For 2-qubits (pure or mixed) states, the Schmidt number (number of Schmidt coefficients) is at most 2. Using this and Peres–Horodecki criterion (for 2-qubits), a state is entangled if its partial transpose has at least one negative eigenvalue. Using the Schmidt coefficients from above, the negative eigenvalue is − − λ λ 0 λ λ 1 . The negativity N = − − λ λ 0 λ λ 1 of this eigenvalue is also used as a measure of entanglement – the state is more entangled as this eigenvalue is more negative (up to − − 1 2 for Bell states). For the state of subsystem A (similarly for B ), it holds that: ρ ρ A = tr B ⁡ ⁡ ( | ψ ψ ⟩ ⟩ A B ⟨ ⟨ ψ ψ | A B ) = λ λ 0 2 | 0 ⟩ ⟩ A ⟨ ⟨ 0 | A + λ λ 1 2 | 1 ⟩ ⟩ A ⟨ ⟨ 1 | A