The transmon runs most of the superconducting quantum computers you can rent in the cloud today. It is simple, reliable, and well understood. But it has a ceiling. Its energy levels sit close together, which makes it vulnerable to stray noise and limits how cleanly its qubit states can be separated. A rival design called the fluxonium takes a different bet: add complexity to the circuit itself in exchange for a qubit that is harder to disturb.
What the circuit actually contains
A transmon is almost spartan. It pairs a Josephson junction with a capacitor, and that is nearly the whole story. A fluxonium keeps the junction but adds something unusual: a very large inductance, built from a long chain of dozens or even hundreds of Josephson junctions wired in series. This chain, sometimes called a superinductor, is the defining feature. It behaves like an enormous inductor without the stray capacitance that normal coils would bring.
That superinductor reshapes the qubit's energy landscape. Instead of the gently curved well of a transmon, the fluxonium sits in a double-well potential when it is biased at its sweet spot, half a flux quantum of magnetic flux threaded through the loop. The two lowest states live in different wells, with very little wavefunction overlap between them. Physically separating the states this way is the whole point.
Why the separation matters
Two numbers explain the appeal. The first is the energy gap between the qubit states. In a fluxonium at its sweet spot, that gap drops to a few hundred megahertz or lower, far below the roughly five gigahertz of a typical transmon. A lower transition frequency means the qubit is less sensitive to certain kinds of high-frequency noise and to the dielectric loss that plagues transmons.
The second is the matrix element, a measure of how strongly noise can drive a transition between states. Because the two fluxonium states barely overlap, many noise channels that would flip a transmon simply cannot get a grip. The upshot is coherence time. Fluxonium devices have demonstrated energy relaxation times stretching into the hundreds of microseconds and in some cases well past a millisecond, several times longer than comparable transmons on the same kind of chip.
Gate quality has followed. Research groups have reported two-qubit gate fidelities for fluxonium that rival or beat the best transmon results, with single-qubit operations reaching extraordinary precision. In the arithmetic of error correction, where every fraction of a percent of error compounds across thousands of qubits, those margins are not cosmetic. They change how expensive a fault-tolerant machine becomes.
The catches
None of this is free. The double-well potential that protects the qubit also makes it harder to control. The low transition frequency complicates readout, because distinguishing two states that sit close together in energy demands careful engineering of the measurement chain. Some fluxonium control schemes rely on driving through higher levels or using the qubit's richer energy structure, which adds complexity to the pulses that a transmon would not need.
The superinductor itself is a fabrication challenge. Stringing together a long array of junctions that all behave consistently is harder than making the single junction a transmon requires. Variation across that chain can shift the qubit's properties in ways that are difficult to predict and correct. Flux noise, the slow drift of magnetic flux through the loop, is another headache, since the qubit must be held precisely at its sweet spot to enjoy its best protection.
Who is chasing it
Fluxonium began as an academic design, pioneered in work associated with Yale and collaborators, and it has spent years maturing in university labs. More recently it has drawn startup interest. Atlantic Quantum, a venture with MIT roots, has publicly built its approach around fluxonium combined with cryogenic control electronics. Academic groups in the United States, Europe, and Asia continue to push record coherence and gate numbers. The large cloud providers remain committed to transmons for now, but the research signals are hard to ignore.
The broader lesson is that the superconducting field is not finished arguing about what a qubit should be. The transmon won the first round by being easy to build and operate at scale. Fluxonium is a reminder that easy is not the same as best. If its fabrication can be tamed and its control simplified, the heavy qubit's deeper protection could matter most exactly where it counts, in the enormous error-corrected machines the whole industry is trying to reach.
