A quantum memory lasting seconds in silicon

Dr John Morton ( Department of Materials, University of Oxford )

The transfer of information between different physical forms is a central theme in communication and computation, for example between processing entities and memory. Nowhere is this more crucial than in quantum computation, where great effort must be taken to protect the integrity of a fragile quantum bit. Nuclear spins are known to benefit from long coherence times compared to electron spins, but are slow to manipulate and suffer from weak thermal polarisation. A powerful model for quantum computation is thus one in which electron spins are used for processing and readout while nuclear spins are used for storage. Here we demonstrate the coherent transfer of a superposition state in an electron spin processing qubit to a nuclear spin memory qubit, using a combination of microwave and radiofrequency pulses applied to 31P donors in an isotopically pure 28Si crystal. We have achieved store-recover fidelities of 90%.

The large number of spins used in these experiments is capable of storing a much larger amount of information if one uses distributed collective modes, as in holography. We demonstrate the storage and retrieval of weak excitations in distributed memories based on donors in silicon. Each excitation is phase encoded and we have stored up to 100 weak microwave excitations in a spin ensemble and recalled them sequentially: a quantum 'stack'. We also demonstrate the storage and retrieval of such multiple excitations into a coupled nuclear spin, for more robust storage.