MIT physicists have observed that LIGO’s 40-kilogram mirrors can move in response to small quantum effects. In this photo, a LIGO optics technician inspects one of the LIGO mirrors. Credit: Matt Heintze / Caltech / MIT / LIGO Lab
Study shows LIGOThe 40-kilogram mirrors can move in response to small quantum effects, revealing the “spooky popcorn of the universe.”
The universe, seen through the lens of quantum mechanics, is a noisy, crackling space where particles constantly flicker in and out of existence, creating a background of quantum noise whose effects are typically too subtle to detect in everyday objects.
Now, for the first time, a team led by researchers from MIT The LIGO Laboratory has measured the effects of quantum fluctuations on objects on a human scale. In an article published on July 1, 2020, in Nature, the researchers report observing that quantum fluctuations, however small, can “kick” an object as large as the 40-kilogram mirrors of the Gravitational Wave Observatory (LIGO) of the laser interferometer of the US National Science Foundation. To a small degree, the team was able to measure.
It turns out that the quantum noise in the LIGO detectors is enough to move the large mirrors by 10-twenty meters – a displacement that was predicted by quantum mechanics for an object of this size, but had never been measured before.
“A hydrogen atom is 10-10 meters, so this mirror shift is to a hydrogen atom what a hydrogen atom is to us, and we measure it, “says Lee McCuller, research scientist at the Kavli Institute for Astrophysics and Space Research at MIT.
The researchers used a special instrument they designed, called the quantum juicer, to “manipulate the detector’s quantum noise and reduce its kicks towards the mirrors, in a way that could ultimately improve LIGO’s sensitivity in detection gravitational waves“Explains Haocun Yu, a graduate student in physics at MIT.
“What is special about this experiment is that we have seen quantum effects on something as large as a human being,” says Nergis Mavalvala, a marble professor and associate chief of MIT’s physics department. “We too, every nanosecond of our existence, are being hit, hit by these quantum fluctuations. It’s just that the restlessness of our existence, our thermal energy, is too great for these quantum vacuum fluctuations to affect our movement in any measurable way. With LIGO mirrors, we have done all this work to isolate them from thermally driven motion and other forces, so that they are now strong enough to be shaken by quantum fluctuations and this creepy popcorn from the universe. “
Yu, Mavalvala, and McCuller are co-authors of the new article, along with graduate student Maggie Tse and Principal Research Scientist Lisa Barsotti at MIT, along with other members of the LIGO Scientific Collaboration.
A quantum kick
LIGO is designed to detect gravitational waves reaching Earth from cataclysmic sources millions or billions of light years away. It includes twin detectors, one in Hanford, Washington, and the other in Livingston, Louisiana. Each detector is an L-shaped interferometer made up of two tunnels 4 kilometers long, at the end of which hangs a 40-kilogram mirror.
To detect a gravitational wave, a laser located at the entrance of the LIGO interferometer sends a beam of light through each tunnel of the detector, where it is reflected in the mirror at the far end, to reach its starting point again. In the absence of a gravitational wave, the lasers should return at the exact same time. If a gravitational wave passes, it would briefly disturb the position of the mirrors, and thus the arrival times of the lasers.
Much has been done to protect interferometers from external noise, so that detectors have a better chance of detecting extremely subtle disturbances created by an incoming gravitational wave.
Mavalvala and his colleagues wondered if LIGO could also be sensitive enough that the instrument could even sense more subtle effects, such as quantum fluctuations within the interferometer, and specifically quantum noise generated between the photons in the LIGO laser.
“This quantum fluctuation in laser light can cause radiation pressure that can really kick an object,” adds McCuller. “The object in our case is a 40-kilogram mirror, which is a billion times heavier than nanoscale objects where other groups have measured this quantum effect.”
Noise juicer
To see if they could measure the movement of LIGO’s massive mirrors in response to small quantum fluctuations, the team used an instrument they recently built to complement interferometers, which they call a quantum juicer. With the juicer, scientists can adjust the properties of quantum noise inside the LIGO interferometer.
The team first measured total noise inside LIGO interferometers, including background quantum noise, as well as “classic” noise or disturbances generated by normal everyday vibrations. They then turned on the juicer and set it to a specific state that specifically altered the properties of quantum noise. They were then able to subtract the classical noise during the data analysis, to isolate the purely quantum noise in the interferometer. Because the detector constantly monitors the displacement of the mirrors for any incoming noise, the researchers were able to see that only quantum noise was enough to displace the mirrors, by 10-twenty meter.
Mavalvala points out that the measurement aligns exactly with what quantum mechanics predicts. “But it’s still remarkable to see it confirmed in something so big,” she says.
Going a step further, the team wondered if they could manipulate the quantum juicer to reduce the quantum noise inside the interferometer. The juicer is designed in such a way that when it is set to a particular state, it “squeezes” certain properties of quantum noise, in this case phase and amplitude. Phase fluctuations can be thought of as arising from quantum uncertainty in the travel time of light, while amplitude fluctuations impart quantum kicks to the mirror surface.
“We think of quantum noise as being distributed along different axes, and we try to reduce noise in some specific way,” says Yu.
When the juicer is set to a certain state, it can, for example, squeeze or reduce the uncertainty in the phase, while at the same time stretching or increasing the uncertainty in amplitude. Squeezing the quantum noise at different angles would produce different amplitude and phase noise ratios within the LIGO detectors.
The group wondered if changing the angle of this compression would create quantum correlations between the LIGO lasers and their mirrors, in a way that they could also measure. Testing their idea, the team configured the juicer at 12 different angles and found that they could indeed measure the correlations between the various quantum noise distributions in the laser and the movement of the mirrors.
Through these quantum correlations, the team was able to squeeze out the quantum noise and the resulting mirror shift, up to 70 percent of its normal level. This measurement, incidentally, is below what is called the standard quantum limit, which, in quantum mechanics, states that a certain number of photons or, in the case of LIGO, a certain level of laser power, will generate a certain quantum minimum. fluctuations that would generate a specific “kick” to any object in its path.
By using squeezed light to reduce quantum noise in the LIGO measurement, the team has made a more accurate measurement than the standard quantum limit, reducing that noise in a way that will ultimately help LIGO detect milder and more distant gravitational wave sources .
Reference: “Quantum correlations between light and LIGO kilogram-mass mirrors” by Haocun Yu, L. McCuller, M. Tse, N. Kijbunchoo, L. Barsotti, N. Mavalvala and members of the LIGO Scientific Collaboration, 1 July 2020, Nature.
DOI: 10.1038 / s41586-020-2420-8
This research was funded, in part, by the National Science Foundation.
