Less than five years ago, Nobel-Prize winning theoretical physicist Frank Wilczek proposed a hypothetical form of matter that added a fourth dimension—movement in time—to a crystal, which he called a “time crystal,” an entirely new form of matter. Wilczek theorized that if crystals have an atomic structure that repeats in space, similar to the carbon lattice of a diamond, why can’t crystals also have a structure that repeats in time—hence, a time crystal? Last year, theoretical physicists at Princeton University and UC Santa Barbara’s Station Q both proved independently of each other that such a crystal could be made. The UC Berkeley group was “the bridge between the theoretical idea and the experimental implementation,” according to Norman Yao, assistant professor of physics at the University of California, Berkeley, who has done a great deal of work on time crystal theory.
Yao took the theory to the next step and published a paper in Physical Review Letters describing how to make and measure the properties of this type of crystal, even predicting the possible “phases” the time crystal should experience – best compared to the liquid and gas phases of ice. These crystals have a remarkable atomic structure that repeats not just in space, but in time, which means that they “move” or, more technically, enter a state of disequilibrium, without any added energy.
Now this mythical sounding concept has been made a reality by two teams of researchers in collaboration with Yao, at the University of Maryland and Harvard University, who have successfully created this new form of matter based off Yao’s blueprint.
A Short History of Time Crystal Theory
But first let’s take a step back to the short history of time crystal theory. Physicists have spent decades trying to understand why certain crystals and magnets appear to violate a fundamental law of physics called time-translation symmetry when they enter their “ground state”—where atoms are at their lowest possible energy—resulting in asymmetry. This means that their repeating structural patterns appear to be the same at certain angles but not others depending upon the vantage point.
“From the perspective of quantum mechanics, electrons can form crystals that do not match the underlying spatial translation symmetry of the orderly, three-dimensional array of atoms,” Yao said. “This breaks the symmetry of the material and leads to unique and stable properties we define as a crystal.”
Time crystals take this a step further. “Time crystals repeat in time because they are kicked periodically, sort of like tapping Jell-O repeatedly to get it to jiggle,” Yao said. It’s essentially incapable of sitting still, engaging in oscillations of various kinds.
Most of us are familiar with “equilibrium matter”—this comprises known materials, such as metals and insulators, which have stable and predictable atomic structures. However, quantum researchers have theorized that there are much more types of matter in the Universe that do not exist in equilibrium, including time crystals. Time crystals offer “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. This new class of materials may open a whole new realm of research for quantum physics.