Quantum observable expectations
A state and observable determine the mean of repeated outcomes.
Key knowledge
A normalized quantum state ψ and self-adjoint observable A give expectation ⟨A⟩ = ⟨ψ|A|ψ⟩. When the second moment exists, its variance is (ΔA)² = ⟨A²⟩ − ⟨A⟩².
Values and equations
⟨A⟩ = ⟨ψ|A|ψ⟩; (ΔA)² = ⟨A²⟩ − ⟨A⟩²
- ψ
- Normalized state vector
- A
- Self-adjoint observable; its own physical unit
- ⟨A⟩, ΔA
- Mean and standard deviation; same unit as A
Conditions and scope
- For unbounded operators, ψ must lie in suitable domains and the required moments must be finite.
- An expectation describes an ensemble mean; it need not be an allowed single-measurement eigenvalue.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Lecture 8 equation 2.28; Lecture 9 equations 2.42–2.48: expectation and variance.
2026-10-10 · Normalized-state expectation and finite variance only; operator domain conditions added explicitly.
Entry details
Permanent link to this entry ↗- Record identity
- physics-quantum-observables-expectation
- Revision
- university-reference.1
- Structured reference record
- University reference · University core expansion