  We have built a hybrid system composed of a superconducting flux qubit (the
processor) and an ensemble of nitrogen-vacancy centers in diamond (the memory)
that can be directly coupled to one another and demonstrated how information
can be transferred from the flux qubit to the memory, stored and subsequently
retrieved. We have established the coherence properties of the memory, and
succeeded in creating an entangled state between the processor and memory,
demonstrating how the entangled state's coherence is preserved. Our results are
a significant step towards using an electron spin ensemble as a quantum memory
for superconducting qubits.
