Fluxonium

Fluxonium is a type of superconducting qubit, one of the tiny circuits that can act as the '0 and 1' of a quantum chip. It is built from a single small Josephson junction shunted by a very large inductance, so a chip designer can think of it as a cousin of the more common transmon, tuned for a different set of tradeoffs.

That large inductance is the trick. Instead of a real coil, it is made from a long chain of dozens to hundreds of junctions in series, a so-called superinductance. This reshapes the qubit's energy levels so that the lowest two states sit far apart from the rest. That large 'anharmonicity' means a control pulse aimed at the 0-to-1 transition is much less likely to accidentally excite the chip into a useless higher state, and the design can also be parked at a sweet spot where it largely ignores magnetic noise, giving long coherence times.

The honest catch is complexity. Fluxonium's working frequency is low, so it often needs a magnetic flux bias and more elaborate control and readout than a transmon, and the long junction array is harder to fabricate reliably. It is a serious, fast-improving alternative to the transmon, but no single qubit design has 'won', and chips of either kind are still small and noisy.