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

TL;DR

This paper demonstrates a fast, microwave-activated controlled-Z gate on low-frequency fluxonium qubits with low error rates, highlighting advantages like long coherence times and simplified control compared to transmon qubits.

Contribution

It introduces a microwave-activated controlled-Z gate on low-frequency fluxonium qubits with competitive speed and error rates, leveraging their unique architectural benefits.

Findings

Gate error of (8±1)×10⁻³ limited by decoherence

Gate duration of 61.6 ns with only 4-8 Larmor periods

Faster than similar gates on transmons despite slower qubits

Abstract

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 $|10\rangle - |20\rangle$ and $|11\rangle - |21\rangle$, during which the qubits complete only $4$ and $8$ Larmor periods, respectively. The measured gate error of $(8\pm1)\times 10^{-3}$ 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.