Quadrature variances
Key insight: Squeezed states satisfy Heisenberg equality Δx·Δp = ħ/2, but redistribute the uncertainty — one quadrature below shot noise (1/2) while the other grows.
Wigner function: W(x,p) can be negative (non-classical!) for Fock states n≥1 and certain superpositions. Negativity is a witness of quantum weirdness.
Squeezing factor: Δx = e⁻ʳ/√2, Δp = eʳ/√2