A two-channel source of spectrally degenerate polarization entangled states on chip

In the last years the increasing complexity of quantum information protocols triggered the development of integrated waveguide technology for quantum applications [1–3] such that stability is intrinsically obtained thanks to the combination of many optical elements on a monolithic chip and scalability is potentially achievable. In this context the realization of integrated sources of quantum light is of fundamental interest. Some experimental implementations using parametric down conversion in nonlinear materials [4], as well as in semiconductors [5] have been reported. However, in the landscape of integrated devices, the realization of one important source of entangled states is still missing: a source of spectrally indistinguishable photon pairs emitted on two different spatial modes, which share entanglement in the polarization degree of freedom without the need of post-selection. This source is widely adopted in bulk setups [6], indeed entanglement is a fundamental requirement for quantum information purposes and the polarization of photons is the most used degree of freedom for quantum information encoding. However the implementation of such a scheme in integrated waveguides is a challenging task due to the combination of orthogonal polarizations and the need of two different spatial modes.