Squeezed Light

Squeezed light is a quantum state of the electromagnetic field in which the uncertainty in one quadrature — amplitude or phase — is reduced below the vacuum level, at the cost of increased uncertainty in the conjugate quadrature. This is not a violation of the Heisenberg uncertainty principle; it is a redistribution of quantum noise, produced by nonlinear optical processes such as second-harmonic generation or parametric down-conversion in crystals like PPKTP. Squeezed light has become a practical technology in precision measurement, most notably in the A+ LIGO gravitational wave detectors, where it reduces quantum noise below the standard quantum limit that would otherwise cap interferometric sensitivity.

The production of squeezed light relies on an optical parametric oscillator driven below threshold, generating correlated photon pairs whose quantum fluctuations are anticorrelated. The degree of squeezing is measured in decibels of noise reduction relative to vacuum, with state-of-the-art sources achieving 10-15 dB of squeezing. The technology bridges quantum optics and macroscopic engineering: the same quantum state that demonstrates foundational nonclassicality in laboratory experiments now shapes the noise budget of tonne-scale interferometers.