Time
Crystals Made of Light Could Soon Escape the Lab
A new, more robust approach to creating these bizarre constructs
brings them one step closer to practical applications
By Karmela Padavic-Callaghan on March 9,
2022
Credit: noprati
somchit/Getty Images
In many respects, scientists are much like detectives, solving
mysteries by sifting through evidence in search of cluelike patterns. For
example, any crystal, whether a granule of table salt or a diamond necklace, is
just a bunch of atoms arranged in a repeating pattern. By glimpsing only a few
of the crystal’s patterned atoms, a sleuth may surmise where all the others
should be.
But what if that pattern was spread across time rather than
space, with the pattern’s constituents related by “when” instead of “where”?
This counterintuitive concept is the basis of “time crystals,” quantum systems that exhibit
crystal-like predictably repetitive behavior. Massachusetts Institute of
Technology physicist and Nobel laureate Frank Wilczek first theorized their existence in 2012. And
after years of arduous work, experimentalists only managed to conclusively
engineer one into existence in 2021. Now a team of physicists led by engineer
Hossein Taheri of the University of California, Riverside, have achieved
another advance by making a time crystal out of light. Their
work, published in Nature Communications in February, could
help time crystals transform from delicate experimental curiosities into more
robust components of practical devices.
Although
a time crystal’s behavior repeats over time, it cannot be considered a mere
ticking clock. Specifically, a clock requires external energy to keep going,
but for a time crystal, the “ticking” is its most natural, stable state. This
is the opposite of physicists’ idea of thermodynamic equilibrium, in which
energy flows into a system only to inevitably dissipate: imagine a pot of water
that is brought to a rolling boil and then returned to room temperature. In
this sense, time crystals are rather like a pot of water that always boils in
the exact same way and never cools down. By some definitions, they thus
represent a new and unique state of matter that is distinguished by a steadfast
persistence in staying out of equilibrium. As such inherent metronomes, time
crystals may be a great future asset to precision timekeeping or quantum
information processing.
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“Time
crystals have gone from being a conceptual idea motivated by highly theoretical
considerations to something that people are trying to use for technology,” says
Wilczek, who was not involved in the new work. But researchers have had to
follow long and thorny paths in the quest to bring time crystals out of the lab
and into the realm of real-world applications. Typically, daunting experimental
setups or the unique scrutiny of powerful quantum computers have been required
to discern whether any given setup constitutes a time crystal at all.
The
team’s breakthrough was arguably using a comparatively simpler approach
centered on piping twin beams of laser
light into a millimeter-wide, disk-shaped crystal cavity. Inside the
cavity, the two beams repeatedly ricocheted off of its sides and collided in
the process. Crucially, the researchers picked a particular cavity design and precisely controlled the properties of
the laser beams so that the salvo of reflected light produced odd patterns that
could never emerge from light emitted by, for example, common household light
bulbs. Inside its crystalline bounce
house, the laser light became a parade of “chunks”, each more like a single
wave crest that never loses shape rather than a broad, wide ripple on the
surface of a perturbed lake. These so-called solitary waves, or solitons, emerged and formed the parade with a
predictable periodicity, marching perfectly on beat, consequently building a time crystal. The physicists
caught on to this “crystallization” by carefully studying the light that
trickled out of the cavity.
If some
tiny version of you stood by the cavity’s exit while holding a light detector,
Taheri explains, initially you would detect periodic variations in the outgoing
light’s intensity linked to the lasers’ properties. Eventually, however, a
light intensity pattern would spontaneously emerge with a starkly different
periodicity set by the solitons parading through the cavity. This would be a
bit like watching a movie on a television that suddenly began playing it in
fast-forward, with the specific frame rate set by some hidden mechanics within
the display rather than any setting you controlled. “Now we see some features
of the [light] wave that are periodic, but their period is actually twice or
three times or some other integer multiple of the periodicity imprinted [on the
light] by the lasers,” Taheri says. This increase revealed a quantum system
that now naturally kept its own time—in other words, a light-based time
crystal.
Andrey
Matsko, a physicist at NASA’s Jet Propulsion Laboratory and a co-author of the
study likens it to growing salt crystals
by suspending a string in salty liquid. “Tuning our lasers is like controlling
the structure of the thread you’d put in the [salt] solution,” he says. In
either case, the laser or the thread helps crystals form, but their
periodicity, their patterning, is fully their own.
Past
studies utilized different building blocks for engineering time crystals, yet using light in the new experiment
proved to have practical advantages. Importantly, the team’s time crystal
operates under relatively normal circumstances. Most quantum phases of matter
reveal their special properties only at cryogenic temperatures or other extreme
conditions and revert to being very ordinary when they are exposed to the world
outside the lab. “From my perspective, this experiment is important because it
works at [relatively] high temperatures,” says Berislav Buca, a physicist at
the University of Oxford, who did not participate in the study. “This makes it
closer to complex processes we see in the real world around us.”
The new
time crystal also proved surprisingly resilient against the real world’s
notorious messiness. According to Taheri, the system’s random losses of energy,
as well as its encroaching noise (a bit like your television heating up and
displaying static in the movie-watching analogy), actually boosted its
stability. Usually “when these two elements are present, they try to ruin the
crystallinity,” he says. To avoid such external perturbations, time crystals
typically have to be stringently isolated from their environment. “But our
system strikes a balance between these counteracting players,” Taheri says.
Igor Lesanovsky, a physicist at the University of Nottingham in England, who
was also not involved with the experiment, agrees that keeping a time crystal
working without closing it off to its surroundings can be tricky. “You really
need a ‘conspiracy’ between different effects,” he says.
Because
dissipation and noise conspire to keep each other’s deleterious effects in
check, the new light-based time crystal is a promising candidate for
integration into practical devices in the future. Making it also requires
relatively few components, notes Lute Maleki, CEO of the photonic technology
company OEwaves and a co-author of the study. “This is really a simple [device]
architecture,” he underscores. “It should be accessible to a lot of [research]
groups.” Maleki’s hope is that future research will push this simple but
resilient design to the center of both investigations of fundamental physics
and applied efforts such as in precision timekeeping.
As a
timekeeping device, the light-based time crystal may be slightly less accurate
than state-of-the-art atomic clocks. But its stability and unfussy components
could make it just right for integration into, for example, communication or
computation devices that require very accurate timekeeping while also being
rugged enough to function outside a lab’s carefully controlled conditions.
Additionally, some common electronics fabrication techniques could possibly
enable the time crystal’s implementation on chips, making it easier to add the
system to existing consumer gadgets.
Additionally,
physicists could study very large time crystals in the same way that more
conventional, spatial crystals have been investigated for decades, says study
co-author Krzysztof Sacha, a physicist at Jagiellonian University in Poland.
Here, physicists could exchange space for time to investigate whether time
crystals engineered with certain defects or bathed with excess energy display
unexpected behaviors. Such behaviors are typically harder to detect in small
crystals, so the ability to make its light-based system large potentially sets
up the team for a foray into a fully new realm. "I think that is really
opening a new [physics research] horizon,” Sacha underlines. Wilczek agrees.
“This is a whole new class of states of matter,” he says. “It is very
conceivable to me that, when you examine them, useful devices and other
surprises will emerge. It’s virgin territory; we are discovering a new world
here.”
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