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Coherent coupling of a superconducting flux qubit to an electron spin ensemble in diamond


During the past decade, research into superconducting quantum bits (qubits) based on Josephson junctions has made rapid progress1. Many foundational experiments have been performed2,3,4,5,6,7,8, and superconducting qubits are now considered one of the most promising systems for quantum information processing. However, the experimentally reported coherence times are likely to be insufficient for future large-scale quantum computation. A natural solution to this problem is a dedicated engineered quantum memory based on atomic and molecular systems. The question of whether coherent quantum coupling is possible between such natural systems and a single macroscopic artificial atom has attracted considerable attention9,10,11,12 since the first demonstration of macroscopic quantum coherence in Josephson junction circuits2. Here we report evidence of coherent strong coupling between a single macroscopic superconducting artificial atom (a flux qubit) and an ensemble of electron spins in the form of nitrogen–vacancy colour centres in diamond. Furthermore, we have observed coherent exchange of a single quantum of energy between a flux qubit and a macroscopic ensemble consisting of about 3 × 107 such colour centres. This provides a foundation for future quantum memories and hybrid devices coupling microwave and optical systems.

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Figure 1: Experimental set-up of a NV diamond sample attached to a flux qubit system.
Figure 2: Photoluminescence spectra.
Figure 3: Energy spectrum of the flux qubit coupled to a NV ensemble.
Figure 4: Vacuum Rabi oscillations of the flux qubit/NV ensemble coupled system.


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We thank T. Tawara, H. Gotoh and T. Sogawa for optical measurements at an early stage of this work. We also thank H. Tanji, Y. Matsuzaki, S. J. Devitt, J. Schmiedmayer and J. E. Mooij for discussions. This work was supported in part by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST), Scientific Research of Specially Promoted Research (grant no. 18001002) by MEXT, a Grant-in-Aid for Scientific Research on Innovative Areas (grant no.22102502), and Scientific Research (A) grant no. 22241025 from the Japanese Society for the Promotion of Science (JSPS). M.S.E. was supported by a JSPS fellowship.

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All authors contributed extensively to the work presented in this paper. X.Z. and A.K. carried out measurements and data analysis on the coupled flux qubit/NV ensemble. N.M., M.K. and K.S. prepared and characterized the NV diamond crystals. X.Z., S.K., S.S. and A.K. designed and fabricated the flux qubit and associated devices. S.S., K.K. and A.K. designed and developed the flux qubit measurement system. W.J.M., A.K., Y.T., H.N., M.S.E. and K.N. provided theoretical support and analysis. X.Z., M.S.E., W.J.M. and K.S. wrote the manuscript, with feedback from all authors. W.J.M. and K.S. designed and supervised the project.

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Correspondence to Kouichi Semba.

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The authors declare no competing financial interests.

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Supplementary Information

This file contains Supplementary Text and Data 1-5, Supplementary Figures 1-3 with legends and additional references. (PDF 360 kb)

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Zhu, X., Saito, S., Kemp, A. et al. Coherent coupling of a superconducting flux qubit to an electron spin ensemble in diamond. Nature 478, 221–224 (2011).

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