III-V MBE heterostructures (InAs–Pb wires, QD sources) fab_mbe

Two unrelated devices share a tool class. The first is a III-V nanowire or 2DEG (two-dimensional electron gas) grown by molecular beam epitaxy (MBE) with a superconductor deposited in the same vacuum, so the interface is never exposed: aluminium through 2025, lead in the 2026 tetron as the higher-gap shell D[21]. The second is GaAs-based quantum dots as single-photon sources, same reactors, different physics fact. The lineage dates to 2015, when Copenhagen grew aluminium epitaxially on InAs nanowires without breaking vacuum Dfact; the field's 2018 headline, quantised Majorana conductance in InSb–Al wires, was retracted on 2021-03-08 D[365].
a mostly fabricated (0.75) · b a process, not a clock · c none · d none
e none · f disorder-dominated · g molecular beam epitaxy

The floor is disorder. Unintentional doping, interface roughness and shell strain produce trivial sub-gap Andreev states whose signatures mimic Majorana zero modes (MZMs), which is the Nature dispute: Legg argues the regions used for parity readout are disordered and gapless D[22]. Lead raises the gaps above aluminium's: in Microsoft's tetron stack the lead film's own gap is ≈1.3 meV and the gap it induces in the wire ≈570 µeV, against 295 µeV and 129 µeV with aluminium, while the topological gap, taken at the top quintile of its measured values, only doubles, from ~30 to ~70 µeV D[21], fact; epitaxial Pb on InAs nanowires holds a hard 1.25 meV gap to 8.5 T (Copenhagen, 2021) Dfact. Moving the floor means a mean free path well above the coherence length, verified by a published mobility or disorder metric rather than device outcomes. Microsoft published such metrics for InAs–Al — mobility 60,000–100,000 cm²/V·s, a charged-defect density of 2.7 × 10¹² cm⁻² Dfact — and for the Pb stack only Hall-bar values of its active region, a surface charge density of ≈2 × 10¹² cm⁻² and a buried-well mobility above 350,000 cm²/V·s D[21]; none is measured on a device, which is why the materials argument runs through transport data.

No yield or uniformity figure is published for either stack, and the Pb one's disorder figures come from Hall bars, not devices; the only public scaling aid is an rf method resolving wire-end-state splitting to µeV for per-device bring-up D[21].

For InAs–Al, Microsoft published an induced gap of 129 µeV, topological gaps of 20–60 µeV, a mobility and a charged-defect density (Physical Review B, 2023) fact, from a gap protocol whose reading Legg disputes D[22]; for the InAs–Pb stack the tetron paper gives the lead film's gap (≈1.3 meV), the induced gap (≈570 µeV), a top-quintile topological gap (~70 µeV) and Hall-bar charge density and mobility [21]. ECCN 3C907 covers only silicon or germanium with an isotopic impurity below 0.08% G[301]; whether a given III-V stack falls under 3C001.d depends on its layers and doping, and no BIS ruling addresses it. Not published: a yield metric for either stack, and a dated revenue or market-share figure for merchant MBE tool vendors.

Growth needs a III-V MBE reactor under ultra-high vacuum with in-situ superconductor deposition, keeping the interface clean enough for a hard gap. Microsoft's recipe is in-house, and since 2025-11-13 the full Majorana chip core is fabricated at Lyngby in Denmark Pfact; nobody outside has grown an equivalent stack, so one recipe in one building is the architecture's single point of failure D[22]. An independent III-V base exists for another geometry: Eindhoven-grown InSb wires carry QuTech's Kitaev devices D[216]. Export exposure predates the quantum rules: MBE growth equipment has long been on the Wassenaar dual-use list (US Export Control Classification Number 3B001.a.3), and a III-V hetero-epitaxial wafer falls, if anywhere, under 3C001.d fact; the BIS rule of 2024-09-06 added 3C907 only for epitaxial layers of isotopically enriched silicon or germanium, which leaves an InAs–Pb stack outside it G[301]P[818].

Everything in the topological architecture stands on this stack: encoding, readout and the unbuilt gate inherit its disorder. It provides no clock and no fidelity, only the ceiling on everyone else's; its one load-bearing fact is negative: no independent replication D[22]. The same tool class supplies quantum-dot single-photon sources to the fusion-based photonic architecture fact. The register lists four machines using it, among them Lucy, Belenos, Majorana 1 and Majorana 2. Verification here is materials verification and does not exist: the Nature exchange of 2026-06-24 disputes transport data from one laboratory's wafers, Microsoft conceding nothing D[22]; no second laboratory has grown the planar Pb stack, though Copenhagen put epitaxial Pb on InAs nanowires in 2021 Dfact.

Who.
| Organisation | Role | Country | What they do | Evidence |
|---|---|---|---|---|
| Microsoft Quantum | developer | US | Grows and consumes the InAs–Pb stack; fab in Lyngby | Pfact |
| DARPA | investor | US | Funds both users: US2QC final stage, QBIT Stage A | fact |

Money. 2025-02-06 · Microsoft · US2QC enters its final stage · undisclosed · DARPA · announced fact. 2025-11-13 · Microsoft · Lyngby (DK) Majorana chip-core fab · >DKK 1 bn national total, Microsoft's outlay undisclosed · announced Pfact. 2026-06-16 · Quandela · DARPA QBIT Stage A selection · announced fact.

Market & supply chain. MBE reactors are a small merchant market, but the tool is not the bottleneck: the recipe is, and it is not for sale. G3/G4 pay for the wires, G6 for the photon sources.

IP & standards. No dated patent count specific to this process as of 4 Sep 2026; no standard covers these interfaces.

Roadmaps & track record. (three wire generations, InSb–Al 2018 → InAs–Al 2025 → InAs–Pb 2026, promised as the base for fault tolerance by 2029; status 4 Sep 2026: materials delivered, no yield metric published) C[215]. Lyngby opened as announced; the retracted 2018 result is why outsiders discount later ones.

Strategic reading. While the recipe stays secret every claim above it is unfalsifiable from outside; independent replication either way is worth more than another Microsoft device. Success means a stack nobody can second-source; failure moves value to the InSb base.

Falsifiable in 12–24 months: an outside lab publishing an independently grown planar InAs–Pb device with parity readout; a published yield, or a disorder metric measured on Microsoft's Pb devices rather than Hall bars. Confirm on the first; without it the dispute is unresolvable. Best case, an academic group reproduces the stack; worst case, the recipe stays proprietary and the architecture unauditable to 2029. Open: the spread of topological gaps behind the top-quintile ~70 µeV; yield per wafer; whether anyone outside Microsoft grows the planar Pb stack.