  Spins in solids are an ideal candidate to act as a memory and interface with
superconducting qubits due to their long coherence times. We spectroscopically
investigate erbium-167-doped yttrium orthosilicate as a possible
microwave-addressed memory employing its microwave frequency transitions that
occur without applying an external magnetic field. We obtain coherence times of
380 $\mu$s in a ground state spin transition and 1.48 ms in an excited state
spin transition. This is 28 times longer compared to previous zero field
measurements, as well as 200 times longer than a previous microwave memory
demonstration in the same material. These long coherence times show that
erbium-167-doped yttrium orthosilicate has potential as a microwave-addressed
quantum memory.
