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[[268,8,12]] d ≤
n
268
k
8
d
12
kd²/n
4.299
w
6
X/Z
1
g
0.0672
r
4.0
layers
1
swaps
1731

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Distance

X/Z asymmetry 1 · d_X ≤ 12, d_Z ≤ 12 · w_X = 6, w_Z = 6 (max(d_X,d_Z)/min(d_X,d_Z); each side carries its own earned tier: = certified exact, ≤ witness upper bound)
d_X 12 · witness weight 12 (claimed upper_bound)
witness found by @dorakingx · gf2_fast RIS ladder (verify/gf2_fast.cpp), both sides searched jointly · found at 2×104 trials · survived 2×107 trials · 2026-10-02
witness operator (support, 12 qubits)
[15, 39, 40, 75, 99, 100, 123, 125, 143, 155, 176, 186]
d_Z 12 · witness weight 12 (claimed upper_bound)
witness found by @dorakingx · gf2_fast RIS in the packaging search; the ladder searched both sides jointly · found at 4000 trials · survived 2×107 trials · 2026-10-02
witness operator (support, 12 qubits)
[84, 86, 96, 106, 214, 221, 222, 235, 236, 238, 246, 260]
certificate none yet · distance stands as a self-certified upper bound (d ≤)

Diagnostics

computed by the verifier from the parity checks, the layout, and the stored witnesses; shown as evidence, not used for ranking
girth H_X 6 · H_Z 6 (shortest cycle of each side’s Tanner graph; longer is friendlier to belief propagation)
check weights H_X 2–6 (mean 5.098) · H_Z 2–6 (mean 5.211)
qubit degrees H_X 1–3 (mean 2.511) · H_Z 1–3 (mean 2.489)
trapping sets H_X (1,1)×38 (2,0)×7 (3,0)×20 (smallest syndrome weight at each size, connected sets of up to 3 qubits)
full (size, syndrome weight): count census for H_X
(1,1): 38 (1,2): 55 (1,3): 175 (2,0): 7 (2,1): 64 (2,2): 167 (2,3): 272 (2,4): 996 (3,0): 20 (3,1): 112 (3,2): 482 (3,3): 1322 (3,4): 2402 (3,5): 7683 (3,6): 250 (3,7): 1140
trapping sets H_Z (1,1)×44 (2,0)×10 (3,0)×22 (smallest syndrome weight at each size, connected sets of up to 3 qubits)
full (size, syndrome weight): count census for H_Z
(1,1): 44 (1,2): 49 (1,3): 175 (2,0): 10 (2,1): 64 (2,2): 185 (2,3): 249 (2,4): 1006 (3,0): 22 (3,1): 137 (3,2): 486 (3,3): 1456 (3,4): 2268 (3,5): 7803 (3,6): 229 (3,7): 1149
witness diameter X 15.2971 · Z 16.1245 (Euclidean support diameter of the stored distance witnesses in the layout; an upper bound on the exhibited logicals’ spread, not a minimum over all logicals)

Verified 2D layout

as measured by the verifier: every check drawn over the submitted coordinates; the interaction radius is the longest dashed pair
r = 4
X checkZ checkqubit site (268)dashed: the pair setting the interaction radiushover a check to isolate its qubits; click to pin — repeated clicks cycle through overlapping checks; click empty space to release
routing cost 1731 nearest-neighbor SWAPs per round in total, at most 8 for one check (heuristic: MST lower bound on the layout, with one lattice step = the minimum qubit spacing 1; not a rank)

Construction & provenance

authors @dorakingx
provenance submitted through the challenge
novelty novelty not audited
construction Reduction of the board's codes/276-8-12.json ([[276,8,12]]) by 8 qubits at unchanged k, unchanged check-weight class and unchanged locality class, by one move applied to a fixpoint: graft a qubit away using a low-weight element of the stabilizer ROW SPACE. Let S be an element of the row space of H_X with support T and let a be in T. Replace one generator of the subset summing to S by S itself -- the span is unchanged, that generator being S plus the rest of the subset -- and add S into every other X row meeting a, so that a survives only in S. Then apply the CNOT fan-out from a to T\{a}, which acts on the check matrices as H_X[:, b] ^= H_X[:, a] and H_Z[:, a] ^= H_Z[:, b] for each b in T\{a}. Only S still meets a, so the first substitution turns S into the weight-1 stabilizer X_a and leaves every other X row alone; the second zeroes column a of H_Z, because commutation forces every Z row to meet T an even number of times. Qubit a is then disentangled and is deleted: n -> n-1, k unchanged, and the Z rows only LOSE an index, so their weights and support diameters cannot rise. The weight of S separates three regimes, and the difference is entirely in the clearing step R ^= S. |S| = 1: nothing is added anywhere, so the weights, the radius AND THE DISTANCE are all preserved exactly, and no distance search is needed -- this is the row-space form of the weight-1 stabilizer cleanup of Liang, Eberhardt and Chen (arXiv:2504.08887 Sec. III D step 4), which the implementation in research/local2d/boundary_engine.py applies only to literal weight-1 generator ROWS. |S| = 2: R loses a and toggles one other index, so its weight changes by 0 or -2 and can never rise. |S| >= 3: R can grow, so every touched row is checked against the code's weight class and its locality radius, measured in the source's own layout. This generalises the capped merge-graft I introduced with [[454,8,17]], which only ever built S from the generators a single qubit happens to lie in. Here the accepted grafts were . Low-weight row-space elements are enumerated as sums of at most three checks with a connected overlap pattern. Every graft with |S| >= 2 was accepted only if k was unchanged and a bit-packed RIS search found nothing lighter than 12, screened once and confirmed twice with independent seeds; those in-loop rungs are a filter, not the evidence. QUBIT POSITIONS ARE THE SOURCE'S: every surviving qubit keeps the coordinate it has in codes/276-8-12.json, so the layout is inherited rather than re-derived, and the reduction only deletes. The surviving qubits' indices into the source numbering are [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 13]... (full list implied by the coordinates in this file).
model Claude Claude Opus 5 (Claude Code) (claimed, not verified)
builds on arXiv:2504.08887
date 2026-10-02
notes Derived from codes/276-8-12.json, not an independent construction; the source's authors are @vprusso and the reduction is mine. The gate reports no exact duplicate and no WL-equivalent entry. It dominates [[275,8,11]], [[276,8,12]], [[288,8,12]] on (n, k, d, w), which therefore leave the frontier of every cell they share with it. The measured interaction radius is unchanged at 4, so the reduced code is no less local than the source. Distance is a witness-backed upper bound: the deepest null result is 20000000 fresh-seed RIS trials. Literature novelty is unverified.
family bivariate bicycle (a tag, not a ranking)
locality 2D-local single (computed from the layout)
weight class weight ≤ 6 (computed)

How this code was found

the research note submitted with this code · raw markdown · all notes

[[268,8,12]] — reduction of the board's [[276,8,12]]

Where the qubits came from

codes/276-8-12.json is @vprusso's construction; this is a reduction of it, following the precedent @mathysrennela set on this board of submitting a reduction of someone else's entry as a contribution in its own right. 8 qubits come out under the general form of the move below, and the accepted grafts were .

The move

Let S be any element of the row space of H_X — an element of the stabilizer group, not necessarily one of the published generators — with support T, and let a ∈ T.

1. Replace one generator of the subset summing to S by S itself. The span is unchanged: that generator equals S plus the rest of the subset. 2. Add S into every other X row meeting a, so a survives only in S. 3. Apply the CNOT fan-out from a to T \ {a}, which acts on the check matrices as H_X[:, b] ^= H_X[:, a] and H_Z[:, a] ^= H_Z[:, b].

After step 2 only S meets a, so step 3 turns S into the weight-1 stabilizer X_a and leaves every other X row alone; and column a of H_Z becomes the XOR of its entries over T, which commutation forces to be 0. Qubit a is disentangled and is deleted: n → n-1, k unchanged, and the Z rows only lose an index, so their weights and support diameters cannot rise.

The weight of S selects the regime, entirely through the clearing step R ^= S:

  • |S| = 1 — nothing is added anywhere, so weight, radius and distance are all

    • preserved exactly and no distance search is needed. This is the row-space form of the weight-1 stabilizer cleanup of Liang, Eberhardt and Chen (arXiv:2504.08887 Sec. III D step 4); research/local2d/boundary_engine.py fires only on a *literal* weight-1 generator ROW, and row-space membership is strictly weaker.

  • |S| = 2 — the row loses a and toggles one other index, so its weight changes by 0

    • or −2 and can never rise.

  • |S| ≥ 3 — the row can grow, so every touched row is checked against the code's weight

    • class and its locality radius, measured in the source's own layout.

Low-weight row-space elements are enumerated as sums of at most three checks with a connected overlap pattern.

Verification

Fresh-seed RIS ladder: 12 at 20k → 200k → 1M → 5M → 20M trials per rung, independent seeds, both sides searched jointly. Nothing lighter than 12 appeared. Each graft was also screened and confirmed twice in the loop with independent seeds; those rungs are a filter, not the evidence. The distance is a witness-backed upper bound — on this project a [[682,10,38]] candidate survived three fresh seeds to five million trials and then fell at twenty million, so a flat ladder is not a proof.

verify/validate_candidate.py passes: board_advancing: true, dominated_by: [], no duplicate and no WL-equivalent entry, cell weight-6 × local-2d-single.

Numbers

Max check weight 6 throughout. Interaction radius unchanged at 4, so the reduced code is no less local than the source; layers unchanged. Qubit positions are the source's — every surviving qubit keeps the coordinate it has in codes/276-8-12.json, and the reduction only deletes. kd²/n 4.174 → 4.299.

It dominates 275-8-11, 276-8-12, 288-8-12 on (n, k, d, w).

Parity checks

X-checks 132 (max weight 6) · Z-checks 128 (max weight 6)
H_X (132 checks, sparse supports)
[0, 133, 134] [1, 134, 135] [2, 135, 136] [3, 136, 137] [4, 137, 138] [5, 138, 139] [6, 139, 140] [7, 140, 141] [8, 141, 142] [9, 142, 143] [0, 10, 145, 146] [1, 11, 146, 147] [2, 12, 147, 148] [3, 13, 148, 149] [4, 14, 149, 150] [5, 15, 150, 151] [6, 16, 151, 152] [7, 17, 152, 153] [8, 18, 153, 154] [9, 19, 154, 155] [2, 10, 21, 132, 157, 158] [3, 11, 22, 133, 158, 159] [4, 12, 23, 134, 159, 160] [5, 13, 24, 135, 160, 161] [6, 14, 25, 136, 161, 162] [7, 15, 26, 137, 162, 163] [8, 16, 27, 138, 163, 164] [9, 17, 28, 139, 164, 165] [18, 29, 140, 165, 166] [12, 21, 33, 144, 168, 169] [13, 22, 34, 145, 169, 170] [14, 23, 35, 146, 170, 171] [15, 24, 36, 147, 171, 172] [16, 25, 37, 148, 172, 173] [17, 26, 38, 149, 173, 174] [18, 27, 39, 150, 174, 175] [19, 28, 40, 151, 175, 176] [20, 29, 41, 152, 176, 177] [30, 42, 153, 177, 178] [23, 33, 44, 156, 180, 181] [24, 34, 45, 157, 181, 182] [25, 35, 46, 158, 182, 183] [26, 36, 47, 159, 183, 184] [27, 37, 48, 160, 184, 185] [28, 38, 49, 161, 185, 186] [29, 39, 50, 162, 186, 187] [30, 40, 51, 163, 187, 188] [31, 41, 52, 164, 188, 189] [32, 42, 53, 165, 189, 190] [35, 44, 56, 167, 192, 193] [36, 45, 57, 168, 193, 194] [37, 46, 58, 169, 194, 195] [38, 47, 59, 170, 195, 196] [39, 48, 60, 171, 196, 197] [40, 49, 61, 172, 197, 198] [41, 50, 62, 173, 198, 199] [42, 51, 63, 174, 199, 200] [43, 52, 64, 175, 200, 201] [46, 56, 68, 179, 203, 204] [47, 57, 69, 180, 204, 205] [48, 58, 70, 181, 205, 206] [49, 59, 71, 182, 206, 207] [50, 60, 72, 183, 207, 208] [51, 61, 73, 184, 208, 209] [52, 62, 74, 185, 209, 210] [53, 63, 75, 186, 210, 211] [54, 64, 76, 187, 211, 212] [55, 65, 77, 188, 212, 213] [58, 68, 79, 191, 215, 216] [59, 69, 80, 192, 216, 217] [60, 70, 81, 193, 217, 218] [61, 71, 82, 194, 218, 219] [62, 72, 83, 195, 219, 220] [63, 73, 84, 196, 220, 221] [64, 74, 85, 197, 221, 222] [65, 75, 86, 198, 222, 223] [66, 76, 87, 199, 223, 224] [67, 77, 88, 200, 224, 225] [70, 79, 91, 202, 227, 228] [71, 80, 92, 203, 228, 229] [72, 81, 93, 204, 229, 230] [73, 82, 94, 205, 230, 231] [74, 83, 95, 206, 231, 232] [75, 84, 96, 207, 232, 233] [76, 85, 97, 208, 233, 234] [77, 86, 98, 209, 234, 235] [78, 87, 99, 210, 235, 236] [81, 91, 103, 214, 238, 239] [82, 92, 104, 215, 239, 240] [83, 93, 105, 216, 240, 241] [84, 94, 106, 217, 241, 242] [85, 95, 107, 218, 242, 243] [86, 96, 108, 219, 243, 244] [87, 97, 109, 220, 244, 245] [88, 98, 110, 221, 245, 246] [89, 99, 111, 222, 246, 247] [90, 100, 112, 223, 247, 248] [93, 103, 114, 226, 250, 251] [94, 104, 115, 227, 251, 252] [95, 105, 116, 228, 252, 253] [96, 106, 117, 229, 253, 254] [97, 107, 118, 230, 254, 255] [98, 108, 119, 231, 255, 256] [99, 109, 120, 232, 256, 257] [100, 110, 121, 233, 257, 258] [101, 111, 122, 234, 258, 259] [102, 112, 123, 235, 259, 260] [105, 114, 237, 261, 262] [106, 115, 238, 262] [107, 116, 239, 263] [108, 117, 126, 240, 263, 264] [109, 118, 127, 241, 264, 265] [110, 119, 242, 265, 266] [111, 120, 128, 243, 266, 267] [112, 121, 129, 244, 267] [113, 122, 130, 245] [116, 249] [117, 250] [118, 251] [119, 126, 252] [120, 127, 253] [121, 254] [122, 128, 255] [123, 129, 256] [124, 130, 257] [125, 131, 258] [126, 261] [127, 262] [128, 263] [129, 264] [130, 265] [131, 266]
H_Z (128 checks, sparse supports)
[21, 132, 144] [0, 22, 133, 145] [0, 1, 23, 134, 146, 156] [1, 2, 24, 135, 147, 157] [2, 3, 25, 136, 148, 158] [3, 4, 26, 137, 149, 159] [4, 5, 27, 138, 150, 160] [5, 6, 28, 139, 151, 161] [6, 7, 29, 140, 152, 162] [7, 8, 30, 141, 153, 163] [8, 9, 31, 142, 154, 164] [9, 32, 143, 155, 165, 166] [33, 144, 156] [10, 34, 145, 157] [10, 11, 35, 146, 158, 167] [11, 12, 36, 147, 159, 168] [12, 13, 37, 148, 160, 169] [13, 14, 38, 149, 161, 170] [14, 15, 39, 150, 162, 171] [15, 16, 40, 151, 163, 172] [16, 17, 41, 152, 164, 173] [17, 18, 42, 153, 165, 174] [18, 19, 43, 154, 166, 175] [19, 20, 155, 176] [20, 177, 178] [44, 156, 167] [21, 45, 157, 168] [21, 22, 46, 158, 169, 179] [22, 23, 47, 159, 170, 180] [23, 24, 48, 160, 171, 181] [24, 25, 49, 161, 172, 182] [25, 26, 50, 162, 173, 183] [26, 27, 51, 163, 174, 184] [27, 28, 52, 164, 175, 185] [28, 29, 53, 165, 176, 186] [29, 30, 54, 166, 177, 187] [30, 31, 55, 178, 188] [31, 32, 189] [32, 190] [56, 167, 179] [33, 57, 168, 180] [33, 34, 58, 169, 181, 191] [34, 35, 59, 170, 182, 192] [35, 36, 60, 171, 183, 193] [36, 37, 61, 172, 184, 194] [37, 38, 62, 173, 185, 195] [38, 39, 63, 174, 186, 196] [39, 40, 64, 175, 187, 197] [40, 41, 65, 176, 188, 198] [41, 42, 66, 177, 189, 199] [42, 43, 67, 178, 190, 200] [43, 201] [68, 179, 191] [44, 69, 180, 192] [44, 45, 70, 181, 193, 202] [45, 46, 71, 182, 194, 203] [46, 47, 72, 183, 195, 204] [47, 48, 73, 184, 196, 205] [48, 49, 74, 185, 197, 206] [49, 50, 75, 186, 198, 207] [50, 51, 76, 187, 199, 208] [51, 52, 77, 188, 200, 209] [52, 53, 78, 189, 201, 210] [53, 54, 190, 211] [54, 55, 212] [55, 213] [79, 191, 202] [56, 80, 192, 203] [56, 57, 81, 193, 204, 214] [57, 58, 82, 194, 205, 215] [58, 59, 83, 195, 206, 216] [59, 60, 84, 196, 207, 217] [60, 61, 85, 197, 208, 218] [61, 62, 86, 198, 209, 219] [62, 63, 87, 199, 210, 220] [63, 64, 88, 200, 211, 221] [64, 65, 89, 201, 212, 222] [65, 66, 90, 213, 223] [66, 67, 224] [67, 225] [91, 202, 214] [68, 92, 203, 215] [68, 69, 93, 204, 216, 226] [69, 70, 94, 205, 217, 227] [70, 71, 95, 206, 218, 228] [71, 72, 96, 207, 219, 229] [72, 73, 97, 208, 220, 230] [73, 74, 98, 209, 221, 231] [74, 75, 99, 210, 222, 232] [75, 76, 100, 211, 223, 233] [76, 77, 101, 212, 224, 234] [77, 78, 102, 213, 225, 235] [78, 236] [103, 214, 226] [79, 104, 215, 227] [79, 80, 105, 216, 228, 237] [80, 81, 106, 217, 229, 238] [81, 82, 107, 218, 230, 239] [82, 83, 108, 219, 231, 240] [83, 84, 109, 220, 232, 241] [84, 85, 110, 221, 233, 242] [85, 86, 111, 222, 234, 243] [86, 87, 112, 223, 235, 244] [87, 88, 113, 224, 236, 245] [89, 90, 247] [90, 248] [114, 226, 237] [91, 115, 227, 238] [91, 92, 116, 228, 239, 249] [92, 93, 117, 229, 240, 250] [93, 94, 118, 230, 241, 251] [94, 95, 119, 231, 242, 252] [95, 96, 120, 232, 243, 253] [96, 97, 121, 233, 244, 254] [97, 98, 122, 234, 245, 255] [98, 99, 123, 235, 246, 256] [99, 100, 124, 236, 247, 257] [100, 101, 125, 248, 258] [101, 102, 259] [102, 260] [104, 105, 126, 240, 252, 261] [105, 106, 127, 241, 253, 262] [107, 108, 128, 243, 255, 263] [108, 109, 129, 244, 256, 264] [109, 110, 130, 245, 257, 265] [110, 111, 131, 246, 258, 266] [111, 112, 247, 259, 267] [88, 89, 225, 246]
Code ID 268-8-12 · download JSON · raw on GitHub