  Optically interfaced solid-state defects are promising candidates for quantum
communication technologies. The ideal defect system would feature bright
telecom emission, long-lived spin states, and a scalable material platform,
simultaneously. Here, we employ one such system, vanadium (V4+) in silicon
carbide (SiC), to establish a potential telecom spin-photon interface within a
mature semiconductor host. This demonstration of efficient optical spin
polarization and readout facilitates all optical measurements of
temperature-dependent spin relaxation times (T1). With this technique, we lower
the temperature from about 2K to 100 mK to observe a remarkable
four-orders-of-magnitude increase in spin T1 from all measured sites, with
site-specific values ranging from 57 ms to above 27 s. Furthermore, we identify
the underlying relaxation mechanisms, which involve a two-phonon Orbach
process, indicating the opportunity for strain-tuning to enable qubit operation
at higher temperatures. These results position V4+ in SiC as a prime candidate
for scalable quantum nodes in future quantum networks.
