Demonstration of entanglement-by-measurement of solid-state qubits

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Projective measurements are a powerful tool for manipulating quantum states1,2,3,4,5,6,7,8,9,10,11,12,13. In particular, a set of qubits can be entangled by measuring a joint property3,4,5,6,7,8,9,10,11,12,13 such as qubit parity. These joint measurements do not require a direct interaction between qubits and therefore provide a unique resource for quantum information processing with well-isolated qubits. Numerous schemes for entanglement-by-measurement of solid-state qubits have been proposed8,9,10,11,12,13, but the demanding experimental requirements have so far hindered implementations. Here we realize a two-qubit parity measurement on nuclear spins localized near a nitrogen-vacancy centre in diamond by exploiting an electron spin as a readout ancilla. The measurement enables us to project the initially uncorrelated nuclear spins into maximally entangled states. By combining this entanglement with single-shot readout we demonstrate the first violation of Bell’s inequality with solid-state spins. These results introduce a new class of experiments in which projective measurements create, protect and manipulate entanglement between solid-state qubits.

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Figure 1: Entanglement by measurement and qubits in diamond.
Figure 2: Projection into a Bell state by a non-destructive qubit parity measurement.
Figure 3: Bell state analysis.
Figure 4: Bell’s inequality violation.


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We thank D. D. Awschalom, L. Childress, L. DiCarlo, V. V. Dobrovitski, G. D. Fuchs and J. J. L. Morton for helpful discussions and comments, and R. N. Schouten for technical assistance. We acknowledge support from the Dutch Organization for Fundamental Research on Matter (FOM), the Netherlands Organization for Scientific Research (NWO), the DARPA QuEST and QuASAR programmes, and the EU SOLID and DIAMANT programmes.

Author information

W.P., T.H.T. and L.R. carried out the experiment. W.P. and T.H.T. analysed the data. H.B. fabricated the sample. M.M. and D.J.T. grew the diamond. W.P., T.H.T. and R.H. wrote the manuscript. All authors commented on the manuscript. R.H. supervised the project.

Correspondence to Ronald Hanson.

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Pfaff, W., Taminiau, T., Robledo, L. et al. Demonstration of entanglement-by-measurement of solid-state qubits. Nature Phys 9, 29–33 (2013) doi:10.1038/nphys2444

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