Tunable generation of entangled photons in a nonlinear directional coupler

The on‐chip integration of quantum light sources has enabled the realization of complex quantum photonic circuits. However, for the practical implementation of such circuits in quantum information applications, it is crucial to develop sources delivering entangled quantum photon states with on‐demand tunability. Here we propose and experimentally demonstrate the concept of a widely tunable quantum light source based on spontaneous parametric down‐conversion in a simple nonlinear directional coupler. We show that spatial photon‐pair correlations and entanglement can be reconfigured on‐demand by tuning the phase difference between the pump beams and the phase mismatch inside the structure. We experimentally demonstrate the generation of split states, robust N00N states, various intermediate regimes and biphoton steering on a single chip. Furthermore we theoretically investigate other regimes allowing all‐optically tunable generation of all Bell states and flexible control of path‐energy entanglement. Such wide‐range capabilities of a structure comprised of just two coupled nonlinear waveguides are attributed to the intricate interplay between linear coupling and nonlinear phase matching. This scheme provides an important advance towards the realization of reconfigurable quantum circuitry. An integrated all‐optically tunable source of two‐photon quantum states based on spontaneous parametric down‐conversion is experimentally demonstrated in a directional coupler consisting of two lithium niobate waveguides. The degree of entanglement and shape of two‐photon spatial correlations is widely tunable by varying the phase mismatch and relative phases of the two pump beams. The operating principle is suitable for practical implementation of reconfigurable photon sources in on‐chip quantum circuits.