Abstract
We construct a quantum low-density parity-check code family from a length-512 Calderbank--Shor--Steane base matrix pair. The base pair is permutation-equivalent to the known SPC(3) product CSS code, and the present affine-coset description gives a direct proof that both Tanner graphs are (3,8)-regular with girth 8. The base code has parameters [[512,174,8]]. We then apply circulant permutation matrix (CPM) lifts. The main decoding experiment uses the CPM-lifted code with lift factor P=32, which has parameters [[16384,4142,≤ 40]], under the code-capacity depolarizing model. A belief-propagation decoder with post-processing achieved frame error rate about 10⁻⁸ at p=0.085, and one observed logical residual of weight 40 gives a decoder-derived upper bound d≤ 40.