  A long-lived multi-mode qubit register is an enabling technology for modular
quantum computing architectures. For interfacing with superconducting qubits,
such a quantum memory should be able to store incoming quantum microwave fields
at the single-photon level for long periods of time, and retrieve them
on-demand. Here, we demonstrate the partial absorption of a train of weak
microwave fields in an ensemble of bismuth donor spins in silicon, their
storage for 100 ms, and their retrieval, using a Hahn-echo-like protocol. The
long storage time is obtained by biasing the bismuth donors at a clock
transition. Phase coherence and quantum statistics are preserved in the
storage.
