Interfacing integrated thin-film lithium niobate optical resonators with rare earth ions

A critical component in the realisation of a global quantum network is the coupling between photons at the optical and microwave frequencies. A well-known promising candidate to build the key quantum light-matter device are rare earth ions (REIs). However, to achieve efficient optical interface with the REIs, a tuneable high quality factor resonator is required to leverage strong light-matter interaction. Here, we focus on the application of thin film lithium niobate (LN) photonic integrated chips, incorporating microring resonators, to interface with an erbium-doped calcium tungstate (CaWO₄) host crystal. We show that by using this heterogeneously integrated platform, we achieved a collective cooperativity beyond 2 with our erbium ensemble at C-band telecom wavelength at millikelvin temperatures, which enables quantum memory protocols with efficient readout.