Fusion-based photonic (FBQC) on the map bar: Photon fusion

The architecture in the report (§8.3)

8.3.10 Photon fusion — Fusion-based photonic (FBQC) (ph_fusion)

The idea. Single photons in path or time-bin encoding are never joined by a gate: Linear-optical fusion (g_fusion) measures two photons from small pre-made resource states in the Bell basis, destroying both, and Fusion-based fault tolerance reads the logical state from the pattern of outcomes. The fact exploited: a photon does not decohere, loss is its dominant error, and a failed fusion announces itself: it erases one of its two measured outcomes, at a known place. The price is determinism. The bet: a foundry can build fault tolerance if each photon loses under 2.7% end to end, the threshold with 6-ring resource states [S][179].

What it needs. Fusion requires number-resolving detection to herald and switching with feed-forward to route the next photon within its flight time; the switches require the Photonic IC foundry, where each costs 0.1–0.2 dB of a ~0.5 dB per-photon budget [D][169]. The code requires the Multi-photon resource-state generator (src_resource), an empty slot: fused states reach 8 photons at under 1 Hz against 24–168 photons at MHz [D] (§7.6). A 2026 re-analysis adds a logical-error floor set by fusion failure alone [S][264]. Seven layers hold one technology each, and nothing inside the architecture replaces or conflicts; fusion itself forecloses the continuous-variable branch, replacing Gaussian gates as counting replaces homodyne detection.

Where it stands. Six machines, three devices (Table 8.3), no logical qubit. Omega chipset (PsiQuantum, February 2025), all ten technologies in slot, is a component set and is counted as such, not as a device: fusion Bell fidelity 99.22%, chip-to-chip Bell 99.72% over 42 m [D][169]; its 7.8×10⁻³ is that single fusion, and its code and decoder cells are flagged no-QEC-cycle. Belenos and Lucy (Quandela, 12 qubits each; Lucy at CEA, 2025 [176]) are the largest gate-capable devices; their 6.2×10⁻² is Ascella's 2023 figure [D][265], and Lucy has 0 of 14 cells documented. The ETRI 8-photon silicon-photonic qubit chip publishes no error rate; Carina (QuiX Quantum, 8 qubits) awaits commissioning at DLR [177]; Quandela's Canopus (24 qubits) is announced for its cloud in 2026 [C][876].

Exceptions and borrowings. No cell falls off the architecture, yet it is drawn wider than its bet. Belenos, Lucy and the ETRI chip occupy the fusion technology with post-selected or probabilistic gates in a reconfigurable mesh, circuit-model linear optics that the register's own cells call "not FBQC fusion"; only Omega and Carina fuse. Quandela's fault-tolerance design is a hybrid of quantum emitters and linear-optical gates [266]. Both Quandela machines carry an Atlas gap, ∅G-twofab: a III-V quantum-dot source and a SiN/silica mesh, two fabrication technologies where the map allows one (layer 10 too narrow). Omega and Belenos list the resource-state generator, an empty slot, as an alternate technology; Belenos's own cell calls the route undemonstrated (mislabelled). The other four have no code, decoder or interconnect: early stage. On other architectures, Aurora, Jiuzhang 4.0 and Borealis use its detector technology.

The next test. By March 2028 (§5.2), a chain of three or more fusions, or a resource state beyond Bell pairs fused into a network, published with an end-to-end success rate, would show that the architecture's real clock, resource-state supply, exists. Demoted in October 2026: PsiQuantum's 2027 target moved — its June 2026 release dates only the cryoplant (second half of 2027) and names no operating date. Promoted back by a dated first system or a fusion-based logical qubit; demoted further if only single-fusion figures remain, fused states stay at eight photons, or PsiQuantum's Stage C validation [181] comes back negative.

What the builders say. PsiQuantum's co-founder, Jeremy O'Brien, argues that a useful quantum computer must fully leverage the semiconductor industry [C][851]. In 2024 PsiQuantum planned its Brisbane site operational by the end of 2027, its first system near a million physical qubits [R][852]; in June 2026, breaking ground at a new site in Moreton Bay, it set cryoplant delivery for the second half of 2027, with commissioning and chip cabinets to follow, naming no operating date [R][853]. Quandela plans 100+ logical qubits in 2030–31 [R][854] and says hundreds of logical qubits will draw under 500 kW [C][854]. QuiX promised a universal machine in 2026 and error correction in 2027 [R][340]; Carina's core hardware, designed for a universal gate set, reached DLR in July 2026 [C][855].

Machines of this architecture (register: 6, of them 3 devices, 3 gate-capable). Quandela — Quandela Canopus (announced, 24), Belenos (deployed, 12), Lucy (deployed, 12); ETRI (with KAIST and the University of Trento) — ETRI 8-photon silicon-photonic qubit chip (demonstrated, 8); PsiQuantum — Omega chipset (Moreton Bay / Chicago systems unnamed) (demonstrated); QuiX Quantum — Carina (announced, 8).

Cells outside the architecture's slots (3). Belenos: L10 ∅G-twofab (Atlas gap — needs two L10 nodes: SiN/silica interferometer meshes (fab_pic) plus GaAs quantum-dot sour); Quandela Canopus: L10 ∅G-twofab (Atlas gap — quantum-dot source (III-V MBE) plus photonic-chip interferometer — two L10 nodes as for Be); Lucy: L10 ∅G-twofab (Atlas gap — needs two L10 nodes: SiN/silica interferometer meshes (fab_pic) plus GaAs quantum-dot sour).