Doug Bonderud

Dec 6th 2021

LIGO My Limits: Quantum Mechanics Goes Big

Quantum mechanics is the science of the small, but it comes with big frustrations as the framework scales up. Sometimes, larger objects don’t seem to follow quantum rule sets, creating a disconnect between what we see and what we know. To help bridge the gap, researchers at the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) repurposed LIGO mirrors with a new goal: pushing quantum limits.

Spooky Action at a Distance

The Einsteinian concept known as “spooky action” refers to some of the strangeness found in quantum mechanics — specifically, the seeming ability of paired particles to influence each other regardless of distance and almost instantaneously. And that’s not the only issue. In the quantum realm, objects can seemingly exist as both particle and wave, in effect creating a second set of physical laws that don’t align with those we see in action every day.

For scientists this is especially frustrating since bigger objects are made up of smaller particles. Shouldn’t some of these quantum traits carry over as size scales up? One potential explanation is the notion of decoherence — that as objects get bigger, the increasing vibrations, in the form of sound or heat, “try” to remain in a quantum state. LIGO mirrors present an opportunity to explore this idea further.

The Heat Is On

As MIT notes, both sound and heat create the same effect in objects: the vibration of atoms and molecules. At lower frequencies, these vibrations produce sound; at higher frequencies, they produce heat. Ongoing work in quantum mechanics has found that this vibrational energy isn’t random; instead, it “must be a multiple of a basic amount of energy, called a quantum, that is proportional to the frequency.” Also called phonons, these basic energy levels effectively describe how much the particles in a object are vibrating in response to conditions in their environment.

In the realm of quantum interactions, the phonon limit is effectively zero; vibrations are virtually absent. This isn’t the case for familiar objects, which are constantly bombarded by light, sound and heat.

LIGO Mirrors on the Wall

For physicist Vivishek Sudhir of MIT and his colleagues, LIGO offered an opportunity to tackle phonons head-on. As Science News notes, while the facility is typically used to measure gravitational waves using laser light that bounces between mirrors, Sudhir saw an opportunity to control for phonon frustrations.

Sudhir leveraged the lasers’ light to detect small movements in LIGO’s four 40-kilogram mirrors. Application of precise electric fields allowed the team to create almost perfect vibrational synchronicity between the mirrors, enabling them to act like a single 10-kilogram object. “It’s almost like noise-cancelling headphones,” says Sudhir, but rather than reducing noise, the electrical fields help to manage motion. In practice, the team was able to isolate LIGO mirrors from both surrounding vibrations and even account for the impact of laser light bouncing off them.

Relative motion in the mirrors was reduced to just 10.8 phonons, which is similar to the motion of particles at just 0.77 nanokelvin — one billionth of a degree warmer than absolute zero — even as the system itself remained at room temperature.

Blurring the Lines of Quantum Mechanics

While the LIGO experiment hasn’t blown the doors off the disconnect between standard and quantum mechanics, the use of precise electric fields to significantly reduce particle vibrations sets the stage for further efforts in this space by blurring the lines between big and small. In effect, the effort is zeroing in on the conditions that cause quantum decoherence. By eliminating vibration variables and scaling up the size of quantum objects, future experiments may be able to pinpoint the exact moment that quantum conditions collapse and standard operations take over.

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