Ion-chain motional bus (all-to-all in chain) cx_bus

Ions in one trap share collective motional modes; a spin-dependent force through a shared mode entangles any pair without moving them, so connectivity is all-to-all by construction. Cirac–Zoller (1995) and Mølmer–Sørensen (1999) gave the mechanism; chain gates date from 2003.
Bus mobility, one shared mode, no transport; MEMS-adjacent trap fabrication.
No control modality or readout of its own; coherent error — mode addressing and heating, not loss.

N ions carry N motional modes in a fixed bandwidth, so mode spacing falls as ~1/N: a gate must resolve one mode spectrally, costing time, or close all with shaped pulses, costing power and calibration. Gate time therefore rises with chain length — 550–883 µs, median 672 µs, on a 30-ion chain against 110 µs for one qubit D[100]. Electrode noise heats the shared mode, and error grows with N as every pair borrows the same excitation. Above a critical anisotropy the chain buckles into a zigzag: long chains need weak axial confinement, hence lower frequencies and slower gates. The floor moves with laser-free electronic gates (2Q 8.4×10⁻⁵ D[102]) and shaped pulses.

Tempo: 100 ions, #AQ 64, no gate time C[101]. Dominant term: gate time, not fidelity.

The chain's motional modes can also serve as a bosonic register rather than only as a gate bus. A three-ion ¹⁷¹Yb⁺ chain encoded a scalar field in one set of transverse modes (2.80–3.05 MHz) and evolved it with spin–phonon gates, avoiding the truncation of bosons into qubits, while Mølmer–Sørensen gates used the other transverse set so that the two do not interfere; measured phonon occupations matched simulation up to eight quanta per mode, and a fourth bosonic mode had to run in a separate circuit D[884]. The register is short-lived: motional coherence is ≲ 10 ms against ≈ 1.5 s of idle spin coherence D[884].

The 100-ion Tempo chain is a company claim; the largest independently benchmarked chain is 30 ions. Not published: Tempo's two-qubit gate time and per-pair fidelity, beyond #AQ 64 and a "99.9% fidelity" headline; KISTI's Tempo-100 contract value; and any heating, mode-spectrum or interconnect data that reconciles IonQ's 10,000-ions-on-one-chip 2027 target with the chain-length trade-off.

IonQ closed its SkyWater acquisition on 2026-07-31, naming microfabricated-trap fabs in Minnesota, Florida and Texas, no wafer size or node disclosed Cfact. Control splits two ways: laser MS gates need several wavelengths and per-ion optics; electronic gates use on-chip current and microwave lines. At 10³ ions the wall is the mode spectrum, not wiring: the answer is many short chains joined by shuttling or photonics — IonQ's first two-system entanglement (2026-04-14) disclosed no rate or fidelity Cfact. Export exposure sits with the finished machine: ECCN 4A906 controls it only when its qubit count and C-NOT error fall in the same band, from 34–99 qubits at ≤ 10⁻⁴ to any error from 2,000 qubits fact; no rule names ion traps G[301].

It is the primary connectivity of two trapped-ion architectures: the linear Paul trap with individual laser addressing and electronic qubit control (laser-free, microwave-driven gates). Requires nothing; provides the non-local checks bivariate bicycle qLDPC and high-rate transversal codes assume, as IonQ's eight codes on one 40-ion chain show, run without hardware reconfiguration, one of them at break-even within error bars D[109]. It competes with QCCD shuttling, which buys chain length with transport time: Helios gets all-to-all from a ring plus junction at ~55 ms per full-width layer D[97], and on H2 transport took ~60% of circuit time on average D[110]. Derived clock = sum of the syndrome round: gate layers + transport + readout + reset ≈ 1.5×10⁻³ s, gate layers 1.4 ms of it. Verification: Forte's figure is an all-pairs benchmark over 435 pairs, so position effects are visible; Tempo publishes #AQ 64 and "99.9% fidelity" with no gate time or per-pair data — a disclosure gap, not a dispute.

Who.
| Organisation | Role | Country | What they do | Evidence |
|---|---|---|---|---|
| IonQ | developer | US | Forte and Tempo chains; owns SkyWater | C[101] |
| Oxford Ionics | developer | UK | Electronic laser-free gates; IonQ unit | D[102] |
| Quantinuum | developer | US/UK | Rival transport architecture | D[97] |
| eleQtron | developer | Germany | MAGIC microwave ion control | C[129] |

Money. 2025-09-17 · IonQ · M&A, Oxford Ionics · $1.075 B USD · closed fact. 2025-10-12 · IonQ · equity · $2.0 B USD at $93/share · closed fact. 2025-11-06 · DARPA · QBI Stage B · up to $15 M each · IonQ and Quantinuum among eleven G[65]. 2026-05-05 · eleQtron · Series A · €57 M · closed C[129]. 2026-07-31 · IonQ · M&A, SkyWater · ~$1.8 B USD · closed fact. 2026-08 · IonQ · Q2-2026 revenue $80.1 M (+287% YoY), cash $3.0 B C[326].

Market & supply chain. The bus buys a trap chip, lasers or microwave lines and one vacuum system; concentration risk sits with narrowband laser and AOM vendors, whom electronic gates remove. It pays into G3 and G5.

IP & standards. PatSnap names IonQ the filing leader on motional-mode engineering Pfact; no standards body.

Roadmaps & track record. (2025-06 · for 2026 · 256 qubits at 99.99% · slipped to H1 2027); (for 2027 · 10,000 on one chip · no heating data); (for 2030 · 2 M physical) R[132]. Fidelity claims land, scale dates do not: the 2020 roadmap promised 4,000 qubits by 2026, missed ~40×.

Strategic reading. If chain all-to-all holds to a few hundred ions, ion vendors keep the connectivity advantage that makes high-rate codes cheap, and IonQ's owned fabs speed trap iteration. If not, the bus is a module-internal detail and value moves to interconnects.

Confirm if IonQ publishes Tempo gate times and per-pair fidelities, or ships 256 qubits in H1 2027; demote if disclosure stays headline-only. Best case 2029: electronic gates on owned-fab traps carry chains past 100 ions. Worst case: chains stall near 30–50 ions. Open: how does mode heating scale at 100 ions?