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

Abstract

We demonstrate a controlled-Z gate between capacitively coupled fluxonium qubits with transition frequencies $72.3\text{MHz}$ and $136.3\text{MHz}$. The gate is activated by a $61.6\text{ns}$ long pulse at the frequency between non-computational transitions $|10⟩-|20⟩$ and $|11⟩-|21⟩$, during which the qubits complete only $4$ and $8$ Larmor periods, respectively. The measured gate error of $(8\pm 1)\times 10^{-3}$ 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 $46\mathrm{kHz}$), and absence of either excessive parameter matching or complex pulse shaping requirements.

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