Cups and Gates I: Cohomology Invariants and Logical Quantum Operations

We take initial steps towards a general framework for constructing logical gates in general quantum CSS codes. Viewing CSS codes as cochain complexes, we observe that cohomology invariants naturally give rise to diagonal logical gates. We show that such invariants exist if the quantum code has a structure that relaxes certain properties of a differential graded algebra. We show how to equip quantum codes with such a structure by defining cup products on CSS codes. The logical gates obtained from this approach can be implemented by a constant-depth unitary circuit. In particular, we construct a Λ-fold cup product that can produce a logical operator in the Λ-th level of the Clifford hierarchy on Λ copies of the same quantum code, which we call the copy-cup gate. For any desired Λ, we can construct several families of quantum codes that support gates in the Λ-th level with various asymptotic code parameters.