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# High-fidelity projective read-out of a solid-state spin quantum register

## Abstract

Initialization and read-out of coupled quantum systems are essential ingredients for the implementation of quantum algorithms1,2. Single-shot read-out of the state of a multi-quantum-bit (multi-qubit) register would allow direct investigation of quantum correlations (entanglement), and would give access to further key resources such as quantum error correction and deterministic quantum teleportation1. Although spins in solids are attractive candidates for scalable quantum information processing, their single-shot detection has been achieved only for isolated qubits3,4,5,6. Here we demonstrate the preparation and measurement of a multi-spin quantum register in a low-temperature solid-state system by implementing resonant optical excitation techniques originally developed in atomic physics. We achieve high-fidelity read-out of the electronic spin associated with a single nitrogen–vacancy centre in diamond, and use this read-out to project up to three nearby nuclear spin qubits onto a well-defined state7. Conversely, we can distinguish the state of the nuclear spins in a single shot by mapping it onto, and subsequently measuring, the electronic spin5,8. Finally, we show compatibility with qubit control: we demonstrate initialization, coherent manipulation and single-shot read-out in a single experiment on a two-qubit register, using techniques suitable for extension to larger registers. These results pave the way for a test of Bell’s inequalities on solid-state spins and the implementation of measurement-based quantum information protocols.

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## References

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## Acknowledgements

L.R. acknowledges support by a Marie Curie Intra European Fellowship within the 7th European Community Framework Programme. L.R., H.B., B.H. and R.H. acknowledge support from the Dutch Organization for Fundamental Research on Matter (FOM) and the European Commission (SOLID). L.C. acknowledges support from Research Corporation for Science Advancement (RCSA).

## Author information

Authors

### Contributions

L.R., L.C. and H.B. conducted the experiments. L.R., L.C., H.B., B.H. and R.H. analysed the data. H.B. and P.F.A.A. fabricated the devices. L.R., L.C. and R.H wrote the paper. All authors commented on the manuscript.

### Corresponding author

Correspondence to Ronald Hanson.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary Information

The file contains Supplementary Text, Supplementary Figures 1-9 with legends and additional references. (PDF 463 kb)

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Robledo, L., Childress, L., Bernien, H. et al. High-fidelity projective read-out of a solid-state spin quantum register. Nature 477, 574–578 (2011). https://doi.org/10.1038/nature10401

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• DOI: https://doi.org/10.1038/nature10401

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