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≈0.7×10^{7}, even though the dielectric loss quality factor of the circuit components is in the low-10^{5} range. The Ramsey coherence time exceeds 100μs, and the average fidelity of Clifford gates is benchmarked to 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.