Fluxonium qubits and long coherence times

Short answer: Fluxonium is a type of superconducting qubit that has shown much longer coherence times than the common transmon in published laboratory experiments. One 2023 paper reports an uncorrected coherence time of 1.48 +/- 0.13 ms and an average gate fidelity of 0.99991(1). A 2022 paper reports single-qubit fidelity of 99.97% and two-qubit fidelity of up to 99.72% on a small fluxonium processor. These are research results, and the authors' comparisons are their own.

Why look beyond the transmon

The transmon has been the leading superconducting qubit platform, and the 2022 paper credits it with achieving multiple milestones. The same paper notes that fault-tolerant quantum computing calls for operations at error rates significantly lower than the state of the art, so alternative superconducting qubits with better error protection have attracted growing interest. It describes fluxonium as a particularly promising candidate, featuring large anharmonicity and long coherence times.

A millisecond-coherence fluxonium qubit

A 2023 Physical Review Letters paper reports a superconducting fluxonium qubit with an uncorrected coherence time T2* of 1.48 +/- 0.13 ms. The authors say this exceeds the state of the art for transmons by an order of magnitude. They benchmark an average gate fidelity of 0.99991(1). They also report that even at the millisecond scale the coherence time is limited by material absorption, and could be improved with more rigorous fabrication. They suggest the demonstration may be useful for suppressing errors in next-generation quantum processors. A summary published with the paper describes the result as a lifetime 10 times longer than the previous best for this system.

A fluxonium processor with high-fidelity gates

A 2022 Physical Review Letters paper describes a fluxonium-based quantum processor that integrates high qubit coherence, fast frequency tunability and individual-qubit addressability for reset, readout and gates. Using simple, fast gate schemes the authors achieve an average single-qubit gate fidelity of 99.97% and a two-qubit gate fidelity of up to 99.72%. They state that this is comparable to the highest values reported in the literature on superconducting circuits, and conclude that fluxonium is an alternative platform competitive with the transmon within superconducting qubits.

What the results do not settle

Both papers are single-group experiments on small devices. Coherence time and gate fidelity are measured under each group's own conditions, so the figures from the two papers should not be added together or compared as if from one device. Neither paper reports a large processor or a full error-corrected computation, and claims of competitiveness with transmons are the authors' own conclusions.

FAQ

What is a fluxonium qubit?

A type of superconducting qubit that the 2022 paper describes as having large anharmonicity and long coherence times.

How long did the fluxonium qubit stay coherent?

The 2023 paper reports an uncorrected T2* of 1.48 +/- 0.13 ms.

How accurate were the gates?

The 2023 paper reports an average gate fidelity of 0.99991(1). The 2022 processor paper reports 99.97% for single-qubit gates and up to 99.72% for two-qubit gates.

What limits the coherence now?

The 2023 authors report that, even at millisecond scale, coherence is limited by material absorption.

Is fluxonium replacing the transmon?

Not according to these papers. They describe it as a competitive alternative in laboratory experiments.