Equation Icon  2πSY Z (Ω)δ(Ω − Ω ′) = ⟨ψ | ˆNY (Ω) ∘Nˆ†Z(Ω ′)|ψ⟩ = ⟨NˆY (Ω ) ∘ Nˆ†Z(Ω′)⟩, (90 )