Counter-propagating spontaneous parametric down-conversion source in lithium niobate on insulator

Lithium niobate on insulator is a promising platform for integrated quantum photonics. Its strong nonlinear coefficient and electro-optic effect allow the integration of photon pair sources and fast reconfigurable interferometers for many applications like boson sampling, QKD and quantum state generation. The photons are generated via Spontaneous Parametric Down-Conversion (SPDC). Two key aspects of the photon pairs are their bandwidth and the possibility to split them deterministically. Here we present a new source type where the generated photons travel in opposite directions and feature a narrow bandwidth of 5 nm in the forward and 20nm in the backward propagation. The reduction of bandwidth and the separability are big advantages over the type-0-SPDC sources. Compared to the well-known type-2-SPDC sources we achieve a 3.6 times higher efficiency with similar bandwidths.