  Multiphoton quantum interference underpins fundamental tests of quantum
mechanics and quantum technologies. Consequently, the detrimental effect of
photon distinguishability in multiphoton interference experiments can be
catastrophic. Here, we employ correlation measurements in the photonic inner
modes, time or frequency, to restore quantum interference between photons
differing in their colors or injection times in arbitrary linear optical
networks, without the need for additional filtering or post selection.
Interestingly, we demonstrate how harnessing the multiphoton inner-mode quantum
information enables to unravel symmetries of multiphoton networks and states
and the generation of an entire class of multipartite entangled states with a
fixed interferometer. These results are therefore of profound interest for
future applications of universal inner-mode resolved linear optics across
fundamental science and quantum technologies with photons with experimentally
different spectral properties.
