A unique phase of matter called a time crystal, which can in theory flip between two configurations forever with no energy input necessary, has been created inside a quantum computer built by Google. It is one of the first real-world problems solved by a quantum computer, and could also be harnessed to improve them.
A crystal can form out of a material when its components form stable, repeating patterns. At this point it is said to have lost spatial symmetry – it no longer looks the same from every angle. In 2012, at the Massachusetts Institute of Technology suggested that as general relativity defines time as a fourth dimension, a material could instead lose temporal symmetry. Such a “time crystal” would move back and forth between two states again and again without using or losing energy. It would have repeating patterns over time, rather than across space.
Wilczek, who wasn’t involved in this new work, admits that this idea of perpetual motion feels like it should violate the laws of thermodynamics, but says it is only because the scientists who penned them long ago “weren’t aware of all the subtleties that open up” in the world of quantum mechanics, which can create frictionless systems and other unusual phenomenon. He says that he “took a lot of grief” in the years after he published his hypothesis from people who thought he had fallen into the trap of describing a perpetual motion machine.
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But the concept quickly became more plausible. Several teams have claimed to have created a time crystal in recent years and now a group of researchers from Google, Stanford University and MIT among others have used Google’s Sycamore quantum computer to achieve what Wilczek says is a time crystal more true to the strict definition than ever before.
The team used a series of 20 qubits within Sycamore to represent a string of data with random spin patterns. These values were then finely balanced using a technique called many-body localisation, in which interference between particles freezes them in place even when they would tend to drift apart due to thermal equilibrium. The researchers demonstrated that the qubits in this delicate state would spontaneously reverse their spin in unison to create a new pattern of data, then revert to their original state, over and over again. The team says that part of the quantum processor had become a time crystal.
Wilczek says it is good to see his hypothesis come to life experimentally. “It’s definitely a milestone in the field,” he says. “And I think even more so in some ways it’s a milestone for quantum computing. I think this is the first reasonably natural problem that a quantum computer has actually helped to solve.”
He jokes that in some ways the experiment is like a traditional computer falling into an accidental infinite loop and crashing, and that time crystals can be thought of as an “interesting failure mode” for quantum computers.
Wilczek believes that time crystals could find applications within quantum computers to increase reliability and accuracy, but are more likely to find use before that as extremely accurate measurement tools because their stability is delicate.
“They can be very, very sensitive probes of certain kinds of external fields so they will give us, in principle, new kinds of exquisitely sensitive devices,” he says. Such probes could be used in all sorts of electrical and magnetic experiments.
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