Rydberg tweezer array — alkaline-earth(-like) (Yb/Sr), erasure-native on the map bar: Alkaline-earth arrays

The architecture in the report (§8.3)

8.3.8 Alkaline-earth arrays — Rydberg tweezer array — alkaline-earth(-like) (Yb/Sr), erasure-native (atom_ae)

The idea. Ytterbium and strontium have a metastable ³P₀ level one clock-line photon above the ground state. Store the qubit there (the Metastable ('omg') erasure encoding on the Alkaline-earth(-like) atom (Yb/Sr) — erasure-native), and ground-state imaging light, dark to the qubit, reveals atoms that decayed out of it: errors arrive located, as erasures. The bet: a worse gate for a cheaper code. At 98% conversion the surface-code threshold rises from 0.94% to 4.15% [S][8]; measured, a metastable CZ errs 0.016(1), 38(6)% flagged, against 0.0060 raw on the ground-state ¹⁷¹Yb qubit [D][135], [141], [142].

What it needs. A relay: the encoding requires the metastable manifold; the Mid-circuit erasure check requires the encoding, since only a qubit dark to imaging can be checked unread; Erasure-adapted codes require the flags and Correlated / loss-aware decoding, since a plain matcher cannot use them. Break a link and the machine is an alkali array with a heavier atom. Five layers have one option; replaces edges name the retreats: ground-state or optical encodings, surface code, millisecond imaging, one-shot loading. One conflict: reload light (a magneto-optical trap, MOT, and a repumper) depletes stored atoms; a repumper-free MOT and ³P₀ shelving, 13 s lifetime, mitigate it [P][259].

Where it stands. Six machines, three devices (Table 8.3). Phoenix (Gen-1) and Gen-2, 1,180 ¹⁷¹Yb atoms (Oct 2023), is the largest gate-capable device, with the only two-qubit figure, 4.0×10⁻³ [C][148]; its toric code ran 90 rounds, but suppression vanished with reloading included [D][148]. Magne, 1,225 physical for ~50 logical, is being installed for early 2027 [R][11], [12], [164], [165]. The metastable Yb-171 erasure array (Princeton, 20 atoms) slowed [[4,2,2]] decay 1.9(4)× with unconditional erasure decoding [D][141], [142]. MAQCS and DiNAQC: a 1,000-qubit demonstrator at TRL 5 [C][161]; the neutral-ytterbium QPU began hardware validation in August 2026 [C][260]. 31 of 67 cells documented.

Exceptions and borrowings. All five fit here best; only Princeton runs the bet's encoding, check, code and decoder as primaries. Phoenix and Magne encode in the ¹⁷¹Yb ground-state nuclear spin (Yaqumo by inference); their erasures are imaged atom losses, as on rubidium [D][4]. Phoenix's metastable-encoding cell is denied by its own note (mislabelled), its code is an Atlas gap, ∅G-toric (code technology too narrow), its row joins ⁸⁷Sr and ¹⁷¹Yb generations. MAQCS encodes on the Sr clock line, 10–100 ms coherence [P][246], [247], in an optical lattice (∅G-lattice, shared with QUIONE), no code, an undisclosed core link; planqc's error-corrected line, LOGIQC, is ytterbium [C][160]. Yaqumo's fast readout is inferred from 17.6 µs imaging of spinless ¹⁷⁴Yb [P][261]: early stage. No machine has the Atom–photon cavity interface. Borrowers: OQC GENESIS (check), Superion 256 (encoding), IBEX Q1 (optical qubit), QUIONE (carrier). The architecture is drawn by species; erasure-native rests on one laboratory.

The next test. Consistent with §5.2: by mid-2027 Magne crosses the 1,180-qubit frontier (§8.5, F1b) and delivers 50 logical qubits, decoded with erasure flags on unconditioned data; loss detection alone confirms the carrier, not the encoding. By end-2027 a group outside Princeton and Caltech converts over 70% of a metastable CZ's error with residual below 3×10⁻³; if none passes 50% and the next Yb memory again loses suppression under reloading, the architecture demotes to a species choice within the alkali bet.

What the builders say. Atom Computing credits its ytterbium nuclear-spin qubits with record coherence, needed for deep algorithms [C][846], and calls its June 2026 toric-code run practical error correction [C][846]. Microsoft expects Magne, more than 1,200 physical qubits from Atom Computing, online in early 2027 [R][847]. Princeton's argument for the metastable qubit is overhead: fewer errors, and the remaining ones shaped to be easier to correct [D][141], [142]. planqc plans MAQCS, its 1,000-qubit demonstrator, as a co-processor at LRZ by the end of 2027 [R][161], and Yaqumo a ytterbium system of several hundred qubits with error correction by fiscal 2027 [R][260].

Machines of this architecture (register: 6, of them 3 devices, 3 gate-capable). Atom Computing — Phoenix (Gen-1) and Gen-2 (deployed, 1,180); Atom Computing + Microsoft, for QuNorth — Magne (announced, 1,225); Princeton (Thompson group) — metastable Yb-171 erasure array (demonstrated, 20); Yaqumo (+ Fujitsu) — neutral-ytterbium QPU (demonstrated); planqc — MAQCS (planned, 1,000); planqc, for the DLR Quantum Computing Initiative — DiNAQC (planned, 100).

Cells outside the architecture's slots (5). Phoenix (Gen-1) and Gen-2: L7 ∅G-toric (Atlas gap — toric code d=4 and d=6, up to 90 syndrome-extraction rounds; lower logical error at larger); DiNAQC: L7 undisclosed where the architecture expects a technology; DiNAQC: L8 undisclosed where the architecture expects a technology; MAQCS: L4 ∅G-lattice (Atlas gap — no Atlas node: optical-lattice grid ('egg carton made of light'), no per-atom tweezer move); MAQCS: L9 undisclosed where the architecture expects a technology.