Time crystals realize new order of space-time
Two teams of experimentalists have created exotic patterns, repeating in time, that represent a new order of space-time previously thought impossible.
—Time crystals exist, and not just as technobabble from a science fiction movie.
Two teams have succeeded in realizing Nobel Prize-winning physicist Frank Wilczek's vision of odd entities called time crystals. Like their spatial counterparts defined by geometric patterns, these crystals feature behavior that repeats in time. While they won’t quite power a DeLorean, this new form of matter is the first known example of a structure that breaks the symmetry of time.
The crystals used in jewelry and technological applications, more properly called spatial crystals, feature atoms arranged in neat and tidy repeating patterns: hexagonal for ice, cubic for diamonds, and many others. Unlike the chaotic jumble of particles in other forms of matter, crystals are precisely formed. One is never lost inside a crystal – from one atom, you always know exactly where to go to find another.
In 2012, MIT's Frank Wilczek wondered why this special arrangement should apply only to space. One need look no further than the Einstein-inspired and Hollywood-abused concept of “the space-time continuum” for evidence that the two share some deep similarities.
In his original description, a time crystal would be a system that had some behavior that occurs regularly and predictably, a characteristic that repeats in time just as diamond atoms repeat in space.
There’s nothing wacky about the idea of time-repetition in and of itself. A few systems that meet this description include a bouncing basketball, those desk toys with ball bearings that swing back and forth, and even a person with a daily commute.
But none of these examples could be called “crystals” because they aren’t in what’s known as a ground state, the situation with the least possible energy. The ground states are where they end up when they run out of steam: sitting still on the court, hanging from their frame, and sleeping on the couch, respectively.
A true time crystal, in the original description, would be a system paradoxically in motion or flux despite having zero excess energy. Such a system could continue ticking indefinitely, since it would have nowhere else to go.
Dr. Wilczek’s claim attracted quite a bit of attention, and even more skepticism, due in no small part to a perceived similarity to the concept of a perpetual motion machine. And critics’ intuition was proved correct with a 2015 paper proving that time crystals, as described, were in fact too good to be true.
Nevertheless, the idea refused to die out, and three papers last year found a loophole: crystal-like behavior could theoretically exist in a system that receives periodic nudges and never settles down into a ground-state equilibrium. These nudges would take the form of a “drive,” something like a cubicle dweller who manually swings the ball bearings back and forth to keep the toy going.
A team at the University of Maryland achieved just such a setup with what Norman Yao of U.C. Berkeley called a “bridge between the theoretical idea and the experimental implementation.” They linked ten ytterbium atoms with interacting electron spins, and periodically hit them with lasers to create a “repetitive pattern of spin flipping.” What sets this atomic system apart from the person-pushed ball bearing system is that the resulting flipping pattern repeated itself at a faster multiple of the nudge period.
Dr. Yao likens the repeated flipping of time crystals to Jell-O being jiggled by regular tapping. “Wouldn’t it be super weird if you jiggled the Jell-O and found that somehow it responded at a different period?” he said in a press release. “But that is the essence of the time crystal. You have some periodic driver that has a period ‘T’, but the system somehow synchronizes so that you observe the system oscillating with a period that is larger than ‘T’.”
As Vedika Khemani, a junior fellow at the Harvard Society of Fellows, explains to The Christian Science Monitor, “The idea is that while the system doesn't absorb any net energy from the drive, the drive is needed to keep the system ‘tickled’ and prevent it from relaxing.”
A Harvard team generated similar behavior with a completely different system a month later, confirming that the physicists had broken new experimental ground.
“This is a new phase of matter, period, but it is also really cool because it is one of the first examples of non-equilibrium matter,” Yao said. “For the last half-century, we have been exploring equilibrium matter, like metals and insulators. We are just now starting to explore a whole new landscape of non-equilibrium matter.”
Dr. Khemani, who has done pioneering theoretical work in the field of time crystals and is an author of the Harvard paper, agrees on the significance of the experiments.
"These phases are a novel example of spatio-temporally ordered systems which certainly represents a new shift in our understanding of possible quantum orders," she says.
One reason for the excitement is that this is the first time physicists have succeeded in breaking the symmetry of time. Like the familiar concept from grade school, symmetry describes a type of resistance to change. Rotate a circle, or flip over a square, and you get back the same shape.
From the point of view of a molecule, a cup of water has a smooth spatial symmetry. No matter how far, or which way you go, you get to another place with another water molecule. There are no special places.
But when water freezes into an ice crystal, that symmetry is broken into a chunkier symmetry. Carefully arranged into lattices, you have to go a certain distance in a certain direction to reach another molecule, which makes that location special.
When the University of Maryland’s ytterbium atoms insist on flipping with a period of twice the pulse of the driving laser, they’re acting in a way physicists describe as “rigid,” because it bestows a uniqueness on certain moments in time, the way atoms in ice create special places in space.
Symmetry breaking has historically been a rich area for progress in physics, describing a wide range of phenomena including superconductors and the Higgs boson. Proving that an analogous concept exists for time could open the door to new understanding.
“Observation of the discrete time crystal … confirms that symmetry breaking can occur in essentially all natural realms, and clears the way to several new avenues of research,” wrote Phil Richerme, of Indiana University, in Physical Review Letters.
While some speculate that the “rigidity” of the time crystals could be useful in steadying the notoriously unstable qubits of quantum computing, scientists are more excited about uncovering the existence of a new aspect of our universe.
The creation of time crystals “has allowed us to add an entry into the catalog of possible orders in space-time, previously thought impossible,” Khemani says.
Editor's note: This story has been corrected to reflect Dr. Khemani's involvement with the Harvard experiment, and to clarify the relationship between Dr. Wilczek's original theory and recent studies.