Sub-Harmonic Control of a Fluxonium Qubit via a Purcell-Protected Flux Line

TL;DR

This paper introduces a fluxonium qubit control scheme that suppresses decay through the control line using a low-pass filter, enabling high-fidelity, scalable quantum control with significantly improved coherence times.

Contribution

The authors demonstrate a Purcell-protected fluxonium control method utilizing sub-harmonic drives and filtering, achieving enhanced coherence and high-fidelity control.

Findings

Five times longer T1 coherence time with filtering

Ten times improved T2-echo coherence time

Sub-harmonic drives achieve >99.94% fidelity

Abstract

Protecting qubits from environmental noise while maintaining strong coupling for fast high-fidelity control is a central challenge for quantum information processing. Here, we demonstrate a control scheme for superconducting fluxonium qubits that eliminates qubit decay through the control channel by suppressing the environmental density of states at the transition frequency. Adding a low-pass filter on the flux line allows for flux-biasing and, at the same time, coherently controlling the fluxonium qubit by parametrically driving it at integer fractions of its transition frequency. We compare the filtered to the unfiltered configuration and find a five times longer $T_1$, and ten times improved $T_2$-echo time in the filtered case. We demonstrate coherent control with up to 11-photon sub-harmonic drives, highlighting the strong non-linearity of the fluxonium potential. Measured Rabi frequencies and drive-induced frequency shifts show excellent agreement with numerical and analytical models. Furthermore, we show the equivalence of a 3-photon sub-harmonic drive to an on-resonance drive by benchmarking sub-harmonic gate fidelities above 99.94$\,$%. These results open up a scalable path for full qubit control through a single Purcell-protected channel, providing strong suppression of control-induced decoherence and enabling wiring-efficient superconducting quantum processors.