A team of physicists say they’ve managed to nearly freeze the motion of atoms across four suspended mirrors. It’s a mind-twisting feat that strains the very definitions of seemingly simple words like “object” and “temperature.” So buckle up.
The setting for this experiment was LIGO, the Laser Interferometer Gravitational-Wave Observatory, where physicists look for ripples in space-time created by the collisions of massive objects like black holes. The observatory relies on four carefully suspended mirrors and laser beams to detect passing gravitational waves, which shift the mirrors ever so slightly, causing the laser beams to briefly wobble. The researchers behind the current experiment took advantage of a break period at LIGO last September to attempt something that had never been done before: cool a human-scale object so much that quantum observations could be made on it. Their results are published today in the journal Science.
You can cool an object by sticking it in a freezer, but when you’re a physicist, you can also cool an object by reducing its motion. Sometimes, that means applying a counteracting force — in this case, laser beams — to the object, in order to slow down its random movement on the atomic scale. Happily, LIGO is already equipped with lasers, so the team didn’t need to worry about messing up the incredibly expensive experimental setup.
“We could actually use the same capability of LIGO to do this other thing, which is to use LIGO to measure the random jiggling motion of these mirrors — use that information which we have about the motion — and apply a counteracting force, so that you know you would stop the atoms from moving,” said Vivishek Sudhir, a quantum physicist at the Massachusetts Institute of Technology and a co-author of the paper, in a video call.
Here’s where it gets weird. The team didn’t laser-cool any one mirror; instead, they cooled the collective motion of all four mirrors down to 77 nanokelvins, or 77-billionths of a kelvin, just above absolute zero. This collective motion is what the physicists call their “object,” even though that doesn’t quite meet your everyday definition of the word. This is now the largest object ever cooled to nearly the quantum motional ground state — in other words, complete rest on the atomic level.
So why would they undertake such an effort? They seek to better understand how the classical world — that is, the stuff you and I are familiar with, like chairs and cats — interacts with the quantum regime. To do that, it would be helpful to have a large, easy-to-observe system (like the mirrors) that behaves like a quantum-scale system. Typically, human-scale objects are far too influenced by things like the rumbles of passing trains, wind, the sound waves of someone talking nearby, etc, to get delicate measurements of very slight forces. Underground and suspended, LIGO is mostly protected from these factors already. But to behave like a quantum system, the team also needed to remove the noise caused by heat. Room temperature means the air is buzzing with energy. But the colder things get, the less movement there is.
“This is an impressive improvement over their previous results on cooling this massive mechanical mode of their mirror system,” said Markus Aspelmeyer, a quantum physicist at the University of Vienna who is unaffiliated with the recent paper, in an email. “I agree with their statement that this is a fantastic system to study decoherence effects on super-massive objects in the quantum regime.” By decoherence, Aspelmeyer means the way objects lose their quantum properties.
Sudhir said that the next step for the team would be to test gravity’s effect on the system. Gravity has not been observed directly in the quantum realm; it could be that gravity is a force that only acts on the classical world. But if it does exist in quantum scales, a cooled system in LIGO — already an extremely sensitive instrument — is a fantastic place to look. Gravity acts more intensely on massive objects, so having such a large object to work with is a big step toward exploring how the force may or may not engage with the quantum world.
For Sudhir, part of what’s so exciting is unpacking the limits of these of the laws of physics. “Why is it that … any physical law that all of us have discovered as humans on the Earth applies equally well far, far away, somewhere in another corner of the universe?” Sudhir said. “That need not be the case. And yet it is.”