The Flux Qubit Revisited to Enhance Coherence and Reproducibility

TL;DR

This paper revisits the superconducting flux qubit design, achieving high coherence and reproducibility, and identifies photon shot noise as a key factor limiting qubit coherence times.

Contribution

The study demonstrates a flux qubit with enhanced coherence, broad tunability, and reproducibility, and reveals photon shot noise as a dominant dephasing source.

Findings

Relaxation times exceeding 40 microseconds at flux-insensitive point.

Consistent modeling of relaxation times with resonator loss and noise sources.

Photon shot noise identified as a key limit to qubit dephasing.

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 $40\,\mu$s 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 $T_2\approx 85\,\mu$s, approximately the $2T_1$ 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.