US scientists have fashioned crystals that seem to defy the laws of physics. Richard A Lovett reports.
by RICHARD A. LOVETT is a Portland, Oregon-based science writer and science fiction author. He is a frequent contributor to COSMOS.
Two American teams of scientists have independently created the world’s first “time crystals”, but don’t order up a trip on the TARDIS anytime soon, because the crystals in question have nothing to do with time travel.
Both sets of research have been published in Nature.
“I’m not responsible for its name,” laughs Mikhail Lukin, a physicist at Harvard University, Cambridge, Massachusetts, lead author on one of the papers.
Chetan Nayak, principal researcher at Microsoft’s Station Q and a professor of physics at the University of California, Santa Barbara, puts it more simply. “What they observed is a new state of matter,” he says.
Nayak is responsible for a third paper in the journal, explaining the significance of the discovery.
What’s unique about the crystals, Lukin says, is that they have properties that repeat over time in a manner analogous to the way the atoms in crystal lattices repeat over space.
Repeating phenomena, of course, aren’t a big deal. “Every year we have spring, summer, and fall,” Lukin notes.
But most repeating phenomena are easily altered. An AC electrical current, for example, can be changed by altering the spin rate of the dynamo that produces it. The length of the Earth’s seasons would change if, heaven forbid, a giant asteroid hit us, altering our orbit.
To understand time crystals, we need to start by considering liquids and gases. In these, Lukin says, molecules are uniformly distributed in a way that makes one point in the liquid or gas basically the same as all other points.
But in crystals, atoms are arranged in repeating patterns that mean that once you know the position of one atom, you can pinpoint the locations of all the others. Furthermore, crystals are rigid. If you bash on one, you aren’t going to see one atom move one way, while another moves a different way, as would happen if you sloshed a tub of water or let the air out of a balloon.
Crystals are common to our normal understanding of nature. Time crystals aren’t. In fact, it was only recently that anyone even hypothesized they might exist.
Their atoms operate in a sort of time-array, as opposed to a physical array. The time crystal created by Lukin’s team was a synthetic black diamond, meaning that it was a diamond with a million or so “nitrogen vacancy” impurities — so many they made it appear black.
The electrons in these impurities have spins: they can react to electromagnetic pulses by flipping 180 degrees, analogous to what happens to nuclei in the human body during magnetic resonance imaging.
Normally, you would expect the spins to flip back and forth in synchronisation with the pulse. But that is not what happened. Instead, when Lukin’s team tried it with their black diamond, the spins flipped only once for every two or three pulses.
Shivaji Sondhi, a theoretical physicist at Princeton University in New Jersey, who was part of the team that in 2015 first theorised that such crystals might be possible, compares the effect to repeatedly squeezing on a sponge.
“When you release the sponge, you expect it to resume its shape,” he says. “Imagine that it only resumes its shape every second squeeze, even though you are applying the same force each time.”
In the second study, a team lead by the Christopher Monroe, physicist at the University of Maryland, used a chain of 14 charged ytterbium ions, but got essentially the same result.
Furthermore, the scientists found, varying the incoming electromagnetic pulse didn’t particularly alter the response. In other words, the time crystal’s response was stable, not strongly affected by variances that would normally scramble it and rapidly lead to disorder.
Applications are up in the air. “It’s very early days,” says Nayak. “I think applications will become more clear as we expand the contexts in which we can create time crystals.”
One possibility is that this might be used in futuristic quantum computers. “What a time crystal is doing is manipulating quantum information in a period manner,” says Nayak. “That’s potentially useful for quantum information processing.”
Lukin says that another potential application is in developing sensing instruments capable of working on very small scales. These instruments could be designed with numerous tiny time crystals, tightly packed.
The crystals would react to electrical or magnetic impulses in their local environment, but would not be easily perturbed by whatever is going on nearby. “We believe these will enable new approaches for [what are] basically quantum sensors,” Lukin says.