Two quantum states ρ₁ and ρ₂ commute exactly when tr(ρ₁²ρ₂²) = tr(ρ₁ ρ₂ ρ₁ ρ₂).
The proof is almost immediate and holds for arbitrary self-adjoint operators acting on a Hilbert space, so we state it in this more general form
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Commutativity from a single Bargmann invariant equality
Two quantum states ρ₁ and ρ₂ commute exactly when tr(ρ₁²ρ₂²) = tr(ρ₁ ρ₂ ρ₁ ρ₂).