Abstract
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 and , during which the qubits complete only four and eight Larmor periods, respectively. The measured gate error of 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 -coupling rate (here 46 kHz), and the absence of either excessive parameter-matching or complex pulse-shaping requirements.
- Received 4 November 2020
- Accepted 17 March 2021
DOI:https://doi.org/10.1103/PhysRevX.11.021026
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Existing superconducting quantum processors are all based on a single type of quantum bit, or qubit, known as transmon, which resembles an oscillator more than it does a two-level system. The weak anharmonicity of transmons introduces many challenges for controlling quantum information, ultimately limiting the errors of logical operations. We describe the first high-fidelity two-qubit gate implemented on strongly anharmonic qubits known as fluxoniums.
A distinctive property of fluxoniums is that the qubit transition frequency—a measure of the energy difference between the two qubit states—is much lower than that of transmons: around 100 MHz compared to about 5–6 GHz. The prevailing intuition suggests that the rate at which qubits interact with one another scales with their frequency, and hence logical operations on the lower frequency fluxoniums should be much slower than on transmons.
Our result defies such intuition. Because fluxonium is not an ideal two-level system, there are transitions to higher energy levels that do not participate in storing information. By driving these “noncomputational transitions,” we demonstrate a type of fundamental quantum logic operation—a universal controlled-Z gate—that has a nearly state-of-the-art error and duration.
Our demonstration motivates research into low-frequency qubits, and it opens a new, fluxonium-based route to scalable quantum information processing.
