Counterpropagating entangled photons in a periodically poled nonlinear waveguide

Entangled photons, which may be generated through the process of spontaneous parametric down-conversion (SPDC) in a crystal with χ⁽²⁾ nonlinearity, have long been in the spotlight for quantum-optics experiments. The generation of entangled photons by quasi-phase matching (QPM) in lithium niobate offers the promise of increased photon-pair production and, with the integration of a waveguide structure, control of the spatial characteristics of the down-converted photons. It turns out that the use of a waveguide structure, in conjunction with periodic poling to achieve quasi-phase matching, offers another critically important feature: the possibility of generating counterpropagating signal and idler photons. We consider the conditions required for generating counterpropagating photon beams and explore some of the unique properties of this source of light using a periodically poled LiNbO₃ waveguide.