Physicists in Australia have programmed a quantum computer half a world away to make, or at least simulate, a record-size time crystal—a system of quantum particles that locks into a perpetual cycle in time, somewhat akin to the repeating spatial pattern of atoms in an actual crystal.
The new time crystal comprises 57 quantum particles, more than twice the size of a 20-particle time crystal simulated last year by scientists at Google. That’s so big that no conventional computer could simulate it, says Chetan Nayak, a condensed matter physicist at Microsoft, who was not involved in the work. “So that’s definitely an important advance.” The work shows the power of quantum computers to simulate complex systems that may otherwise exist only in physicists’ theories.
The notion of a time crystal emerged 10 years ago, when Frank Wilczek, a Nobel Prize–winning theoretical physicist at the Massachusetts Institute of Technology, mused about an ordinary crystal’s striking spatial pattern of atoms. Where does the pattern come from? It’s not explicitly specified by the equations for the forces among the atoms, which would seem to allow any atom to be anywhere with equal probability. Rather, it emerges spontaneously if the atoms cool sufficiently. Once a few of the atoms nestle next to one another, then the position of the next one becomes predictable, and a pattern that’s only implicit in the forces emerges.
Wilczek wondered whether something similar might happen in time. He envisioned a system of quantum particles, interacting through forces that didn’t vary in time, that somehow managed to execute some cyclic evolution even in its least energetic state. That turned out to be impossible. However, in 2016, two different groups revived the notion by considering a system prodded repeatedly by some external stimulus. Under the right conditions, they found, it could lock into a pattern of change over time that repeats at a different, lower frequency than the stimulus. That lower frequency response is the signature of a time crystal.
The system consists of a chain of tiny quantum mechanical magnets that can point up, down, or, thanks to the strange rules of quantum mechanics, both ways at once. In the chain, neighboring magnets tend to align in opposite directions to lower their energy, whereas a randomly chosen local magnetic field makes each magnet tend to point more one way or the other. A steady stream of magnetic pulses also periodically flips the magnets up to down and vice versa. The idea is that under the right conditions, any configuration of the magnets will turn over and over, once for every two pulses. Experimenters have demonstrated the idea in systems ranging from electrons in a diamond to ions caught in a trap to quantum bits, or qubits, in a quantum computer.
Now, Philipp Frey and Stephan Rachel, theorists at the University of Melbourne, offer a much bigger qubit demonstration. They performed the simulation remotely, using quantum computers built and run by IBM in the United States. The qubits, which can be set to 0, 1, or 1 and 0 at once, can be programmed to interact like magnets. For certain settings of their interactions, the researchers found, any initial setting of the 57 qubits, such as 01101101110 …, remains stable, returning to its original state every two pulses, the researchers report today in Science Advances.
At first blush, that observation may seem a tad underwhelming. After all, if the magnets didn’t interact, each pulse would flip them 180°, creating exactly this half-frequency response. What makes the system a time crystal, however, is the way the interactions among the magnets stabilize the pattern, explains Dominic Else, a condensed matter theorist at Harvard University. That makes the system immune to imperfections, such as pulses that aren’t long enough to flip the spins all the way over. “It’s really a phase of matter that’s stabilized by the many body interactions,” Else says.
Curiously, just cranking up the strength of the magnets’ interactions isn’t enough. The interactions also have to vary randomly from one pair of neighbors to the next, Rachel explains. If all the magnets interact with the same strength, then if one magnet goes wrong, it may cause others down the chain to flip the wrong way, too, he explains. The randomness actually keeps such mistakes from spreading and stabilizes the time crystal, Rachel says.
Whereas more than 100 researchers worked on the Google simulation, Frey and Rachel worked alone to perform their larger demonstration, submitting it to the IBM computers over the internet. “It was just me, my graduate student, and a laptop,” Rachel says, adding that “Philipp is brilliant!” The entire project took about 6 months, he estimates.
The demonstration isn’t perfect, Rachel says. The flipping pattern ought to last indefinitely, he says, but the qubits in IBM’s machines can only hold their states long enough to simulate about 50 cycles. Ultimately, the stabilizing effect of the interactions might be used to store the state of a string of qubits in a kind of memory for a quantum computer, he notes, but realizing such an advance will take—what else?—time.