  The digital revolution was enabled by nanostructured devices made from
silicon. A similar prominence of this material is anticipated in the upcoming
quantum era as the unrivalled maturity of silicon nanofabrication offers unique
advantages for integration and up-scaling, while its favorable material
properties facilitate quantum memories with hour-long coherence. While small
spin-qubit registers have exceeded error-correction thresholds, their
connection to large quantum computers is an outstanding challenge. To this end,
spin qubits with optical interfaces offer key advantages: they can minimize the
heat load and give access to modular quantum computing architectures that
eliminate cross-talk and offer a large connectivity via room-temperature photon
routing. Here, we implement such an efficient spin-photon interface based on
erbium dopants in a nanophotonic resonator. We thus demonstrate optical
single-shot readout of a spin in silicon whose coherence exceeds the
Purcell-enhanced optical lifetime, paving the way for entangling remote spins
via photon interference. As erbium dopants can emit coherent photons in the
minimal-loss band of optical fibers, and tens of such qubits can be spectrally
multiplexed in each resonator, the demonstrated hardware platform offers unique
promise for distributed quantum information processing and the implementation
of a quantum internet based on integrated silicon devices.
