Enabling long-coherence quantum photonic systems with rare-earth ion doped ceramics

Trivalent erbium ions doped in polycrystalline Y₂O₃ ceramics have telecom band transitions with long optical and spin coherence properties comparable to the best single crystal counterparts, making it a promising candidate for photonic quantum memory and spin-photon quantum interfaces. However, challenges remain to integrate ceramic systems with on-chip photonics owing to potential scattering loss at grain boundaries. Here we report an integrated photonic platform based on nanophotonic SiC waveguides and resonators coupled to an Er:Y₂O₃ polycrystalline ceramic substrate. We measure high Q factors of microring resonators up to 1.64 million at cryogenic temperatures at telecom wavelengths, overcoming a major hurdle in integrated photonics using ceramic materials. We confirm coupling of near-surface erbium ions to the waveguides by photoluminescence excitation measurements and observe an optical lifetime of 8.1 ms. Photon echo measurements are used to assess the optical homogenous linewidths and spectral diffusion rates of near surface erbium ions, which are compared with the sub-kilohertz erbium linewidths previously reported for bulk Er:Y₂O₃ ceramics. In future work, taking advantage of the long optical and spin coherence times of erbium in Y₂O₃, we will demonstrate storage of entanglement using atomic frequency comb memory for telecom quantum networks.