  Ensembles of solid-state optical emitters enable broadband quantum storage
and transduction of photonic qubits, with applications in high-rate optical
quantum networks for secure communications, global time-keeping, and
interconnecting future quantum computers. To realize coherent quantum
information transfer using ensembles, spin rephasing techniques are currently
used to mitigate fast decoherence resulting from inhomogeneous broadening. Here
we use a dense ensemble of neodymium rare-earth ions strongly coupled to a
nanophotonic resonator to demonstrate that decoherence of a single photon
excitation is near-completely suppressed via cavity protection- a new technique
for accessing the decoherence-free subspace of collective coupling. The
protected Rabi oscillations between the cavity field and the atomic
superradiant state thereby enable ultra-fast transfer of photonic frequency
qubits (~50 GHz bandwidth) into the ions, followed by retrieval with 98.7%
fidelity. By coupling the superradiant excitation to other long-lived
rare-earth spin states, this technology will enable broadband, always-ready
quantum memories and fast optical-to-microwave transducers.
