TY - JOUR A1 - Setzpfandt, Frank A1 - Solntsev, Alexander S. A1 - Titchener, James A1 - Wu, Che Wen A1 - Xiong, Chunle A1 - Schiek, Roland A1 - Pertsch, Thomas A1 - Neshev, Dragomir N. A1 - Sukhorukov, Andrey A. T1 - Tunable generation of entangled photons in a nonlinear directional coupler JF - Laser & Photonics Reviews N2 - 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. KW - ARRAYS KW - directional coupler KW - entanglement KW - GATES KW - nonlinearity KW - PAIR GENERATION KW - photon pairs KW - WAVE KW - waveguides Y1 - 2016 U6 - https://doi.org/10.1002/lpor.201500216 VL - 10 IS - 1 SP - 131 EP - 136 PB - WILEY-VCH ER -