Integer Fluxonium Qubit

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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mi>Q</a:mi><a:mo>≈</a:mo><a:mn>0.7</a:mn><a:mo>×</a:mo><a:msup><a:mn>10</a:mn><a:mn>7</a:mn></a:msup></a:math>, even though the dielectric loss quality factor of the circuit components is in the low-<d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><d:msup><d:mn>10</d:mn><d:mn>5</d:mn></d:msup></d:math> range. The Ramsey coherence time exceeds <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><g:mn>100</g:mn><g:mspace width="0.2em"/><g:mtext fontfamily="times">μ</g:mtext><g:mrow><g:mi mathvariant="normal">s</g:mi></g:mrow></g:math>, and the average fidelity of Clifford gates is benchmarked to <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><m:mrow><m:mi mathvariant="script">F</m:mi></m:mrow><m:mo>></m:mo><m:mn>0.999</m:mn></m:math>. 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. Published by the American Physical Society 2024