Quantum Noise and the Vacuum
Even in complete darkness, the electromagnetic field fluctuates — these vacuum fluctuations set a fundamental noise floor called the standard quantum limit. For coherent laser light, intensity and phase noise are equal and symmetric, forming a circular noise distribution in phase space. Squeezing breaks this symmetry, compressing noise in one direction while stretching it in the perpendicular direction.
Phase-Space Ellipse
In the Wigner function representation, a coherent state appears as a circular Gaussian blob displaced from the origin. Squeezing deforms this circle into an ellipse: the minor axis represents the reduced-noise quadrature, and the major axis the amplified-noise quadrature. The area of the ellipse is preserved by the Heisenberg uncertainty principle — you cannot reduce total noise, only redistribute it.
Generating Squeezed Light
Optical parametric amplifiers (OPAs) are the workhorse for producing squeezed light. A nonlinear crystal pumped by a strong laser correlates photon pairs, reducing fluctuations in one quadrature. Modern OPAs in bow-tie cavities routinely achieve 10-15 dB of squeezing. Four-wave mixing in atomic vapors and optomechanical systems offer alternative approaches.
LIGO and Beyond
The most dramatic application of squeezed light is in gravitational wave detection. Since 2019, both LIGO detectors inject squeezed vacuum to reduce shot noise, effectively increasing their range by ~15%. Future detectors plan frequency-dependent squeezing using filter cavities to reduce both shot noise (high frequency) and radiation pressure noise (low frequency) simultaneously.