Overcoming the Rotating-Wave Approximation in Fluxonium with Circularly Polarized Driving (Part 2)

Fluxonium qubits are an attractive candidate for gate-based quantum computing due in part to their long coherence times. Two general features of a typical fluxonium are its low qubit frequency (less than 1 gigahertz) and its high anharmonicity (several gigahertz). One consequence, however, is related to the single-qubit gate speed: as the qubit drive power is increased, the rotating wave approximation breaks down before the leakage into non-computational states dominates the gate error. This is notably different with transmon qubits, which have a high qubit frequency but low anharmonicity.



In this work, we circumvent the aforementioned limitation by simultaneously performing linear charge and flux drives on a fluxonium superconducting qubit to create the circuit-quantum-electrodynamics analogue of circularly-polarized light. With this, we show both theoretically and experimentally that the rotating-wave approximation can be completely bypassed and use this technique to calibrate single-qubit gates as short as 10 ns with above 99.99% fidelity. In this second part of the talk, we detail gate calibration procedures and discuss current factors limiting gate fidelities.