środowiskowe Seminarium z Informacji i Technologii Kwantowych

Multiphoton indistinguishability is a key resource for photonic quantum technologies, yet its characterization typically relies on resource-intensive methods. In this work, we develop two efficient and experimentally friendly protocols to estimate or bound the fidelity F_ind of an N-photon state to the closest perfectly indistinguishable state. The first protocol applies to sources preparing separable states, uses a single Fourier interferometer together with photon-number-resolving detection, and yields tight two-sided bounds on F_ind. The second protocol combines randomized implementations of linear-optical interferometers with photon counting, enabling direct estimation of F_ind for arbitrary N-photon states. Both protocols can certify F_ind = 1 - O(eps) using provably optimal O(1/eps) samples, in contrast to previous approaches which required prior assumptions on the model of partial distinguishability. Our methods, based on a multiphoton generalization of the Hong–Ou–Mandel test, bring the rigorous and operationally meaningful certification of multiphoton indistinguishability within reach of current photonic technologies.