Entanglement without indistinguishability: Routing of coupling-induced path-entangled photon pairs

Realizing an on-chip reconfigurable source of path-entangled photons is of critical importance for the advancement of quantum information processing and networking. Achieving this goal has proven challenging to date. We present an on-chip scheme for the deterministic creation of co-propagating or counter-propagating path-entangled photon pairs that can be routed in multiple configurations by tuning a classical parameter. The simplest manifestation of this approach makes use of two $\mathit{\text{coupled}}$ waveguides: a $\mathit{\text{nonlinear}}$ waveguide that produces photon pairs via spontaneous parametric downconversion from an externally incident unguided optical pump, and an auxiliary $\mathit{\text{linear}}$ waveguide. Although the photon pairs are born in only one waveguide, which alone cannot create path-entanglement, linear coupling over an extended length to the passive waveguide introduces unexpected indistinguishability that induces path-entanglement. Tuning of the classical pump spatial profile allows routing the photon pairs over all possible configurations. The proposed device is a building block for future quantum-optical networks.