Quantum Fluctuations Can "Kick" Objects on the Human Scale

For the first time, researchers have measured the effects of quantum fluctuations on an object at the human scale. In a paper published today in Nature, they report the observation that quantum fluctuations, tiny as they may be, can nonetheless "kick" an object as large as the 40-kilogram mirrors of the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO), causing them to move by an infinitesimal degree. The team was able to measure these minuscule movements.

The new study, led by MIT and including several researchers from the LIGO Laboratory headquarted at Caltech, was carried out at the LIGO Livingston Observatory in Louisiana.

It turns out that the quantum noise in LIGO's detectors is sufficient to move the large mirrors by 10^-20 meters, a displacement that was predicted for an object of this size by quantum mechanics, but had never before been measured. In order to measure this motion, the team used a special instrument they designed called a "quantum squeezer" to manipulate the detector's quantum noise and reduce its kicks to the mirrors; by reducing the quantum noise, they were able to determine how much it contributed to the movement of the mirrors.

"It's really impressive to see that squeezed light, a beam of light with just a handful of photons per second, can actually reduce the motion of these huge mirrors that weigh as much as a small person," says co-author Sheila Dwyer, a Caltech scientist working at the LIGO Hanford facility in Washington. "At these frequencies, there are so many noise sources that cause the mirror to move, so it's impressive that the impact is really clear in this result."

Rana Adhikari, professor of physics at Caltech and a member of the LIGO Laboratory, explains that squeezed light reduces the amount of unwanted quantum noise by pushing the noise into the amplitude of the light waves instead of the phase of the light waves, which is what they want to measure.

"It's the amplitude of the light that kicks the mirrors around," he said. "We've taken advantage of a loophole in nature that allows us to push the noise into an area we are not interested in."

By using squeezed light to reduce the quantum noise in the LIGO measurement, the team has made a measurement more precise than the standard quantum limit; this noise reduction will ultimately help LIGO to detect fainter, more distant sources of gravitational waves.

"Even farther into the future, this kind of research could be used to improve smartphones and self-driving cars and other types of technology," says Adhikari. "The team of researchers at LIGO Livingston in Louisiana has been key to making these obervations happen."

Read more at https://www.ligo.caltech.edu/news/ligo20200701.