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Solid-state quantum memory using the 31P nuclear spin


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 crystal4,5. 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.

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Figure 1: The level structure of the coupled electron and nuclear spins and scheme for the transfer of a logical qubit within the two physical spin qubits.
Figure 2: Coherent storage of an electron-spin state in a nuclear-spin state, using a 31 P-doped 28 Si-enriched silicon single crystal.
Figure 3: Observing the nuclear-spin coherence during the storage process.
Figure 4: Density matrix tomography for original and recovered states.

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We thank G. A. D. Briggs for comments and support and R. Weber, P. Höfer and Bruker Biospin for support with instrumentation. We thank P. Weaver of Advanced Silicon Materials, Inc. for zone-refining and H. Riemann of the Institut für Kristallzüchtung for float-zone processing of the 28Si crystals used in this work. This research is supported by the National Security Agency (MOD 713106A) and the EPSRC through the Quantum Information Processing Interdisciplinary Research Collaboration (GR/S82176/01) and CAESR (EP/D048559/1). J.J.L.M. is supported by St John’s College, Oxford. A.A. and B.W.L. are supported by the Royal Society. Work at Princeton received support from the US National Science Foundation through the Princeton MRSEC (DMR-0213706). Work at Lawrence Berkeley National Laboratory was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the US Department of Energy (DE-AC02-05CH11231).

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Correspondence to John J. L. Morton.

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This file contains (A) calculations which confirm the effect of radiofrequency and microwave phases stated in the main text; (B) electron and nuclear relaxation model; and (C) density matrix tomography for all initial states (incorporating Supplementary Figures 1-3). (PDF 863 kb)

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Morton, J., Tyryshkin, A., Brown, R. et al. Solid-state quantum memory using the 31P nuclear spin. Nature 455, 1085–1088 (2008).

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