What Is Optical Squeezing?

Optical squeezing is the process of redistributing the quantum uncertainty of a light field so that fluctuations in one measurable property fall below the vacuum level at the expense of increased fluctuations in a conjugate property. The resulting field is called a squeezed state, or squeezed light, and it belongs to the family of nonclassical states studied in quantum optics. The underlying constraint is the Heisenberg uncertainty relation between the two quadratures of the electromagnetic field, the amplitude-like and phase-like components that oscillate ninety degrees apart. A coherent state distributes uncertainty equally between them; a squeezed state deforms that distribution into an ellipse without shrinking its area.

The concept was developed theoretically in the 1970s and first observed experimentally in 1985 by Slusher and colleagues at Bell Laboratories using four-wave mixing in sodium vapor. What makes squeezing more than a curiosity is that many precision measurements are limited by vacuum fluctuations entering an instrument through an unused port. Replacing that vacuum with a squeezed vacuum reduces the noise in whichever quadrature the measurement actually reads.

Quadrature Noise and the Uncertainty Bound

A single-mode field can be written in terms of two quadrature operators whose commutator fixes a minimum product of variances. Squeezing is quantified in decibels below the shot noise level of a coherent state at the same optical power, so a source described as 10 dB squeezed has one quadrature variance ten times smaller than vacuum. Because the uncertainty product is conserved, the orthogonal quadrature carries a correspondingly larger variance, and any measurement that samples the wrong quadrature is worse off than it would have been with ordinary light. The orientation of the noise ellipse, called the squeeze angle, must therefore be locked to the measurement quadrature, and phase noise in that lock is a leading source of degradation.

Generating and Preserving Squeezed Light

Practical squeezed sources rely on parametric nonlinearity. A degenerate optical parametric oscillator operated below threshold, typically a periodically poled crystal in a resonant cavity pumped at the second harmonic, correlates photon pairs in a way that suppresses one quadrature, and modern devices of this type reach more than 10 dB of noise reduction across the audio band. Four-wave mixing in atomic vapors and Kerr nonlinearity in fibers and microresonators provide alternative routes. Squeezing is fragile: every percent of optical loss admits an equivalent fraction of ordinary vacuum, so escape efficiency, detector quantum efficiency, and mode matching bound the delivered result more tightly than the source does. Mode mismatch and thermally induced aberrations are known degradation mechanisms in gravitational-wave detectors and receive dedicated engineering attention.

Frequency-Dependent Squeezing

A gravitational-wave interferometer is limited by shot noise at high frequency and by quantum radiation pressure noise at low frequency, and these two demand opposite squeeze angles. Rotating the noise ellipse as a function of frequency resolves the conflict, and the standard method reflects the squeezed vacuum from a detuned filter cavity before injection. A suspended 300-meter cavity was demonstrated as a frequency-dependent squeezed vacuum source, and the technique was then implemented in the observatories themselves, yielding broadband quantum enhancement of the LIGO detectors. Squeezed light injection is now a permanent part of the LIGO instrument development program.

Applications

Optical squeezing has applications in a range of fields, including:

• Gravitational-wave astronomy, where it extends the detectable volume of the universe
• Quantum-enhanced interferometry and optical phase estimation
• Continuous-variable quantum key distribution
• Measurement-based and Gaussian boson sampling photonic quantum computing
• Biological measurement, including particle tracking below the shot noise limit
• Atomic magnetometry and optical atomic clocks