  Photonic interference is a key quantum resource for optical quantum
computation, and in particular for so-called boson sampling machines. In
interferometers with certain symmetries, genuine multiphoton quantum
interference effectively suppresses certain sets of events, as in the original
Hong-Ou-Mandel effect. Recently, it was shown that some classical and
semi-classical models could be ruled out by identifying such suppressions in
Fourier interferometers. Here we propose a suppression law suitable for
random-input experiments in multimode Sylvester interferometers, and verify it
experimentally using 4- and 8-mode integrated interferometers. The observed
suppression is stronger than what is observed in Fourier interferometers of the
same size, and could be relevant to certification of boson sampling machines
and other experiments relying on bosonic interference.
