Protected Fluxonium Control with Sub-harmonic Parametric Driving

A key challenge in quantum information processing is protecting qubits from environmental noise while maintaining the strong coupling necessary for fast, high-fidelity control. Here, we demonstrate a novel control scheme for superconducting fluxonium qubits that eliminates qubit decay through the control channel by adding a low-pass filter on the flux line.

This configuration 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 experimentally compare the filtered to the unfiltered configuration and find a five times longer T1, and ten times improved T2-echo time in the filtered case.

Furthermore, we demonstrate coherent control with up to 11-photon sub-harmonic drives and determine Rabi frequencies and drive-induced frequency shifts for different sub-harmonics in agreement with numerical and analytical calculations. In addition, we show that a 3-photon sub-harmonic drive can achieve gate fidelities exceeding 99.94%, comparable to that of an on-resonance drive. These results establish a scalable approach to full qubit control through a single protected channel, substantially reducing qubit decoherence from the control line.