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Description
Multiparticle interference phenomena have been crucial to the understanding of quantum physics. In two-particle systems, Hong, Ou, and Mandel showed how particles’ indistinguishability forbids retrieving information about the pairwise exchange process, playing a key role in witnessing interference. Contrarily, in systems of $N\geq3$ partially distinguishable particles, multiple interference terms originate from the different exchange processes, enabling the observation of genuine $N$-particle interference that is no longer fully determined by pairwise indistinguishability. Here, we introduce yet another fundamental feature of quantum physics, i.e., quantum entanglement, to demonstrate the genuine four-particle interference of photons which, however, only interfere in pairs at two separate and independent beamsplitters, thus suggesting a nonlocal collective interference.
