Ion–photon link ic_ionphoton

An ion decaying from an excited state emits a photon entangled with its qubit state; interfere two such photons from two traps on a beamsplitter and a coincident detection projects the distant ions into a Bell pair — heralded and probabilistic, unlike an in-trap gate, and the only mechanism coupling ions in separate vacuum systems. First shown in 2007 by Monroe's group (Michigan, later Maryland/Duke) — the lineage that holds today's rate record, not the Wineland/NIST line.

Attributes. Mobility: flying — the photon, not the ion, carries entanglement. Control modality @ placement: optical, room temperature — collection, interference and detection sit outside the trap's vacuum envelope.

The rate is the whole problem. Success probability per attempt goes as η², where η is collection solid angle × branching ratio × fibre coupling × detector efficiency, since both photons must survive; two in η is four in rate. With free-space objectives η sits in the low per cent, so published rates are 9.7 s⁻¹ over a 2-m link D[115] and 250 s⁻¹ at the record D[116] against the ~10⁴ s⁻¹ a QEC cycle would consume — 40× at best, closing quadratically through collection, not electronics.

Fidelity is the healthier budget: two-photon interference visibility, detector dark counts and jitter, and residual ion motion cap heralded fidelity at 94–97% D[115], [116]. Because failures are heralded, the error the code sees is loss, so low η costs throughput, not infidelity — an erasure channel at the interconnect layer. What moves the floor: cavity (Purcell) enhancement, waveguide-integrated collection in photonic-chip traps, telecom frequency conversion, emitter multiplexing per module, and quantum memories that let attempts accumulate rather than retry in lockstep.

Best demonstrated is a distributed computation, not just a Bell pair: Oxford ran a teleported CZ at 86.2(9)% and a distributed Grover search across a 2-m link D[115].

Dominant gap: throughput. Published rates sit 40× (the record) to ~10³× below the ~10⁴ s⁻¹ a networked QEC cycle needs; fidelity is not the blocker.

No fab and no wafer statistic — the "yield" is optical collection efficiency, an alignment and numerical-aperture figure. A link is a high-NA objective or in-vacuum cavity, single-mode fibre and polarisation control, a Bell-state-measurement apparatus with two single-photon detectors, and coincidence logic. Traps come from elsewhere: Infineon is the merchant trap foundry for IonQ/Oxford Ionics, eleQtron and Universal Quantum, Honeywell fabricates Quantinuum's in house Cfact. The concentrated input is detectors: the merchant SNSPD base is five vendors — Single Quantum, ID Quantique (owned by IonQ), Photon Spot, Quantum Opus and Moscow's Scontel Pfact. Cost per link is not quotable. Export exposure is indirect: the BIS rule of 2024-09-06 names no interconnect optics G[301]; it reaches only the machine, under 4A906, and 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.

Per link: one collection channel per ion, two detectors, a phase-stable optical path, a coincidence gate. Heralding latency is sub-µs, so the electronics limit nothing — the attempt rate does. At ~10 modules a handful of point-to-point links works; at 10⁴ any-to-any needs a switch fabric plus enough pair generation to feed inter-module lattice surgery, and no published design states a link count. Quantinuum's in-trap alternative exchanges ions at 2.5 kHz and transports through junctions at 4 m/s D[113] — an order faster than the best photonic link, which is why QCCD vendors have not adopted the technology.

It requires the trapped atomic ion and single-photon detection (SNSPD/TES) and provides for nothing downstream: a terminal capability, the exit from single-trap scaling. Architectures served: all three trapped-ion architectures, as primary — QCCD (Quantinuum), the linear Paul trap with laser addressing (AQT, Maryland/Duke, Quantum Art) and electronic-gate, chip-controlled ions (IonQ/Oxford Ionics, eleQtron). The graph lists no replacement or conflict, but in-trap transport is the functional rival and currently wins. Derived-clock contribution: dominant where used — at 250 s⁻¹ one inter-module pair costs 4.0×10⁻³ s, comparable to a full-width ion layer; at 9.7 s⁻¹ it is 1.0×10⁻¹ s.

Heralded fidelity is tomography or parity oscillation conditioned on a successful click, so 94–97% describes the links that happened, not the channel including loss; the honest figure of merit is rate × fidelity, which nobody reports. Published rates come from different species, distances and collection geometries and are not directly comparable. The weakest evidence is commercial: IonQ's release states it "validated the generation, transmission, and detection of photons used to enable quantum entanglement" — wording that stops short of reporting a measured Bell state, with no rate, fidelity or distance and no replication C[589]. Among peer-reviewed results there is no conflict.

The ">20 patents and patent applications" Lightsynq brought is IonQ's own count, with no family names, assignees or filing years. Not published: the Lightsynq deal terms, and a link count or switch-fabric topology for a 10⁴-module ion machine.

Money.

• 2025-05-06 · IonQ · M&A, ID Quantique (single-photon detectors) · undisclosed · — · closed Pfact
• 2025-06-03 · IonQ · M&A, Lightsynq (interconnect, quantum memory, >20 patents) · undisclosed · Harvard/AWS founders · closed C[776]
• 2025-09-17 · IonQ · M&A, Oxford Ionics · $1.075 B · — · closed C[18]
• 2025-11-06 · DARPA · QBI Stage B · up to $15 M each · IonQ and Quantinuum among eleven G[65]
• 2026-06-03 · Quantinuum · IPO, Nasdaq QNT · $1.68 B gross · — · closed G[123]
• 2026-07-31 · IonQ · M&A, SkyWater Technology · ~$1.8 B · — · closed C[19]

Market & supply chain. There is no market yet — five merchant detector houses and a few optics suppliers — and the largest natural customer owns one of the five. IonQ has bought the vertical: Lightsynq for interconnect and memory, ID Quantique for detectors, Oxford Ionics for gates, SkyWater for fabrication. Unit economics are not quotable. G6 pays directly; G4 pays as the way past single-trap ceilings; the rest do not.

IP & standards. The only dated IP fact is IonQ's statement that Lightsynq brought "over 20 patents and patent applications" in photonic interconnect and quantum memory — a company count with no family, assignees or filing years C[776]. No named patent family was found for ion–photon link hardware; no standards body governs it.

Roadmaps & track record. IonQ (promised 2025-06 · 2 M physical, 40–80 k logical by 2030 · status 2026-09-03: needs an interconnect ~40× faster than the best published rate) R[132]; its 2020 roadmap missed 4,000 qubits by 2026 by ~40×. Quantinuum (promised 2024-09 · Sol 2027, Apollo 2029 · no dated photonic-interconnect commitment) R[118]; Helios launched on schedule, so its dates carry weight.

Strategic reading. If heralded links reach production rates the single-trap ceiling stops mattering and every ion vendor becomes modular; IonQ wins that world because it owns each layer, and merchant detector vendors lose their largest customer to a competitor. If rates stay put, Quantinuum wins by ignoring the technology — in-trap transport at 4 m/s is already faster. Supplier power is negligible today and would rise only if link counts grew with module counts.

Confirm by end-2027 if any group publishes above 10³ s⁻¹, or IonQ discloses rate, fidelity and distance. Demote the networking-ready framing if by end-2028 nothing exceeds 10³ s⁻¹, four times the 250 s⁻¹ record. Best case by 2029: cavity or waveguide collection plus telecom conversion reaches 10³–10⁴ s⁻¹ and a three-node network appears. Worst case: rates stay at 10²–10³ s⁻¹ and modular ion machines use in-trap transport instead. Open questions: what did IonQ's link achieve; does cavity enhancement cost interference visibility; can frequency conversion survive the fidelity budget?