Optical narrow-line qubit (ion S–D, atom clock) enc_opt

The ion realisation stores |1⟩ in |4S₁/₂, m = −1/2⟩ and |0⟩ in |3D₅/₂, m = −1/2⟩ of ⁴⁰Ca⁺, coupled by the 729 nm electric-quadrupole line D[253]. Innsbruck ran the Cirac–Zoller CNOT on two such ions in 2003 D[376]; the encoding carried its fault-tolerant universal gate set of 2022 D[255] and AQT's products. ⁸⁸Sr⁺ at 674 nm is the sister ion, its D₅/₂ lifetime measured on a single ion at NPL D[377]. The atom realisation uses ¹S₀ and ³P₀ of neutral strontium, the lattice-clock line, run in tweezers since 2019 D[378], with alkaline-earth Rydberg entanglement from 2020 D[379]. Attributes: natural carrier; no time, mobility or control of its own; errors Pauli plus leakage.

Three ceilings. Lifetime: τ(D₅/₂, ⁴⁰Ca⁺) = 1.168(7) s D[253], so T2 ≤ 2τ ≈ 2.3 s; decay from m = −1/2 may land in S₁/₂, m = +1/2, outside the qubit pair — leakage G. The Sr ³P₀ level outlives the >3 s atomic coherence measured in a tweezer clock D[378]. Field: ⁴⁰Ca⁺ has no hyperfine structure, hence no field-insensitive line; with g(D₅/₂) = 6/5 and g(S₁/₂) ≈ 2.002 the qubit line moves 0.40 μB B ≈ 5.6 kHz per µT, so 0.1 µT of noise shifts it 560 Hz, a radian in 0.3 ms S. Mitigations: shielding, mains feed-forward, dynamical decoupling, decoherence-free pairs. The J = 0 → J = 0 clock line has no first-order electronic Zeeman shift G. Laser: every gate references the laser phase, so T2 is the laser's coherence over the circuit. AQT measured Ramsey T2 = 90(30) ms on the optical qubit against 18(1) ms on the ground-state Zeeman qubit, naming magnetic noise from mains and neighbouring magnets beyond 25 ms D[253] — 26× below the lifetime bound S.

The shipped figure: IBEX Q1, 12 qubits, 1Q 99.97(1) %, 2Q 98.7(3) % over all pairs, QV 128 C[382]; MARMOT, 20 qubits, 2Q (98.47 ± 0.3) % on an 11-qubit register C[383]. Atoms lead on fidelity, ions on deployment.

No fabricated part; the item is the narrow-line laser — 729 nm (Ca⁺), 674 nm (Sr⁺), 698 nm (Sr) — locked to a high-finesse reference cavity G. AQT's demonstrator uses seven wavelengths (375, 397, 423, 515, 729, 854, 866 nm); the 854 nm quench laser exists only because the qubit level is metastable D[253]. IBEX Q1 fills two 19-inch racks, 2 m², <2 kW, at (22.0 ± 1.5) °C C[382].

One phase-stable laser per species, split into individually addressed 729 nm beams on the ion chain D[255]; fibre and modulator phase noise lands directly on the qubit S. Readout is electron shelving — D₅/₂ stays dark on the 397 nm cycling line — by camera or photodiode, ~99.9 % in 300 µs D[253]; atoms add ancilla-based readout with non-destructive conditional reset D[381].

Slot 2 of Trapped ions — linear Paul trap with individual laser addressing, and of Rydberg tweezer array — alkaline-earth(-like) (Yb/Sr), erasure-native, beside enc_hf and enc_omg. It requires either ion (S–D quadrupole transition) or ae_atom (¹S₀–³P₀ of Sr/Yb). It is replaced by enc_hf, whose field-insensitive ground-state pair trades the one-laser drive for Raman or microwave control. It provides single-laser universality and shelving readout, and the metastable manifold that the omg erasure scheme reuses S[333]; it defines the channel metric τ(D₅/₂) = 1.17 s. Register: IBEX Q1 (Marmot / PINE / LYNX) ✅ and University of Innsbruck / IQOQI research trap ✅; PIAST-Q 🔎 (20 AQT qubits, species unnamed — consistent with MARMOT); MAQCS and DiNAQC 🔎. Gap G-enc-clock is answered: the ledger's 10–100 ms is planqc's range in a press profile P[247], 30–300× below published clock-qubit arrays D[380], so it measures a laser, not the encoding.

AQT's 98.7(3) % is an all-pairs product average C[382]; a secondary 97.7 % in the register stays unreconciled. The demonstrator's 0.997(6) is a two-ion Bell-state fidelity, not randomised benchmarking D[253]. Caltech's 99.62(3) % is averaged over symmetric input states D[381]. LYNX publishes QV without qubit count or fidelity C[128]. No vendor reports T2 beside τ, as of 2026-09-26.

Not published: planqc's computing species and transition — its technology and LRZ pages name neither strontium nor the clock transition — and the species and product of PIAST-Q.

Money.

• 2024-07 · planqc · Series A · €50 M · closed P[247]
• 2026-09-09 · planqc / LRZ · MAQCS, first hardware delivered · almost €20 M (BMFTR) · under construction C[161]

Market & supply chain. One ultrastable laser and cavity per species is the concentration point. Pays into G2, G3 and G7.

IP & standards. No patent search was run; no standard defines reporting of laser-limited T2, as of 2026-09-26.

Roadmaps & track record. (AQT · LYNX first units · Q4 2026) C[128]; (planqc · MAQCS co-processor · end-2027, TRL 5 today, 6–7 targeted) R[161].

Strategic reading. On ions the encoding is AQT's legacy, its depth capped by τ; on atoms it is the best-gated clock qubit, and a clock-grade laser on a processor is the prize.

Confirm the atomic branch if planqc publishes clock-qubit T2 and CZ fidelity on MAQCS hardware by 2027-12-31; demote it to metrology if not. Confirm the ion branch if LYNX ships in Q4 2026 with per-pair 2Q figures. Open questions. (1) What laser linewidth would bring ⁴⁰Ca⁺ T2 to its 2.3 s bound? (2) Is planqc's 10–100 ms the lattice, the laser or the field? (3) How much D₅/₂ decay leaks versus flips? (4) Will AQT move to a hyperfine or omg species? (5) Can one clock laser serve 10³ sites?