Diagonalize ρ ρ and form the m × × ( d ∗ ∗ 3 ) matrix T , which rows are the m eigenvectors associated with the m largest eigenvalues e α α of ρ ρ . So T is formed by the most significant eigenstates of the reduced density matrix. You choose m looking to the parameter P m ≡ ≡ ∑ ∑ α α = 1 m e α α : 1 − − P m ≅ ≅ 0 .