Fast logic with slow qubits: microwave-activated controlled-Z gate on low-frequency fluxoniums

We demonstrate a controlled-Z gate between capacitively coupled fluxonium qubits with transition frequencies 72.3 textrm{MHz} and 136.3 textrm{MHz}. The gate is activated by a 61.6 textrm{ns} long pulse at the frequency between non-computational transitions |10rangle - |20rangle and |11rangle - |21rangle, during which the qubits complete only 4 and 8 Larmor periods, respectively. The measured gate error of \(8pm1\)times 10^-3 is limited by decoherence in the non-computational subspace, which will likely improve in the next generation devices. Although our qubits are about fifty times slower than transmons, the two-qubit gate is faster than microwave-activated gates on transmons, and the gate error is on par with the lowest reported. Architectural advantages of low-frequency fluxoniums include long qubit coherence time, weak hybridization in the computational subspace, suppressed residual ZZ-coupling rate here $46 kHz$, and absence of either excessive parameter matching or complex pulse shaping requirements.