Physicists used LIGO’s mirrors to approach a quantum limit

Quantum mechanics usually applies to very small objects: atoms, electrons and the like. But physicists have now brought the equivalent of a 10-kilogram object to the edge of the quantum realm.

Scientists with the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, reduced vibrations in a combination of the facility’s mirrors to nearly the lowest level allowed by quantum mechanics, they report in the June 18 Science.

The researchers quelled differences between the jiggling of LIGO’s four 40-kilogram mirrors, putting them in near-perfect sync. When the mirrors are combined in this way, they behave effectively like a single, 10-kilogram object.

LIGO is designed to measure gravitational waves, using laser light that bounces between sets of mirrors in the detector’s two long arms (SN: 2/11/16). But physicist Vivishek Sudhir of MIT and colleagues instead used the laser light to monitor the mirrors’ movements to extreme precision and apply electric fields to resist the motion. “It’s almost like a noise-canceling headphone,” says Sudhir. But instead of measuring nearby sounds and canceling out that noise, the technique cancels out motion.

The researchers reduced the mirrors’ relative motions to about 10.8 phonons, or quantum units of vibration, close to the zero-phonon quantum limit.

The study’s purpose is not to better understand gravitational waves, but to get closer to revealing secrets of quantum mechanics. Scientists are still trying to understand why large objects don’t typically follow the laws of quantum mechanics. Such objects lose their quantum properties, or decohere. Studying quantum states of more massive objects could help scientists pin down how decoherence happens.

Previous studies have observed much smaller objects in quantum states. In 2020, physicist Markus Aspelmeyer of the University of Vienna and colleagues brought vibrations of a nanoparticle to the quantum limit (SN: 1/30/20). LIGO’s mirrors are “a fantastic system to study decoherence effects on super-massive objects in the quantum regime,” says Aspelmeyer.