The flux qubit revisited to enhance coherence and reproducibility.

Research paper by Fei F Yan, Simon S Gustavsson, Archana A Kamal, Jeffrey J Birenbaum, Adam P AP Sears, David D Hover, Ted J TJ Gudmundsen, Danna D Rosenberg, Gabriel G Samach, S S Weber, Jonilyn L JL Yoder, Terry P TP Orlando, John J Clarke, Andrew J AJ Kerman, William D WD Oliver

Indexed on: 04 Nov '16Published on: 04 Nov '16Published in: Nature communications


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.