  Quantum mechanically, multiple particles can jointly be in a coherent
superposition of two or more different states at the same time. This property
is called quantum entanglement, and gives rise to characteristic nonlocal
interference and stays at the heart of quantum information process. Here,
rather than interference of different intrinsic properties of particles, we
experimentally demonstrated coherent superposition of two different birthplaces
of a four-photon state. The quantum state is created in four probabilistic
photon-pair sources, two combinations of which can create photon quadruplets.
Coherent elimination and revival of distributed 4-photons can be fully
controlled by tuning a phase. The stringent coherence requirements are met by
using a silicon-based integrated photonic chip that contains four spiral
waveguides for producing photon pairs via spontaneous four-wave mixing. The
experiment gives rise to peculiar nonlocal phenomena without any obvious
involvement of entanglement. Besides several potential applications that
exploit the new on-chip technology, it opens up the possibility for fundamental
studies on nonlocality with spatially separated locations.
