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arXiv:2407.15783 · PRX Quantum

24 Days-Stable CNOT Gate on Fluxonium Qubits with Over 99.9% Fidelity

Wei-Ju Lin, Hyunheung Cho, Yinqi Chen, Maxim Vavilov, Chen Wang, Vladimir Manucharyan

Published Mar 11, 2025• 27 citations
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Abstract

Fluxonium qubit is a promising elementary building block for quantum information processing due to its long coherence time combined with a strong anharmonicity. In this paper, we realize a 60-ns direct gate on two inductively coupled fluxoniums, which behave almost exactly as a pair of transversely coupled spin- 1 / 2 systems. The -gate fidelity, estimated using randomized benchmarking, was as high as 99.94%. Furthermore, the fidelity remains above 99.9% for 24 days without any recalibration between measurements. Compared with the 99.96% fidelity of a 60-ns identity gate, our data brings the investigation of the nondecoherence-related errors during logical operations down to 2 × 10 − 4 . The present result adds a simple and robust two-qubit gate into the still relatively small family of the “beyond three nines” gates on superconducting qubits.

DOI
10.1103/prxquantum.6.010349

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PRX Quantum•May 2, 2024•63 citations
Fluxonium qubit is a promising elementary building block for quantum information processing due to its long coherence time combined with a strong anharmonicity. In this paper, we realize a 60-ns direct gate on two inductively coupled fluxoniums, which behave almost exactly as a pair of transversely coupled spin- 1 / 2 systems. The -gate fidelity, estimated using randomized benchmarking, was as high as 99.94%. Furthermore, the fidelity remains above 99.9% for 24 days without any recalibration between measurements. Compared with the 99.96% fidelity of a 60-ns identity gate, our data brings the investigation of the nondecoherence-related errors during logical operations down to 2 × 10 − 4 . The present result adds a simple and robust two-qubit gate into the still relatively small family of the “beyond three nines” gates on superconducting qubits.
The superconducting fluxonium qubit has a great potential for high-fidelity quantum gates with its long coherence times and strong anharmonicity at the half flux quantum sweet spot. However, current implementations of two-qubit gates compromise fluxonium's coherence properties by requiring either a temporary population of the non-computational states or tuning the magnetic flux off the sweet spot. Here we realize a fast all-microwave cross-resonance gate between two capacitively-coupled fluxoniums with the qubit dynamics well confined to the computational space. We demonstrate a direct CNOT gate in 70 ns with fidelity up to F=0.9949(6)\mathcal{F}=0.9949(6)F=0.9949(6) despite the limitations of a sub-optimal measurement setup and device coherence. Our results project a possible pathway towards reducing the two-qubit error rate below 10−410^{-4}10−4 with present-day technologies.
The superconducting fluxonium qubit has a great potential for high-fidelity quantum gates with its long coherence times and strong anharmonicity at the half-flux-quantum sweet spot. However, current implementations of two-qubit gates compromise fluxonium's coherence properties by requiring either a temporary population of the noncomputational states or tuning the magnetic flux off the sweet spot. Here we realize a fast all-microwave cross-resonance gate between two capacitively coupled fluxoniums with the qubit dynamics well confined to the computational space. We demonstrate a direct controlled not (cnot) gate in 70 ns with fidelity up to F=0.9949(6)\mathcal{F}=0.9949(6)F=0.9949(6) despite the limitations of a suboptimal measurement setup and device coherence. Our results project a possible pathway towards reducing the two-qubit error rate near to or below ${10}^{\ensuremath{-}4}$ with present-day technologies.
The fluxonium qubit is a promising candidate for quantum computation due to its long coherence times and large anharmonicity. We present a tunable coupler that realizes strong inductive coupling between two heavy-fluxonium qubits, each with ∼50\sim50∼50MHz frequencies and ∼5\sim5∼5 GHz anharmonicities. The coupler enables the qubits to have a large tuning range of XX\textit{XX}XX coupling strengths (−35-35−35 to 757575 MHz). The ZZ\textit{ZZ}ZZ coupling strength is <3<3<3kHz across the entire coupler bias range, and <100<100<100Hz at the coupler off-position. These qualities lead to fast, high-fidelity single- and two-qubit gates. By driving at the difference frequency of the two qubits, we realize a iSWAP\sqrt{i\mathrm{SWAP}}iSWAP​ gate in 258258258ns with fidelity 99.72%99.72\%99.72%, and by driving at the sum frequency of the two qubits, we achieve a bSWAP\sqrt{b\mathrm{SWAP}}bSWAP​ gate in 102102102ns with fidelity 99.91%99.91\%99.91%. This latter gate is only 5 qubit Larmor periods in length. We run cross-entropy benchmarking for over 202020 consecutive hours and measure stable gate fidelities, with bSWAP\sqrt{b\mathrm{SWAP}}bSWAP​ drift (2σ2 σ2σ) <0.02%< 0.02\%<0.02% and iSWAP\sqrt{i\mathrm{SWAP}}iSWAP​ drift <0.08%< 0.08\%<0.08%.
High-fidelity two-qubit gates have been demonstrated in systems of two fluxonium qubits; however, the realization of scalable quantum processors requires maintaining low error rates in substantially larger architectures. In this work, we analyze a system of four inductively coupled fluxonium qubits to determine the impact of spectator qubits on the performance of a \textsc{cnot} gate. Our results show that spectator-induced errors are strongly suppressed when the transition frequencies of the spectator qubits are sufficiently detuned from those of the active qubits. We identify favorable frequency configurations for the four-qubit chain that yield \textsc{cnot} gate errors below 10−410^{-4}10−4 for gate times shorter than 100 ns. Leveraging the locality of the nearest-neighbor coupling, we extrapolate our findings to longer fluxonium chains, suggesting a viable path toward scalable, low-error quantum information processing.
Superconducting qubits provide a promising path toward building large-scale quantum computers. The simple and robust transmon qubit has been the leading platform, achieving multiple milestones. However, fault-tolerant quantum computing calls for qubit operations at error rates significantly lower than those exhibited in the state of the art. Consequently, alternative superconducting qubits with better error protection have attracted increasing interest. Among them, fluxonium is a particularly promising candidate, featuring large anharmonicity and long coherence times. Here, we engineer a fluxonium-based quantum processor that integrates high qubit coherence, fast frequency tunability, and individual-qubit addressability for reset, readout, and gates. With simple and fast gate schemes, we achieve an average single-qubit gate fidelity of 99.97% and a two-qubit gate fidelity of up to 99.72%. This performance is comparable to the highest values reported in the literature of superconducting circuits. Thus our work, within the realm of superconducting qubits, reveals an alternative qubit platform that is competitive with the transmon system.
The fluxonium qubit is a promising candidate for quantum computation due to its long coherence times and large anharmonicity. We present a tunable coupler that realizes strong inductive coupling between two heavy-fluxonium qubits, each with approximately 50 -MHz frequencies and approximately 5 -GHz anharmonicities. The coupler enables the qubits to have a large tuning range of X X coupling strengths ( − 35 to 75 MHz). The Z Z coupling strength is < 3 kHz across the entire coupler bias range and < 100 Hz at the coupler off position. These qualities lead to fast high-fidelity single- and two-qubit gates. By driving at the difference frequency of the two qubits, we realize a i SWAP gate in 258 ns with fidelity 99.72%, and by driving at the sum frequency of the two qubits, we achieve a b SWAP gate in 102 ns with fidelity 99.91%. This latter gate is only five qubit Larmor periods in length. We run cross-entropy benchmarking for over 20 consecutive hours and measure stable gate fidelities, with b SWAP drift ( 2 σ ) < 0.02 % and i SWAP drift < 0.08 % . Published by the American Physical Society 2024