A Tiny Glass Bead Goes as Still as Nature Allows

Inside a small metal box on a laboratory table in Vienna, physicist Markus Aspelmeyer and his team have engineered, perhaps, the quietest place on Earth.

The area in question is a microscopic spot in the middle of the box. Here, levitating in midair—except there is no air because the box is in vacuum—is a tiny glass bead a thousand times smaller than a grain of sand. Aspelmeyer’s apparatus uses lasers to render this bead literally motionless. It is as still as it could possibly be, as permitted by the laws of physics: It’s reached what physicists call the bead’s “motional ground state.” “The ground state is the limit where you cannot extract any more energy from an object,” says Aspelmeyer, who works at the University of Vienna. They can maintain the bead’s motionlessness for hours at a time.

This stillness is different from anything you’ve ever perceived—overlooking that lake in the mountains, sitting in a sound-proofed studio, or even just staring at your laptop as it rests on the table. As calm as that table seems, if you could zoom in on it, you would see its surface being attacked by air molecules that circulate via your ventilation system, says Aspelmeyer. Look hard enough, and you’ll see microscopic particles or tiny pieces of lint rolling around. In our day-to-day lives, stillness is an illusion. We’re simply too large to notice the chaos.

Kahan Dare and Manuel Reisenbauer, physicists at the University of Vienna, adjust the apparatus where the levitated nanoparticle sits.

Photograph: Kahan Dare, Lorenzo Magrini, Yuriy Coroli/University of Vienna

But this bead is truly still, regardless of whether you are judging it as a human or a dust mite. And at this level of stillness, our conventional wisdom about motion breaks down, as the bizarre rules of quantum mechanics kick in. For one thing, the bead becomes “delocalized,” says Aspelmeyer. The bead spreads out. It no longer has a definite position, just like a ripple in a pond, which stretches over an expanse of water rather than being at a particular location. Instead of maintaining a sharp boundary between bead and vacuum, the bead’s outline becomes cloudy and diffuse.

Technically, although the bead is at the limit of its motionlessness, it still moves about a thousandth of its own diameter. “Physicists have a cool name for it. It’s called the ‘vacuum energy of the system,’” says Aspelmeyer. Put another way, nature does not allow any object to have completely zero motion. There must always be some quantum jiggle.

The bead’s stillness comes with another caveat—Aspelmeyer’s team has only forced the bead into its motional ground state along one dimension, not all three. But even achieving this level of stillness took them 10 years. One major challenge was simply getting the bead to stay levitated inside the laser beam, says physicist Uroš Delić of the University of Vienna. Delić has worked on the experiment since its nascence—first as an undergraduate student, then a PhD student, and now as a postdoctoral researcher.

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