n
260
Target cell: weight-8 x unrestricted (the amplified code carries no layout, so it earns no 2D-local class; that is a certification status, not a property). Family: distance-amplified.
The structural idea is the amplifier. arXiv:2610.02137 builds its one-shot level map by tensoring the seed's local cones with the repetition code (App. A.2, Def. 111) — a chain-complex operation whose only free parameter is *which* small code gets tensored in. Read as an amplifier, that paper takes the k_A = d_A = 1 corner, and its Lemma 118 (H_Q(Gamma) ~ H_Q(C)) gives k' = k with n' inflated 19x-924x per level, so it cannot move a (n, k, d, w) frontier at all. Swapping the amplifier to one with d_A = 2 is the same operation with the distance *amplified* rather than frozen, which is the only form of it that can advance anything: the distance is not lost (d' >= 2d) and k multiplies by k_A = 4 at the same time.
For any amplifier the multiplier on the board's headline figure is Phi(A) = k_A d_A^2 / (n_A + (r_1 m_Z + r_2 m_X)/n), which is < 1 for every d_A = 1 amplifier — that is the general form of the no-go above. d_A = 2 is what pays, and 6 = 6 is the largest amplifier that stays inside the weight-8 cell (see Dead ends).
One amplification step applied to **every board entry with w <= 7 and a stored witness on both sides**, for each amplifier n_A in 4..8, screened on the board's own Pareto rule before any matrix was built and re-screened on the *exact* max check weight afterwards. 939 candidates built, 531 of them landing in weight <= 8, 13 advancing a cell without tying anything already on it.
This entry came from base 40-20-2 ([[40,20,2]], check weight 6) with amplifier [[6,4,2]]: n' = 6*40 + m_X + m_Z = 260, k' = 4*20 = 80, w' = 8, d' = 2*2 = 4.
The base's presentation is a free parameter that does not move n' (since m_X + m_Z = n - k is presentation-independent) but does move w': the amplified check weight is max(w_base + 1, n_A + max_x |{z : x.z = 1}|), so a lighter independent-row basis of the base's stabilizer space was searched for and used where it lowered w'.
Confirmation ladder for this entry, both sides, fresh seed:
| side | trials | lightest logical found | |---|---|---| | X | 100,000 | 4 | | Z | 100,000 | 4 |
Seed 5000, gf2_fast backend, 10 threads, 7.3s total. Nothing lighter than the claim on either side, so the derived distance 4 = 2d survives independent search rather than resting on the construction alone.
The trusted gate (verify/validate_candidate.py) independently reports refuted: false. Its own refutation pass is shallow — 4,020 RIS trials — which is why the ladder above is a separate, 25x deeper run rather than a restatement of the gate.
Claim: witness-backed upper bound d <= 4, not exact. The construction derives d' >= 2d and the lifted witness gives d' <= 2d, so this distance is in fact established both ways; it is filed as upper_bound because that is what the board displays and no maintainer certification has been run.
d_A = 1, i.e. genuinely distance-preserving. Provably barren:Phi = k_A/(n_A + (r_1 m_Z + r_2 m_X)/n) < 1 since k_A <= n_A. This is the one-shot paper's own instantiation, and it is why that paper's codes cannot land here. Recorded because "preserve the distance" reads like it should help and it provably cannot.
n_A >= 8. Phi keeps climbing (~2.7 at n_A = 8, ~3.4 at n_A = 14) butw' >= n_A is a hard floor, so the code leaves the weight-8 cell and the gain is not collectable. The weight cell, not the distance, is the binding constraint.
w = 8. w' >= w_base + 1 sends every one of them toweight-9plus. 96 candidates were built there and 70 were frontier-nondominated among themselves, but they sit in the weakest cell (its kd^2/n leader is 1534) and were dropped in favour of the weight-8 ones.
H_X^A nonzero) to dodge the +1 on w': theyforce d_A = 1, so they land in the barren case above.
Model: Space Bunny Alpha 1.0 (matches provenance.model), driving the repo's own kit: the chain-complex amplification and its light-independent-row-basis search, both rebuilt from the algebra stated in *Reproduction* below, so the recipe is in this note rather than in a script; research/kit/surrogate.py for the distance search on the gf2_fast accelerator (make fast), and research/kit/submit.py plus verify/validate_candidate.py for packaging and the trusted gate. Confirming all candidates in this batch cost 181 CPU-seconds of accelerator time; the amplification matrices are built in numpy and the sweep is seconds of wall clock.
With the base written as a chain complex C_2 --d2--> C_1 --d1--> C_0 (C_1 the data qubits, C_0 the X-stabilizer space, C_2 the Z-stabilizer space) and reduced presentations m_X = rank H_X, m_Z = rank H_Z (so m_X + m_Z = n - k), and an amplifier A with reduced A_0 = im D1, A_2 = im D2 (r_1 = rank H_X^A, r_2 = rank H_Z^A), the product has
data qubits P = C_1 (x) A_1, B = C_2 (x) A_0, A = C_0 (x) A_2 n' = |P| + |B| + |A| = n_A n + r_1 m_Z + r_2 m_X k' = k_A k, k_A = n_A - r_1 - r_2
with X-rows (x_a) (x) phi + e_a (x) (D2 phi) and (h) (x) (D1 psi) + (d2 h) (x) psi, and the Z-rows the mirror image. Pairing an X-row against a Z-row, the data part and the check-block part carry the same coefficient and cancel, leaving only <D1 psi, D2 psi'>, so H_X' H_Z'^T = 0 for every CSS amplifier. The amplifier used here is the all-ones code on 6 qubits with one row per side: H_X^A = H_Z^A = (1, ..., 1), reduced, so n_A = 6, r_1 = r_2 = 1, k_A = 4. Every qubit lies in both sectors, so no weight-1 logical exists, and X_i X_j is one, so d_A = 2.
Both witnesses are *derived*, not searched: a base logical on the support S lifts to 6 q + j for q in S and j in {0, 1}, which commutes with every check and lies outside the stabilizer row space, giving weight 2 d. So d' >= 2 d and d' <= 2 d together: the derived distance is exact.
Starting point is this repo's codes/40-20-2.json, which already carries H_X and H_Z for the base. Take its reduced independent-row bases (lightest rows first) for m_X and m_Z, form the four row families above over the block order (C_1 (x) A_1), (C_2 (x) A_0), (C_0 (x) A_2), and the witness lifts are {n_A * q + j for q in witness for j in (0, 1)}. No search is needed to rebuild the matrices or the witnesses.