Fast flux gates through Josephson energy tuning in a heavy fluxonium

The fluxonium-type device, inductively shunted transmon (IST), reported in [1] demonstrated relaxation times of the order of hours between the two lowest fluxon states close to half-flux, establishing it as a promising device for information storage. The long relaxation times are achieved due to the strongly disjoint wavefunctions of the fluxon states created by a high potential barrier. However, this feature prevents efficient manipulation of the qubit states through microwave drives, due to the suppressed matrix elements. We present a fluxonium circuit with tunable Josephson energy, achieved by controlling the flux bias through a dc-SQUID. The flux tuning allows the circuit to transition between a ‘heavy’ regime, characterized by long relaxation times, and a ‘light’ regime, in which the qubit states can be manipulated. By tuning the qubit into the light regime, a continuous X-rotation in the qubit subspace can be performed while changing the external flux of the fluxonium rf-SQUID yields a continuous Z-rotation. Through simultaneous control of the flux biasing in the two loops of the circuit, a universal single-qubit gate set has been implemented. For an efficient implementation of the gate, the flux pulses are optimized to counter the distortion in the drive line and intrinsic dependence of the flux bias in the two loops. We present results characterizing gate fidelity and leakage errors in the experimental implementation of such a tunable fluxonium circuit. This approach eliminates the need for microwave drives in implementing gate operations, showcasing an efficient and dynamic approach to qubit control.

[1] Hassani, Farid, et al. Nature Communications 14.1 (2023): 3968.