# gst: the physics of squeezing; how to squeeze out (quantum) noise

<< 'Squeezing' is used in physics, among other things, to improve the resolution of measuring instruments. It allows disturbing noise to be suppressed in a way that smaller signals can be detected more sensitively. (..) (AA) has now been able to show how such a squeezed state can be measured in a much simpler way than with the existing methods. Moreover, the new method allows examining squeezed states in systems where such measurements were not possible before. >> [1]

<< In the experiment (..) the thermal fluctuations of a vibrating nanomechanical string resonator are squeezed. The nanostring can be thought of as a tiny guitar string, a thousand times thinner and shorter than a human hair. (..)  If the string is deflected far enough, it ceases to behave linearly. This means that the force that deflects the string is no longer proportional to the force that pulls it back to its original position. The strong drive alters the thermal fluctuations as a result of a violation of the time reversal symmetry. In phase space, they no longer look like a circle but like an ellipse: At least in one direction, its diameter, i.e. the noise, becomes significantly smaller—it is squeezed. >> [1]

<< Quantum squeezing was a theory that was first proposed in the 1980s, the general idea being that quantum vacuum noise can be represented as a sphere of uncertainty along two main axes: phase and amplitude. If this sphere were squeezed, like a stress ball, in a way that constricted the sphere along the amplitude axis, this would in effect shrink the uncertainty in the amplitude state of a vacuum (the squeezed part of the stress ball), while increasing the uncertainty in the phase state (stress ball's displaced, distended portion). Since it is predominantly the phase uncertainty that contributes noise to LIGO, shrinking it could make the detector more sensitive to astrophysical signals. (..) The heart of the squeezer is an optical parametric oscillator, or OPO — a bowtie-shaped device that holds a small crystal within a configuration of mirrors. When the researchers direct a laser beam to the crystal, the crystal's atoms facilitate interactions between the laser and the quantum vacuum in a way that rearranges their properties of phase versus amplitude, creating a new, "squeezed" vacuum that then continues down each of the detector's arm as it normally would. This squeezed vacuum has smaller phase fluctuations than an ordinary vacuum, allowing scientists to better detect gravitational waves. >> [2]

[1] - Measure squeezing in a novel way. University of Konstanz. Jun 25, 2020.   https://phys.org/news/2020-06-measure-squeezing-in-a-novel.html ; 

J. S. Huber, G. Rastelli, et al. Spectral Evidence of Squeezing of a Weakly Damped Driven Nanomechanical Mode.  Phys. Rev. X 10, 021066 – Jun 23,  2020.   https://journals.aps.org/prx/abstract/10.1103/PhysRevX.10.021066 

[2] - Jennifer Chu. New instrument extends LIGO’s reach. Technology "squeezes" out quantum noise so more gravitational wave signals can be detected. MIT. Dec 5, 2019.   https://news.mit.edu/2019/ligo-reach-quantum-noise-wave-1205