The Fluxonium Qubit
Fluxonium shunts a Josephson junction with a large superinductance, reaching high anharmonicity and long coherence at low frequency.
Beyond the transmon
Fluxonium replaces the transmon's capacitor shunt with a superinductor, an array of many Josephson junctions in series that behaves as a very large inductance without adding much dissipation. The result is a qubit with strong anharmonicity, often larger than its transition frequency, and reduced sensitivity to several noise channels.
Level structure
Fluxonium is typically operated at the flux sweet spot where its 0-1 transition sits low, sometimes below 1 GHz, and is first-order insensitive to flux noise. The low frequency reduces some loss channels; the large anharmonicity means the higher levels are far away, so leakage is suppressed and pulses can be simple.
Coherence
- Reported T1 and T2 into the hundreds of microseconds and beyond in careful devices
- High single- and two-qubit gate fidelities demonstrated at small scale
- Sensitivity to residual quasiparticles and flux noise off the sweet spot
Costs
The superinductor array is more complex to fabricate than a single junction, and the low operating frequency can complicate control and readout because thermal population is higher at a given temperature. Multi-qubit fluxonium processors are less mature than transmon arrays.
Fluxonium is a leading contender if transmon coherence and gate errors plateau. It illustrates a general theme in superconducting design: you can move along the anharmonicity-versus-noise axis by choosing a different circuit topology, not just different parameter values, and each topology brings its own fabrication and control demands.