IT’S LIKE something out of a bad dream. You’re stuck in a dance hall performing an interminable waltz. The hours go by and the dance continues. The hours melt into days, years, centuries, millennia. Eventually, billions of years have passed in which the universe has transformed into a featureless void populated only by you and your fellow indefatigable waltzers, dancing throughout eternity.
The vision is surreal, nightmarish – and entirely against the laws of physics. Anything that repeats on loop without an external energy source to power it seems to bend the cast-iron laws of thermodynamics, which govern how energy flows and can be exploited. So when five years ago, Nobel laureate Frank Wilczek speculated about a type of material that he called time crystals whose components could, in fact, do just that, he faced a wave of scepticism. “I took a lot of grief,” he says.
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In the time since, Wilczek’s brainchildren have been championed, vilified, proved to be impossible, and now, apparently, made in the lab. If so, it’s the birth of an entirely new phase of matter, one that is fundamentally bizarre, perhaps confounding – and possibly even useful.
Time crystals might still be waiting to be invented if Wilczek were not the sort of person who gets bored easily. He won his Nobel prize in 2004 for theoretical insights into the nature of the strong force, which determines how fundamental particles interact within the atomic nucleus. He once described the experience of waiting for experimental verification of his theory as akin to watching grass grow. So when at some point his employer, the Massachusetts Institute of Technology, appointed him to teach a graduate course involving the structure of solid crystals, he was soon looking for ways to spice up the curriculum. “It’s a beautiful subject, but kind of cut and dried by now,” says Wilczek. “I wanted to hint at something different.”
Wilczek ended up throwing around ideas about symmetry in crystals with a former student of his, Alfred Shapere, now at the University of Kentucky in Lexington. All crystals, from salt flakes to diamonds, are made up of some basic unit of atoms that repeats over and over again in space. The temptation is to think of such a pattern as symmetrical, but to a mathematician or physicist, this structure actually breaks a kind of symmetry.
To see why, imagine asking for random samples of a carpet. If the carpet is plain, any pair of samples will be impossible to distinguish. But if the carpet is patterned, it’s likely no two are the same: if you overlay them, the patterns will not line up, just as they won’t if you take two random chunks out of a repeated solid crystal. In physicists’ speak, both the carpet and the crystals break translational symmetry in space.
So far, so ordinary. But Shapere challenged Wilczek to think of an object that would break translational symmetry in time: in other words, it would repeat naturally not in space, but in time.
Such things do not ordinarily exist on their own. A heart may constantly beat, and the hands on a clock may continuously move round the dial, but they need energy to power them – and hearts eventually run down and clocks cease to work. Claim to have built something that can run on a never-ending loop, with no power source, for as long as you please and you will get laughed out of the patent office. What you have designed is a perpetual motion machine: a long-discredited, thermodynamically impossible device for getting something out of nothing.
Wilczek’s new form of matter, however, exploited a loophole. Time crystals would do no useful work: they wouldn’t travel anywhere or power anything, and so technically would need no external energy source to run. For good measure, Wilczek imagined time crystals arising in a superconductor, a material that already exhibits unusual behaviour by allowing electrons to flow without resistance.
When Wilczek and Shapere published a paper outlining their thought experiment, the pick-up was immediate. “It generated a lot of excitement,” recalls Vedika Khemani, a researcher at Harvard University.
At the time, Haruki Watanabe was a PhD student at the University of California, Berkeley. One of the questions during the oral examination for his degree was about what would happen if symmetry broke in the way that Wilczek proposed. Watanabe didn’t know, and the question festered.
As is clear from the title of the paper he published two years later, ““, the answer he came up with was unambiguous. Time crystals as proposed by Wilczek were still thermodynamically impossible. The problem, Watanabe showed, lay in Wilczek’s proposal that the dancers be in constant, seamless motion, entirely independent of the outside world. These dancers would be in thermal equilibrium, said Watanabe, a state he was able to painstakingly prove could never display periodic behaviour.
“That was that,” says Khemani. “Everyone thought that time crystals were impossible.” But the vagueness of Wilczek’s thought experiment offered a lifeline: a time crystal might still exist, just not in thermal equilibrium.
Khemani ended up exploiting that get-out clause almost by chance. When Watanabe’s paper came out, she was at Princeton, working on a problem called many-body localisation, which can be summed up as “a bunch of tiny particles get stuck”. An example might be if all the air particles in a room, instead of filling the space evenly with gas, were hanging out in one corner. That’s not a normal happening: thermodynamics says that, left to their own devices, things tend to evolve towards messy, featureless, randomly distributed states.
But tickle a system in precisely the right way, Khemani found – gently, not imparting any energy to it – and keep on tickling it in a regular rhythm, and you could prevent that happening. In fact, you could get a bunch of particles to move in space at a rhythm different to the one you’re poking it at. It’s a bit like a sponge that you squeeze and release, says Khemani, except that the material doesn’t follow the rhythm of your squeezing, but expands and contracts at only half the rate, say, as though you were squeezing it half as quickly.
Loophole to hula hoop
It wasn’t until Khemani’s paper reached peer review that someone pointed out the connection with time crystals – although it was of a new sort, as her system was not in thermal equilibrium. With the loophole now expanded into a hula hoop, all time crystals needed was the right experimental home. And Soonwon Choi, a graduate student at Harvard, thought he could provide it. While listening to Khemani lecture about her idea in 2016, it occurred to him that the experiment his colleagues were using to observe strange quantum effects could be an ideal test bed for a time crystal.
Choi and his group were working with a black diamond, a close cousin of those clear rocks that adorn engagement rings, but with a few million carbon atoms swapped out for nitrogen atoms paired with little empty spaces. These impurities, meticulously inserted over months, give them their distinctive colour and extravagant price. More than that, however, they offer just the kind of disorder necessary to allow for the weird effects Khemani had envisioned.
Each nitrogen atom/empty space pair in the diamond has a property called spin, which can be made to flip between two values using va microwave signal. Choi and his colleagues found that they could send the microwave signal twice for every spin flip, just like the sponge moving half as fast as the rhythm of the hand squeezing it.
The Harvard experiment wasn’t unique. Nearly 7 hours’ drive south on I-95, in experimentalist Chris Monroe’s lab at the University of Maryland, another group had built their own version of a time crystal. It was another possible rebel child of Wilczek’s theory, based on a slightly different theoretical blueprint. It consisted of just a handful of atoms – ytterbium ions to be exact – flipping spins in unison inside a chamber the size of a softball. The atoms were responding to lasers pulsing at twice the frequency of the flips.
“It’s as weird as a roomful of dancers leaving the party, going home, and continuing to dance in perfect lockstep”
“My god,” Wilczek recalls thinking when he read the papers this January. To him, this was a major justification of his idea. What’s more, he’d had no idea that the experiments were under way. “It was a very delightful surprise.”
Watanabe was unconvinced. To him, an object that needs an external signal to loop in time is no more a time crystal than a beating heart or a swinging pendulum. “In my opinion, the phenomenon should occur by itself,” he says. The fact that time crystals were popping up in Massachusetts and Maryland represented little more than overenthusiastic branding.
Wilczek himself doesn’t entirely disagree, and nor, for that matter, does Khemani. But she does see an important distinction between her work and the everyday phenomena Watanabe describes. “It is unfair to say that they are just pendulum-like,” says Khemani. That’s because the atoms in time crystals are separated in space, and yet are all still flipping together, cooperating at a distance. Pendulums and hearts don’t do that. It would be as weird as a roomful of dancers leaving the party, going home, and then continuing to dance in perfect lockstep.
That feature makes the creations interesting in their own right, Watanabe admits. And now that they exist, says Wilczek, it’s easier to see the implications of time crystals. Monroe’s set-up, for example, is one he typically uses to create elements for quantum computers. Time crystals could, one day, fit into that category. The fact that the spins align and appear to cooperate means that they could serve as a clock for a future machine or, because they have the capacity to maintain their state as time plods on, a way to store information.
They could also help perform MRI scans on tiny things, says Mikhail Lukin, the principal investigator at the Harvard lab. A time crystal could respond to a relatively weak signal coming from, say, a cell or even an individual protein, and flip millions of spins in unison as a result, amplifying the signal.
Happily ever after?
But for now, Monroe points out, the time crystal doesn’t last long enough to do anything more than let us observe that it exists. Thanks to imperfect equipment, the processes used in the lab are “like deliberately throwing sodium and chlorine crystals together one at a time,” he says, technically capable of creating salt crystals, but ones far too small to use as seasoning.
“Of course, naturally, salt crystals appear all the time,” says Monroe. That means there could be larger time crystals out there in the wild.
Wilczek still believes that time crystals could hypothetically last forever. “There’s no clear upper limit,” he says. Experimentalists are more sceptical – and there are some practical aspects to consider, too. Phil Richerme, who has been watching the emerging time crystal research from his post at Indiana University at Bloomington, drily notes that “in practical terms, the sun is going to explode and take out the lab.”
In the immediate future, the hope is that the properties of this new phase of matter can be explored in more detail. “It’s like we’ve stumbled on to some new territory,” says Wilczek. “We don’t know if it’s an exotic island, or a continent.”
For Khemani, the biggest consequences of the experiment are more subtle: it means that interesting things happen out of equilibrium. “Phases have been studied for so long,” she says – the handful we knew about seemed to be all that were out there. Now, she says, “people are thinking about phases in this out-of-equilibrium setting.” It opens up a whole new realm of matter, one sure to keep physicists dreaming for some time to come.
Forbidden no longer
Time crystals aren’t the only seemingly impossible thing that physicists have created
Teleportation
Quantum entanglement, the phenomenon that Albert Einstein called “spooky action at a distance”, allows information and quantum states to be transmitted apparently instantaneously across space. Today, the world quantum teleportation distance record stands at over 100 kilometres.
Invisibility cloaks
Metamaterials bend waves of light in unaccustomed ways, allowing them to pass around objects in your field of vision and potentially hide them from view. While human-sized objects can never be made totally invisible, similar principles might be used to divert seismic waves and shield entire cities from earthquakes.
Negative temperatures
According to the strict thermodynamic definition, temperature is a measure of order: the quieter and more ordered something is, the lower its temperature. So tidying up atoms already cooled to near absolute zero gives scientists a sneaky way of creating “impossible” negative temperatures.
Matter married with antimatter
Matter and antimatter are supposed to violently annihilate when they come into contact. But so-called Majorana fermions would be their own antiparticles, capable of self-annihilating under the right conditions. Some researchers claim to have created ones in the lab, by tearing electrons out of superconductors and manipulating them alongside the holes they leave behind.
This article appeared in print under the headline “Never-ending story”
