  We propose a design for a quantum interface exploiting the electron spins in
crystals to swap the quantum states between the optical and microwave. Using
sideband driving of a superconducting flux qubit and a combined
cavity/solid-state spin ensemble Raman transition, we demonstrate how a
stimulated Raman adiabatic passage (STIRAP)-type operation can swap the quantum
state between a superconducting flux qubit and an optical cavity mode with a
fidelity higher than $90\%$. We further consider two distant superconducting
qubits with their respective interfaces joined by an optical fiber and show a
quantum transfer fidelity exceeding $90\%$ between the two distant qubits.
