Source: Science News

A newly proposed type of time crystal could stand alone.

Time crystals are structures that repeat regularly in time, just as a standard crystal is composed of atoms arranged in a regularly repeating pattern in space. Scientists first created time crystals in 2016. But those crystals require periodic blasts from a laser to initiate their rhythmic behavior.

Now, two scientists have sketched out a theoretical blueprint for a new version of the odd state of matter. Their time crystal would persist without any input from the outside world, the pair reports in the Nov. 22 Physical Review Letters.

First proposed in 2012 by theoretical physicists Frank Wilczek of MIT and Alfred Shapere of the University of Kentucky in Lexington, the idea of time crystals was initially controversial. Researchers soon proved a no-go theorem stating that, under typical conditions, time crystals couldn’t exist.

But wiggle room remained: Two situations not included in the no-go theorem left open the possibility of creating the unusual materials. One exception was systems for which energy is input from the outside, for example, via lasers. That’s what’s known in physics terminology as “driving” the system, and it’s how scientists had created all time crystals until now.

But theoretical physicists Oleksandr Kyriienko of the University of Exeter in England and Valerii Kozin of the University of Iceland in Reykjavik wanted to design a self-sustaining time crystal. “We said, ‘We don’t want to drive the system at all,’” Kyriienko says.  

The pair exploited the second exception to the no-go rule — systems that involve very long-range interactions, in which atoms or other tiny particles separated by large distances could influence one another. Such long-range effects don’t typically occur in nature: Two atoms on opposite sides of a room normally don’t exert forces on one another, for example.

Based on such interactions, the researchers came up with a new time crystal scenario, consisting of a collection of many such particles, each with a spin — a quantum version of angular momentum. Interactions between the particles’ spins would be configured so that particles near and far would influence one another simultaneously, via some unspecified quantum gymnastics in the laboratory. And particles in the time crystal would be highly entangled with one another, meaning they share quantum links that can persist at large distances. 

Under such conditions, distant parts of the time crystal could affect one another. The result is that the correlation between the spins — whether neighboring particles’ spins were aligned or not — would endlessly oscillate in time in a regular pattern, producing a time crystal, the researchers say.

Scientists have typically studied systems of particles in which the interactions are short-range, or local. But researchers have long known that “something weird occurs once the locality is violated,” says physicist Haruki Watanabe of the University of Tokyo, one of the researchers who proved the no-go theorem. “So I wouldn’t be surprised by these kinds of behaviors of long-range interacting systems,” he says.

But it’s unclear whether such systems could be created in the laboratory. It’s not an easy feat to produce long-range interactions between many particles at once. “I don’t think it is possible to realize the long-range interacting system they proposed,” Watanabe says. But Shapere is optimistic, suggesting that scientists might use quantum computers or cold atoms to create the proposed time crystal or one like it.

When Wilczek and Shapere first came up with the idea of time crystals, the pair had envisioned a system that would operate without any outside input. “This paper brings us much closer to that original idea,” Shapere says.

Source: Science News