Letter

1. Department of Materials, Oxford University, Oxford OX1 3PH, UK
   
2. CAESR, Clarendon Laboratory, Department of Physics, Oxford University, Oxford OX1 3PU, UK
   
3. Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA
   
4. Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA
   
5. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA

Correspondence to: John J. L. Morton^1,2 Correspondence and requests for materials should be addressed to J.J.L.M. (Email: john.morton@materials.ox.ac.uk).

The transfer of information between different physical forms—for example processing entities and memory—is a central theme in communication and computation. This is crucial in quantum computation¹, where great effort² must be taken to protect the integrity of a fragile quantum bit (qubit). However, transfer of quantum information is particularly challenging, as the process must remain coherent at all times to preserve the quantum nature of the information³. 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 radio-frequency pulses applied to ³¹P donors in an isotopically pure ²⁸Si crystal^{4, 5}. The state is left in the nuclear spin on a timescale that is long compared with the electron decoherence time, and is then coherently transferred back to the electron spin, thus demonstrating the ³¹P nuclear spin as a solid-state quantum memory. The overall store–readout fidelity is about 90 per cent, with the loss attributed to imperfect rotations, and can be improved through the use of composite pulses⁶. The coherence lifetime of the quantum memory element at 5.5 K exceeds 1 s.