Subharmonic Control of a Fluxonium Qubit via a Purcell-Protected Flux Line
J. Schirk, Florian Wallner, L. Huang, Ivan Tsitsilin, N. Bruckmoser, L. Koch, D. Bunch, Niklas J. Glaser, Gerhard B. P. Huber, Martin Knudsen, Gleb Krylov, Achim Marx, F. Pfeiffer, L. Richard, Federico Roy, João H. Romeiro, Meenakshi Singh, Lasse Södergren, E Dionis, Dominique Sugny · PRX Quantum · 2025
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 5-times-longer T 1 , and a 10-times-improved T 2 -echo time in the filtered case. We demonstrate coherent control with up to 11-photon subharmonic drives, highlighting the strong nonlinearity 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 subharmonic drive to an on-resonance drive by benchmarking subharmonic 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.