The flux qubit revisited to enhance coherence and reproducibility

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 T 1 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 T 2 ≈85 μs, approximately the 2 T 1 limit. In addition to realizing an improved flux qubit, our results uniquely identify photon shot noise as limiting T 2 in contemporary qubits based on transverse qubit–resonator interaction.