A doubling construction for self-orthogonal codes

A simple construction of quaternary hermitian self-orthogonal codes with parameters $[2n+1,k+1]$ and $[2n+2,k+2]$ from a given pair of self-orthogonal $[n,k]$ codes, and its link to quantum codes is considered. As an application, an optimal quaternary linear $[28,20,6]$ dual containing code is found that yields a new optimal $[[28,12,6]]$ quantum code.