  Optically active solid-state spin qubits thrive as an appealing technology
for quantum interconnect and quantum networking, owing to their atomic size,
scalable creation, long-lived coherence, and ability to coherently interface
with flying qubits. Trivalent erbium dopants in particular emerge as a
compelling candidate with their telecom C band emission and shielded 4f
intra-shell spin-optical transitions. However, prevailing top-down architecture
for rare-earth qubits and devices has not allowed simultaneous long optical and
spin coherence necessary for long-distance quantum networks. Here we
demonstrate dual erbium telecom spin-photon interfaces in an epitaxial
thin-film platform via wafer-scale bottom-up synthesis. Harnessing precise
controls over the matrix purity, dopant placement, and symmetry unique to this
platform, we simultaneously achieve millisecond erbium spin coherence time and
$<$3 kilohertz optical dephasing rate in an inversion-symmetry protected site
and realize both optical and microwave control in a fiber-integrated package
for rapid scaling up. These results demonstrate a significant prospect for
high-quality rare-earth qubits and quantum memories assembled using a bottom-up
method and pave the way for the large-scale development of quantum light-matter
interfaces for telecommunication quantum networks.
