  Quantum memories capable of storing and retrieving coherent information for
extended times at room temperature would enable a host of new technologies.
Electron and nuclear spin qubits using shallow neutral donors in semiconductors
have been studied extensively but are limited to low temperatures ($\le$10 K);
however, the nuclear spins of ionized donors have potential for high
temperature operation. We use optical methods and dynamical decoupling to
realize this potential for an ensemble of 31P donors in isotopically purified
28Si and observe a room temperature coherence time of over 39 minutes. We
further show that a coherent spin superposition can be cycled from 4.2 K to
room temperature and back, and report a cryogenic coherence time of 3 hours in
the same system.
