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 |10 ⟩-|20 ⟩ and |11 ⟩-|21 ⟩, during which the qubits complete only four and eight Larmor periods, respectively. The measured gate error of (8 ±1 )×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 low-frequency fluxoniums include long qubit coherence time, weak hybridization in the computational subspace, suppressed residual Z Z -coupling rate (here 46 kHz), and the absence of either excessive parameter-matching or complex pulse-shaping requirements.