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A valley–spin qubit in a carbon nanotube


Although electron spins in III–V semiconductor quantum dots have shown great promise as qubits1,2,3, hyperfine decoherence remains a major challenge in these materials. Group IV semiconductors possess dominant nuclear species that are spinless, allowing qubit coherence times4,5,6 up to 2 s. In carbon nanotubes, where the spin–orbit interaction allows for all-electrical qubit manipulation7,8,9,10, theoretical predictions of the coherence time vary by at least six orders of magnitude and range up to 10 s or more11,12. Here, we realize a qubit encoded in two nanotube valley–spin states, with coherent manipulation via electrically driven spin resonance2,3 mediated by a bend in the nanotube. Readout uses Pauli blockade leakage current through a double quantum dot13,14,15. Arbitrary qubit rotations are demonstrated and the coherence time is measured for the first time via Hahn echo, allowing comparison with theoretical predictions. The coherence time is found to be 65 ns, probably limited by electrical noise. This shows that, even with low nuclear spin abundance, coherence can be strongly degraded if the qubit states are coupled to electric fields.

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Figure 1: Valley–spin resonance in a bent nanotube.
Figure 2: Spin resonance spectroscopy.
Figure 3: Coherent qubit manipulation
Figure 4: Universal control and measurement of coherence times.

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The authors thank G.A. Steele, K. Flensberg, J. Klinovaja, D. Loss, A. Pályi, J. van den Berg, S.M. Frolov and V.S. Pribiag for discussions. This work was supported by the Netherlands Organization for Scientific Research (NWO)/the Dutch Organization for Fundamental Research on Matter (FOM).

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F.P. fabricated and characterized the device. E.A.L. performed the experiment. All authors prepared the manuscript.

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Correspondence to L. P. Kouwenhoven.

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The authors declare no competing financial interests.

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Laird, E., Pei, F. & Kouwenhoven, L. A valley–spin qubit in a carbon nanotube. Nature Nanotech 8, 565–568 (2013).

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