Understanding and mitigating decoherence in fluxonium qubits – Energy relaxation

Fluxonium qubits with long coherence times and large anharmonicities are one of the promising alternatives to transmons for achieving higher-fidelity gates. While the coherence time of fluxonium qubits has improved greatly in the past few years to the hundreds of microseconds level, theoretical and numerical evidence indicates that even longer coherence times can be achieved. Here, we experimentally study the energy relaxation of 2D fluxonium qubits to understand and then mitigate decoherence sources. Using a high-bandwidth flux control, we measure the relaxation time over a wide range of flux biases, mapping out the noise spectrum over a wide frequency range. We find that spurious tunneling two-level systems and excess quasiparticles are the dominant sources of energy relaxation. By improving the device fabrication process and the shielding against stray high-energy radiation, we achieve relaxation times on the order of hundreds of microseconds for 2D qubits in the light-fluxonium regime. We will also discuss other decoherence mitigation techniques for fluxonium qubits.