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Nat Commun. 2016 Nov 3;7:12964. doi: 10.1038/ncomms12964.

The flux qubit revisited to enhance coherence and reproducibility.

Author information

1
Research Laboratory for Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
2
Department of Physics, University of California, Berkeley, California 94720-7300, USA.
3
MIT Lincoln Laboratory, Quantum Information and Integrated Nanosystems Group, 244 Wood Street, Lexington, Massachusetts 02420, USA.
4
Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
5
Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.

Abstract

The scalable application of quantum information science will stand on reproducible and controllable high-coherence quantum bits (qubits). Here, we revisit the design and fabrication of the superconducting flux qubit, achieving a planar device with broad-frequency tunability, strong anharmonicity, high reproducibility and relaxation times in excess of 40 μs at its flux-insensitive point. Qubit relaxation times T1 across 22 qubits are consistently matched with a single model involving resonator loss, ohmic charge noise and 1/f-flux noise, a noise source previously considered primarily in the context of dephasing. We furthermore demonstrate that qubit dephasing at the flux-insensitive point is dominated by residual thermal-photons in the readout resonator. The resulting photon shot noise is mitigated using a dynamical decoupling protocol, resulting in T2≈85 μs, approximately the 2T1 limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting T2 in contemporary qubits based on transverse qubit-resonator interaction.

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