Fluxonium fluxonium

A superconducting circuit shunted by a large junction-array inductance, putting the 0→1 transition at 0.2–1 GHz with gigahertz anharmonicity; Manucharyan, Koch, Glazman and Devoret, 2009. Charge dispersion vanishes.
Attributes: fabricated carrier; deterministic entangling ~50 ns (10^-7.3 s); dispersive readout ~0.3 µs, non-destructive, mid-circuit-capable.
Static mobility; low-frequency flux control at room temperature; Pauli plus coherent error; superconducting lithography.

Dielectric loss scales with ω, so T1 of 160–260 µs at ~300 MHz is routine where a transmon fights the same bath at 5 GHz D[39]. But kT/h ≈ 420 MHz at 20 mK: equilibrium leaves tens of percent in |1⟩, making active reset (~98% D[315]) mandatory. The floor is 1/f flux dephasing — T2* 20–110 µs against those T1 D[39], [316] — second-order at the sweet spot and untunable. In the record CNOT the dominant term is instead coherent: off-resonant control-qubit flips, residual error below 2×10⁻⁴ D[39]. A quieter bias generated closer to the chip is what moves the floor.

All one- or two-qubit devices; no fluxonium lattice or QEC cycle exists as of 2026-09-04.

D-Wave quotes 200 bias wires (2026-01-06) and ~300 bias lines (2026-01-23) for the same annealer scheme fact. Not published: the gate duration of MIT's 99.922% CZ, which the PRX abstract does not state, and a dated fluxonium-specific patent family.

Same Nb/Al junction lithography as transmon, inheriting its foundries and its TLS problem; record devices come from MIT Lincoln Laboratory and no merchant supplier exists. The I/O cost is drive, readout and a flux line per qubit: a fridge carries a few hundred DC bias lines, so the burden bites at 10³, and at 10⁴–10⁶ the bias must be generated cold. D-Wave flip-chips a multiplexed Φ-DAC die across a ~7 µm gap at 10 mK with no resolvable added decoherence (T1 ≈ 200 µs D[316]), key parts from NASA JPL C[281]; Shenzhen runs XY and Z down one warm line per qubit D[315]. Export exposure is generic superconducting (US EAR ECCN 3A901).

A drop-in Layer-1 alternative on the transmon lattice with tunable couplers, replacing transmon where coherence beats control simplicity; nothing downstream requires it. Its 60 ns gate does not set the derived clock — dispersive readout at ~0.3 µs does, as for transmon — so fluxonium buys error, not speed. Verification: the headline numbers are RB averages on isolated pairs, blind to lattice crosstalk, flux drift and fluxon leakage, and 99.94% is unreplicated; the main report's 99.92% record D[40] is superseded by it, and 99.922% is itself an RL-optimised mean (unassisted peaks 99.85–99.9%).

Money.
2025-10-03 · Google · acquisition of Atlantic Quantum · terms undisclosed · closed P[55]fact
2025-11-06 · Atlantic Quantum · absent from QBI Stage B (11 teams, ≤ $15 M each) · DARPA · official fact
2026-08-06 · D-Wave · H1-2026 revenue $5.9 M, cash $546.2 M · reported C[15]fact

Market & supply chain. No commercial supply chain: transmon's kit plus cryogenic DAC/SFQ control, built only in-house (D-Wave) or at government laboratories (JPL, MIT Lincoln Laboratory) — a single point of failure on the one scalable control path. Pays for G3/G4 and G7.

IP & standards. No dated fluxonium-specific patent family found as of 2026-09-04; PatSnap's 2026-06-30 superconducting-device counts are not broken out by carrier Pfact.

Roadmaps & track record. Atlantic Quantum (2025-04 · fluxonium machine under QBI · absorbed, no roadmap survives). D-Wave (2026-06-01 · DR17/DR49/DR181 · dual-rail cavity qubits, not fluxonium Pfact) — its fluxonium work is control IP with no product attached.

Strategic reading. The advantage over a good transmon is thin — Toshiba's double-transmon coupler gives CZ 99.90% in 48 ns D[37] — so fluxonium wins only if cold flux control lands first. Then cryogenic-control vendors and die-bonding capacity capture the value and power sits with the control-chip supplier, not the qubit fab.

Confirm by end-2027: a ≥ 4-qubit fluxonium under on-chip flux control with published two-qubit RB, or a replication of 99.94%; demote if neither appears. Best case 2029: 2Q error near 10⁻⁴ cuts the surface-code budget for a given Λ; worst case, switching noise from the control die eats the coherence advantage. Does the flux-noise floor survive multiplexing? Can 0.2–1 GHz readout reach 99.5% in a lattice? What became of the Atlantic Quantum team?