Classifying Logical Gates in Quantum Codes via Cohomology Operations and Symmetry

Abstract: We systematically construct and classify fault-tolerant logical gates implemented by constant-depth circuits for quantum codes using cohomology operations and symmetry. These logical gates are obtained from unitary operators given by symmetry-protected topological responses, which correspond to generators of group cohomology and can be expressed explicitly on the lattice using cohomology operations including cup product, Steenrod squares and new combinations of higher cup products called higher Pontryagin powers. Our study covers most types of the cohomology operations in the literature. This hence gives rise to logical $C^{n-1}Z$ gates in $n$ copies of quantum codes via the $n$-fold cup product in the usual color code paradigm, as well as several new classes of diagonal and non-diagonal logical gates in increasing Clifford hierarchies beyond the color code paradigm, including the logical $R_k$ and multi-controlled $C^m R_k$ gates for codes defined in projective spaces. Implementing these gates could make it more efficient to compile specific types of quantum algorithms such as Shor's algorithm. We further extend the construction to quantum codes with boundaries, which generalizes the folding approach in color codes. We also present a formalism for addressable and parallelizable logical gates in LDPC codes via higher-form symmetries. We further construct logical Clifford gates in expander-based codes including the asymptotically good LDPC codes and hypergraph-product codes. As a byproduct, we find new topological responses of finite higher-form symmetries using higher Pontryagin powers.