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 and 136.3 MHz. The gate is activated by a 61.6-ns-long pulse at a frequency between noncomputational transitions vertical bar 10 > - vertical bar 20 > and vertical bar 11 > - vertical bar 21 >, during which the qubits complete only four and eight Larmor periods, respectively. The measured gate error of (8 +/- 1) x 10(-3) is limited by decoherence in the noncomputational subspace, which will likely improve in the next-generation devices. Although our qubits are about 50 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 lowfrequency fluxoniums include long qubit coherence time, weak hybridization in the computational subspace, suppressed residual ZZ-coupling rate (here 46 kHz), and the absence of either excessive parameter-matching or complex pulse-shaping requirements.