A topological theory for qLDPC: non-Clifford gates and magic state fountain on homological product codes with constant rate and beyond the $N^{1/3}$ distance barrier

Abstract: We develop a unified theory for fault-tolerant quantum computation in quantum low-density parity-check (qLDPC) and topological codes. We show that there exist hidden simplicial complex structures encoding the topological data for all qLDPC and CSS codes obtained from product construction by generalizing the Freedman-Hastings code-to-manifold mapping. This is achieved by building manifolds corresponding to high-dimensional topological expanders from the Tanner graphs of the skeleton classical or quantum codes, which further form a product manifold and an associated thickened product code defined on its triangulation with only a constant qubit overhead. This suggests that qLDPC or more generally CSS codes obtained from product constructions are topological, and hence can admit cohomology operations such as cup products, physically corresponding to higher symmetries in the underlying topological quantum field theory. When applying this mapping to a 3D hypergraph product code obtained from the product of 3 copies of good classical expander codes, we obtain the first non-Clifford logical CCZ gates via constant depth circuits on a code with constant stabilizer weight $w=O(1)$, constant rate $K=\Theta(N)$, and polynomial distance $D=\Omega(N^{1/3})$. When applied to 3D homological product codes consisting of the product of a pair of good quantum and classical LDPC codes, we can further improve the distance to $D=\Omega(\sqrt{N})$ exceeding the $N^{1/3}$ distance barrier implied by the Bravyi-K\"onig bound for conventional topological codes. Our work suggests that it is feasible to apply native logical non-Clifford gates on qLDPC codes or directly inject high-fidelity magic states as resources (`magic state fountain') without the distillation process. For the homological product construction, the fountain can inject $\Theta(\sqrt{N})$ magic states in parallel in a single round.