Multi-photon resource-state generator (RSG; 6-ring etc.) src_resource

The source-plus-fusion subsystem that must manufacture, at code rate, the encoded graph states — canonically the 6-ring — that fusion-based fault tolerance consumes and destroys. Browne–Rudolph type-II fusion (2005) made it possible; Bartolucci et al. fixed the 6-ring as reference resource and set its loss budget S[179]. Best instance: MPQ Garching fused deterministically emitted photons into 6-rings and 7-trees, 8 photons, 0.4–2.3 coincidences/minute D[688]. Attributes: flying photons from fabricated emitters, heralded entangling, no fixed clock; electro-optic room-temperature control, pure-loss errors, photonic-IC fabrication.

An n-photon graph from k-photon primitives costs ~n/k fusions, each 50% unboosted or 75% boosted; failure is heralded and erases one of the two measured outcomes, but every attempt burns a photon that must be indistinguishable and lossless. Yield is brightness × indistinguishability × fusion success × transmission over the whole state — the exponent is photon count, not depth. Hence the code's tolerance: 2.7% loss per photon with 6-rings, ~17% only with 168-photon states nobody can build S[179]. A 2026 re-analysis sharpens this: fusion failure alone sets a logical-error floor at zero loss, forcing very large distance for 10⁻¹⁰, while emitter-mediated fusion raises the threshold to 7.0–7.3% against an unencoded 6-ring's 0.38–0.82% S[264]fact.

Rate is the gap: components clear 99%, but optical fusion has not passed eight photons since 2024.

In the microwave domain, fusion is already deterministic: a superconducting circuit fused on-demand time-bin cluster states by non-destructive Bell projection of flying microwave photons, with qubit reset and reuse, into graph states with genuine multipartite entanglement across 13 photonic qubits D[885]. Those photons cannot leave the cryostat without a microwave–optical transducer, itself an empty slot, so the result does not fill this optical slot.

Quandela's 12-qubit Belenos/Lucy delivery to CEA is dated 2025-10 and the Lucy/Joliot-Curie HPC launch 2026-04-14; no single source reconciles the two. No independent group has replicated the 2024 MPQ 8-photon record as of 4 Sep 2026.

No commercial resource-state generator exists; the nearest base is 300 mm photonic IC — GlobalFoundries' Quantum Technology Solutions Cfact and PIXEurope fact. Deterministic sources are the single point of failure: Sparrow Quantum, sole merchant vendor as of 4 Sep 2026, ships 20–35% system efficiency against its own 55.3% best device and USTC's 71.2% record Dfact; no cost or yield per state is public. I/O scales with photon count, not qubit count: a pump, switch tree and detectors per source, feed-forward within the photon's flight time per fusion — ~0.1–0.2 dB per switch against a ~0.5 dB budget at multiplexing depth ~10 D[169]. At 10³ states/s the wall is switch loss; at 10⁶, thousands of cryogenic detector channels under GHz feed-forward. ECCN 4A906 catches the machine; none names photon sources fact.

In the photonic fusion-based architecture (PsiQuantum, Quandela, QuiX): requires linear-optical fusion and single photons, provides the only input fusion-based fault tolerance accepts. It sets the architecture's clock — on a fusion architecture there is no syndrome round (gate layers + transport + readout + reset); its place is taken by generation at ~0.4–2.3 min⁻¹ D[688] against the ~10⁹ s⁻¹ a teraquop machine (10¹² reliable operations) needs; the 24–168-photon slot next door is empty. Verification: the record is coincidence-counted tomography with fidelity 0.34–0.85 by graph type, unreplicated; nearest independent work is 4-photon states D[824]. Every threshold quoted is simulation.

Money.
2025-04-10 · Sparrow Quantum · Series A · EUR 21.5 M · lead undisclosed · closed Pfact
2025-09 · PsiQuantum · Series E · USD 1 B at USD 7 B · closed Pfact
2026-07-22 · PsiQuantum · DARPA QBI Stage C expansion · USD 125 M · definitive fact

Market & supply chain. No merchant product; money goes to the inputs — source chips (Sparrow, a chokepoint) and fusion-capable PICs. Unit economics unquotable; only G3/G4 pay.

IP & standards. No dated patent family specific to resource-state generation as of 4 Sep 2026; the adjacent grant is ORCA's US 12,437,225 fact. No throughput standard exists.

Roadmaps & track record. (2025-07 · QuiX error-corrected generation 2027 · pending); (2025 · Quandela first logical qubit · missed; QBIT Stage A entered 2026-06-16 fact). No dated rate milestone exists.

Strategic reading. Generation rate, not fidelity, decides fusion photonics; a source vendor that also owns fusion gains leverage over integrators building sources in-house. If it fails, the architecture loses to CV/GKP and to matter qubits.

Confirm by 2028: a fused state above 16 photons published with fidelity and rate; demote if still single digits. Best case 2029: merchant emitters plus on-chip fusion reach ≥10³ states/s. Worst case: the subthreshold floor holds and all-linear-optics 6-rings are abandoned. Open: can multiplexed sources beat emitters per useful state; does Sparrow close its 20–35%-to-55% gap. Watch: any rate figure in DARPA Stage C V&V.