The Robertson–Schrödinger uncertainty relation may be generalized in a straightforward way to describe mixed states. σ σ A 2 σ σ B 2 ≥ ≥ | 1 2 tr ⁡ ⁡ ( ρ ρ { A , B } ) − − tr ⁡ ⁡ ( ρ ρ A ) tr ⁡ ⁡ ( ρ ρ B ) | 2 + | 1 2 i tr ⁡ ⁡ ( ρ ρ [ A , B ] ) | 2 .