Researchers See Tiny Quantum Fluctuations Move A Human-Sized Mirror

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The effect and phenomena of quantum mechanics are on a scale much smaller than what we experience with our senses, but it doesn’t mean they aren't there. Researchers have just shown that the quantum influence on the macroscopic world is measurable even in human-sized objects.

As reported in Nature, researchers have measured the motion of a 40-kilogram (88 pounds) mirror due to the effect of a laser inside LIGO, the gravitational wave observatory. The noise from the quantum fluctuation “kicks” the mirror a tiny fraction of an atom, 1 billion billion times smaller than a thumbnail or 10-20 meters. This is the first time we have measured this effect for a macroscopic object.

"A hydrogen atom is 10-10 meters, so this displacement of the mirrors is to a hydrogen atom what a hydrogen atom is to us – and we measured that," co-author Lee McCuller, a research scientist at MIT's Kavli Institute for Astrophysics and Space Research, said in a statement.

What the team actually measured is the quantum correlation between the mirror and the photons (particle of light) in the laser. Photons have no mass but they carry momentum, this is why we can employ them in solar sails. This momentum is transferred to the mirror.

In quantum mechanics, there’s a pesky little phenomenon known as Heisenberg’s uncertainty principle. The more accurately you know the position of something, the less accurately you know its momentum. And vice versa.

To detect gravitational waves,  knowing the position of the mirror constantly and well is very important. That’s how you show if things have moved in space-time. But the action of the photons messes things up by adding this momentum. This creates a quantum noise, a limit to the precision at which the detector can operate.

There are particular ways to avoid the limit and reduce the quantum noise a bit and that’s what they demonstrate in this work. The quantum noise is there but it is less than expected. The goal of this work is to "manipulate the detector's quantum noise and reduce its kicks to the mirrors, in a way that could ultimately improve LIGO's sensitivity in detecting gravitational waves," explained lead author Haocun Yu, a physics graduate student at MIT.

The two LIGO detectors are laser interferometers. They are L shapes, with each arm extending for 4 kilometers ( 2.5 miles). A laser is split and shot down each arm, where it bounces off a mirror at the end. When it comes back, the laser is put together again. If the paths of the two lasers have different lengths, for example, due to the effect of space-time distortion, a  gravitational wave passed through the detector.

There are a lot of uncertainties since the instruments capture motions much smaller than an atom, and so scientists have been very careful in minimizing the many sources of noise. With this, they are also reducing the effect of quantum noise.

"What's special about this experiment is we've seen quantum effects on something as large as a human," said senior author Nergis Mavalvala, the Marble Professor and associate head of the physics department at MIT. "We too, every nanosecond of our existence, are being kicked around, buffeted by these quantum fluctuations. It's just that the jitter of our existence, our thermal energy, is too large for these quantum vacuum fluctuations to affect our motion measurably.

"With LIGO's mirrors, we've done all this work to isolate them from thermally driven motion and other forces, so that they are now still enough to be kicked around by quantum fluctuations and this spooky popcorn of the universe."

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