A new, theoretical type of time crystal could run without outside help

0

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 (SN:
10/26/16
). 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 (SN:
2/16/12
), 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 (SN:
5/4/18
).

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 (SN: 6/15/17). 

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.

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