Pure and mixed density operators
A density operator represents quantum statistical states.
Key knowledge
In finite dimensions, a density operator is Hermitian, positive semidefinite and has trace one. A pure state has ρ = |ψ⟩⟨ψ|; mixtures have ρ = Σ_j p_j|ψ_j⟩⟨ψ_j|.
Values and equations
ρ = Σ_j p_j|ψ_j⟩⟨ψ_j|; ρ† = ρ; ρ ≥ 0; Tr ρ = 1
- ρ
- Dimensionless density operator
- p_j
- Ensemble weights
- ψ_j
- Normalized state vectors
- †
- Hermitian adjoint
- Tr
- Trace
Conditions and scope
- States ψ_j are normalized and p_j ≥ 0 with Σ_j p_j = 1.
- A mixture's ensemble decomposition need not be unique; a coherent superposition is a different construction.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Chapter 3 sections 2.2.1, 2.2.4 and 3: pure states, decomposition ambiguity and density conditions.
2026-10-10 · Finite-dimensional state definition only; infinite-dimensional trace-class extensions excluded.
Entry details
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- physics-density-operator
- Revision
- university-reference.1
- Structured reference record
- University reference · University core expansion