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Friday, December 1, 2023

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 a 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.

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—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 postdoc researcher.

The group published their results today in Science. In the paper they describe how they slow the bead by pelting it with infrared photons. It seems counterintuitive to slow an object by pummeling it, but the reason it works is similar to how you slow down on a playground swing set, says physicist Lukas Novotny of ETH Zurich, who was not involved in the work. You push your legs against the motion of the swing to slow down. Similarly, to slow a jiggling bead, the researchers time the infrared photons so they happen to hit the bead when it is moving toward them.

They’re not the first to force an object into the motional ground state; in the past, physicists have achieved this in single atoms and clouds of atoms. They’ve also managed it in similar-sized objects that have been clamped to surfaces. But this is the first time anyone has slowed a levitating solid into its motional ground state, says Aspelmeyer.

Yet a levitated motionless solid is a key ingredient for many physicists’ ambitious ideas. These beads can be used as extremely precise sensors, says Andy Geraci of Northwestern University. For example, Geraci is running an experiment in which he monitors the motion of a similar levitated bead to look for tiny forces predicted by theories that attempt to unify the laws of physics. So far, no one has found compelling evidence that these forces exist, but it could be because they are still too weak for current instruments to detect. A nanoparticle in the motional ground state could be sensitive to even smaller forces, says Geraci.

Physicists can also perform subtle gravity experiments on the bead. Both Aspelmeyer and Novotny, whose groups have been working on parallel projects for the past decade, are working toward an experiment in which they drop such a bead and observe what happens. Theory predicts that when they release the bead from the laser’s levitational hold, its fuzzy outline will spread further to become an even larger, more diffuse cloud. They think they can make the bead actually become a quantum superposition of two different beads, in two different locations. One of their goals is to understand the trajectory of specific configurations of this cloud-née-bead as it falls. The results of such an experiment could offer ideas on how to make the theory of quantum mechanics compatible with the theory of gravity.

But Aspelmeyer and Novotny anticipate that these experiments will take many more years to achieve. One major difficulty is that measuring a quantum object inherently alters the object. This is the central catch-22 of quantum mechanics: By looking for information about the bead, you destroy that information. The researchers will need to develop a technique to follow the bead’s behavior without watching it.

The broader goal is to “measure where no one has measured before,” says Novotny. And creating this tiny, serene bead is their first step into the unknown.

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