Integer Fluxonium Qubit

Abstract

We describe a superconducting qubit derived from operating a properly designed fluxonium circuit in a zero magnetic field. The qubit has a frequency of about 4 GHz and an energy relaxation quality factor $Q \approx 0.7\times 10^7$, even though the dielectric loss quality factor of the circuit components is in the low $10^5$ range. The Ramsey coherence time exceeds 100 $\mu$s, and the average fidelity of Clifford gates is benchmarked to $\mathcal{F} > 0.999$. These figures are expected to improve with optimized fabrication and measurement procedures. Our work establishes a ready-to-use ``partially protected" superconducting qubit functioning in the frequency range of conventional transmons.