Gatemonium: A Voltage-Tunable Fluxonium

We present a new style of fluxonium qubit, gatemonium, based on an all-superconductor-semiconductor hybrid platform. The linear inductance is achieved using <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mn>600</a:mn></a:math> planar <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><d:mi>Al</d:mi></d:math>-<g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><g:mrow><g:mi>In</g:mi><g:mi>As</g:mi></g:mrow></g:math> Josephson junctions (JJs) in series. By tuning the single junction with a gate voltage, we demonstrate electrostatic control of the effective Josephson energy, tuning the weight of the fictitious phase particle. One- and two-tone spectroscopy of the gatemonium transitions further reveal details of the hybrid plasmon-fluxon spectrum. Accounting for the nonsinusoidal current-phase relation of the single junction, we fit the measured spectra to extract charging and inductive energies. We conduct time-domain characterization of the plasmon modes and find that energy-relaxation times are limited by inductive loss, possibly in the thin aluminum film. We discuss future directions for this platform in gate-voltage-tunable high-plasma-frequency enhanced-impedance junction arrays, and enhanced coherence times for voltage-tunable architectures.