  Spins in silicon that are accessible via a telecom-compatible optical
transition are a versatile platform for quantum information processing that can
leverage the well-established silicon nanofabrication industry. Key to these
applications are long coherence times on the optical and spin transitions to
provide a robust system for interfacing photonic and spin qubits. Here, we
report telecom-compatible Er3+ sites with long optical and electron spin
coherence times, measured within a nuclear spin-free silicon crystal (<0.01%
29Si) using optical detection. We investigate two sites and find 0.1 GHz
optical inhomogeneous linewidths and homogeneous linewidths below 70 kHz for
both sites. We measure the electron spin coherence time of both sites using
optically detected magnetic resonance and observe Hahn echo decay constants of
0.8 ms and 1.2 ms at around 11 mT. These optical and spin properties of Er3+:Si
are an important milestone towards using optically accessible spins in silicon
for a broad range of quantum information processing applications.
