As highlighted in the work of Goscinski and Palma 9 (GP in the following), the eigenvalues and eigenvectors of ρ el (R, t) are easily determined by noting that [ρ el (R, t)] 2 = Tr[ρ el (R, t)]ρ el (R, t), such that the matrix ρ el (R, t) rescaled by its trace is idempotent, i.e.,