Squeezed Vacuum Source

Empty space, rearranged by quantum mechanics until one dimension of its uncertainty is quieter than nothing.

A squeezed vacuum source exploits a nonlinear optical process — most often spontaneous parametric down-conversion (SPDC) inside an optical parametric oscillator (OPO) — to generate pairs of entangled photons whose quantum fluctuations are correlated. The resulting optical field has reduced noise in one quadrature (amplitude or phase) below the standard quantum limit, at the expense of increased noise in the conjugate quadrature, in strict accordance with the Heisenberg uncertainty principle. This squeezed vacuum is injected into precision measurement instruments — most famously LIGO's gravitational-wave detectors — to reduce quantum noise and improve sensitivity. The same resource underlies quantum magnetometry, continuous-variable quantum computing, and quantum key distribution.

A laser beam at frequency 2ω is focused into a nonlinear crystal (typically periodically poled potassium titanyl phosphate, PPKTP, or lithium niobate) inside a bow-tie or linear optical cavity operating below threshold — this is the OPO. The pump photon spontaneously splits into two photons at frequency ω whose creation times are correlated: their quantum phase fluctuations are anti-correlated and cancel, while their amplitude fluctuations reinforce. The emerging field has its noise ellipse stretched in the anti-squeezed direction and compressed in the squeezed direction. To use the squeezed vacuum in a gravitational-wave detector, it is injected into the output port of the Michelson interferometer, where it replaces the ordinary vacuum that normally enters and adds shot noise to the measurement. In LIGO's O4 run, a 300-meter filter cavity preceding the injection point rotates the squeezing angle as a function of frequency — frequency-dependent squeezing — so that the right noise quadrature is squeezed across the entire detection band, reducing both shot noise at high frequencies and quantum radiation-pressure noise at low frequencies. The OPO record for squeezing level is 15 dB below the shot-noise limit, achieved at 1064 nm by a group at AEI Hannover in 2016.

In plain terms

Imagine a lump of quantum noise that looks like a perfect circle. The Heisenberg uncertainty principle says you cannot shrink the circle's area, but you can squash it into an oval — thinner in one direction, fatter in the other. A squeezed vacuum source does exactly that to the noise of empty space. If your measurement only cares about the thin direction, you beat the noise limit that seemed fundamental.

Also called: squeezed light source, OPO squeezer, quantum noise reduction, sub-shot-noise light.

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A short history

The theoretical framework for squeezed states was developed by Yuen, Walls, and Milburn in the late 1970s. The first experimental demonstration of squeezed light was by Slusher et al. at Bell Labs in 1985 using four-wave mixing. OPO-based squeezing, which produces much higher levels, was pioneered in the late 1980s. GEO600 in Hannover became the first gravitational-wave detector to operate routinely with squeezed light in 2010. LIGO first used squeezing in its 2011–2012 Enhanced LIGO run, achieving 2.15 dB improvement. Frequency-dependent squeezing, first proposed theoretically by Kimble et al. in 2001, was demonstrated at full scale at LIGO during the O4 run beginning in 2023.

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