Polarization entanglement generation in silicon nitride waveguide-coupled dual microring resonators

Polarization-entangled photon pair sources exhibiting nonlocal quantum correlations are crucial to developments of quantum computing, quantum communications, quantum cryptography, and quantum sensing technologies. On-chip polarization entanglement generation thus constitutes one enabling component for integrated quantum photonic circuits. Here, we present to our knowledge the first polarization-entangled photon pair sources in a silicon nitride platform for integrated quantum photonic circuits. We demonstrate the generation of a polarization-entangled state by adopting a configuration comprising dual microring resonators, with nearly degenerate transverse electric and transverse magnetic polarized cavity resonances for the two resonators coupled in series to a common bus waveguide. We measure two-photon interference and quantum state tomography to characterize the polarization entanglement of the generated state and to reconstruct the density matrix. Our experiments reveal a visibility of 96.4 o +/- 3.1 o and of 86.7 o +/- 3.2 o with the |H > and |V > bases, respectively (and a visibility of 89.4 o +/- 6.6 o and 81.3 o +/- 7.3 o with the |D > and |A > bases), and a fidelity of similar to 75.7 o from the tomographic reconstructed density matrix.