The Diagonal Distance of CWS Codes

Abstract: Quantum degeneracy in error correction is a feature unique to quantum error correcting codes, unlike their classical counterpart. It allows a quantum error correcting code to correct errors even when they can not uniquely pinpoint the error. The diagonal distance of a quantum code is an important parameter that characterizes if the quantum code is degenerate or not. If code has a distance more than the diagonal distance, then it is degenerate, whereas if it is below the diagonal distance, then it is nondegenerate. We show that most of the CWS codes without a cycle of length four attain the upper bound of diagonal distance d+1, where d is the minimum vertex degree of the associated graph. Addressing the question of degeneracy, we give necessary conditions on CWS codes to be degenerate. We show that any degenerate CWS code with graph $G$ and classical code C will either have a short cycle in the graph $G$ or will be such that the classical code C has one of the coordinates trivially zero for all codewords.