Multimode Storage of Quantum Microwave Fields in Electron Spins over 100 milliseconds*

We report long coherence times (up to 300 ms) for near-surface bismuth donor electron spins in silicon coupled to a superconducting microresonator, biased at a clock transition [G. Wolfowicz, Nat nano, 8, 561 (2013)]. This enables us to demonstrate the partial absorption of a train of weak microwave fields in the spin ensemble, their storage for 100 ms, and their retrieval, using a Hahn-echo-like protocol. We also discuss the phase coherence and quantum statistics of the retrieved states.