Towards entangling remote erbium qubits with millisecond coherence

Remote entanglement of two distant spin qubits is an essential step towards long-distance quantum networks and distributed quantum information processing at scale. Er3+ ions in solids emerge as a promising candidate because of their telecom C-band transition, prospects for spectral multiplexing and long-lived coherence properties. Previously, we reported an epitaxial thin-film Er:Y2O3 platform integrated with a tunable Fiber Fabry Perot Cavity (FFPC) and a superconducting microwave resonator that demonstrated millisecond spin coherence times and <3 kilohertz optical dephasing rate with a high optical cyclicity in an inversion symmetry protected site. Built on this platform, here we report our progress towards single Er spin coherence beyond milliseconds, and >90% fidelity spin readout via dark-count free photon detection and improved photon collection efficiency. We also describe the setup for two Er qubits in separate fridges connected by a 50 km long telecom fiber, and strategies to improve photon indistinguishability between separate Er emitters. This work advances the development of scalable quantum networks by laying the foundation for telecom-band matter qubit quantum repeaters.