Inverse design of a polarization demultiplexer for on-chip path-entangled photon-pair sources based on single quantum...

Epitaxial quantum dots can emit polarization-entangled photon pairs. If orthogonal polarizations are coupled to independent paths, then the photons will be path-entangled. Through inverse design with adjoint method optimization, we design a quantum dot polarization demultiplexer, a nanophotonic geometry that efficiently couples orthogonally polarized transition dipole moments of a single quantum dot to two independent waveguides. We predict 95% coupling efficiency, cross talk less than 0.1%, and Purcell radiative rate enhancement factors over 11.5 for both dipoles, with sensitivity to dipole misalignment and orientation comparable to that of conventional nanophotonic geometries. We anticipate our design will be valuable for the implementation of triggered, high-rate sources of path-entangled photon-pairs on chip.