Under the name prime decomposition (pd), a unique decomposition of an
arbitrary $N$-dimensional density matrix $\rho$ into a sum of seperable density
matrices with dimensions given by the coprime factors of $N$ is introduced. For
a class of density matrices a complete tensor product factorization is
achieved. The construction is based on the Chinese Remainder Theorem and the
projective unitary representation of $Z_N$ by the discrete Heisenberg group
$H_N$. The pd isomorphism is unitarily implemented and it is shown to be
coassociative and to act on $H_N$ as comultiplication. Density matrices with
complete pd are interpreted as grouplike elements of $H_N$. To quantify the
distance of $\rho$ from its pd a trace-norm correlation index $\cal E$ is
introduced and its invariance groups are determined.