Squeezed Light

Sub-Poissonian noise: beating the standard quantum limit in one quadrature
Phase Space (X vs P Quadrature)
ΔX = 0.71 (SQL = 0.707)  |  ΔP = 0.71
Squeezing: 0.0 dB  |  ΔX·ΔP = 0.50 (min = 0.50)
Time-Domain Noise Trace
Coherent: ΔX = ΔP = √(ħ/2) — circle in phase space
Squeezed: ΔX = e⁻ʳ√(ħ/2), ΔP = eʳ√(ħ/2)
ΔX · ΔP = ħ/2 — Heisenberg saturated
Squeezed states are produced by optical parametric amplifiers (OPAs) and four-wave mixing. LIGO uses 15 dB of squeezing to push below the standard quantum limit, enabling detection of gravitational waves that would otherwise be buried in shot noise.
The Wigner function of a squeezed state is a Gaussian — still positive. Only Fock states and superpositions show negative Wigner functions (true non-classicality).