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Add example: logical error rates under coherent rotation (CSS(RM(1,4),RM(1,4))) - #568

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Add example: logical error rates under coherent rotation (CSS(RM(1,4),RM(1,4)))#568
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@sdoygb sdoygb commented Aug 13, 2026

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Closes #434 (part 2).

Exact state-vector verification of coherent-rotation logical error rates on
CSS(RM(1,4), RM(1,4)) = [[16,6,4]] (directionally complete, d = 4); companion
to the ReedMullerCSSCode class from #567.

  • Exp A: single-qubit rotation detection rate = sin²(θ/2), all Pauli types (dev < 3e-16)
  • Exp A1: weight-1 errors perfectly corrected — post-recovery loss < 2e-16
  • Exp B: logical Xbar flip = sin²(θ/2); Zbar flip = 0 (global phase)
  • Exp C: all-qubit coherent rotations → loss ~ θ^d, d = 4 (log-log slope 4.05)
  • Exp C2/D: closed-form coefficient c = C(16,2)·fail(2)·κ/16 = 3.0 (measured 2.98)
  • Exp E: random-Pauli (incoherent) control → loss ~ p² (slope → 2)

All numbers are machine-precision exact: symbolic |0_L>, exact rotations,
stabilizer projections, min-weight lookup-table decoding, coherent loss
formula — no Monte Carlo, no Pauli twirling. Full run ≈ 66 s.

sdoygb added 2 commits August 13, 2026 21:16
Self-orthogonal CSS code family CSS(RM(r,m), RM(r,m)) with parameters
[[2^m, 2^m - 2*sum_{j<=r} C(m,j), 2^(r+1)]] for 2r < m-1.

- Stabilizer generators: rows of the RM(r,m) generator matrix
  (pairwise orthogonal since RM(r,m) is strictly self-orthogonal)
- Closed-form distance 2^(r+1) via Reed-Muller minimum-weight theorem,
  making distance evaluation O(1) instead of exponential in block length
- X/Z distances equal; promise_equal_distance_xz=True
- Closes qLDPCOrg#566
…nt rotation

Exact (machine-precision) state-vector experiments on [[16,6,4]]:
- Exp A: single-qubit detection rate = sin^2(theta/2), all Pauli types
- Exp A1: weight-1 post-recovery loss < 1e-16 (directional completeness)
- Exp B: logical Xbar flip = sin^2(theta/2), Zbar flip = 0
- Exp C: all-qubit random rotations, loss ~ 0.138 theta^4.05 (slope 4 = d)
- Exp C2: uniform-angle R_X, slope 3.975, c -> 3.0 (closed form)
- Exp D: weight-2 degeneracy: fail(2)=0.875, kappa=0.457, c_pred=3.0000
- Exp E: random-Pauli control: loss ~ p^1.8 (incoherent, slope -> 2)

Theory: 10.29 / 10.31 / 10.35 / 10.36 (geometric theory framework).
@sdoygb
sdoygb requested a review from perlinm as a code owner August 13, 2026 14:15
@perlinm

perlinm commented Aug 14, 2026

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I am about to go on vacation, so I apologize advance that this may take me a few weeks to get to 🙂

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Add example: computing logical error rates of individual logical operators in a memory experiment.

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