The heat-death of the universe need not bring an end to the computing age. A strange device known as a time crystal can theoretically continue to work as a computer even after the universe cools. A new blueprint for such a time crystal brings its construction a step closer.
Ordinary crystals are three-dimensional objects whose atoms are arranged in regular, repeating patterns – just like table salt. They adopt this structure because it uses the lowest amount of energy possible to maintain.
Earlier this year, Frank Wilczek, a theoretical physicist at the Massachusetts Institute of Technology, speculated that a similar structure might repeat regularly in the fourth dimension – time.
To translate the spatial symmetry of a regular crystal into the fourth dimension, the atoms in such a “time crystal” would have to constantly rotate and return to their original location. Crucially, they would also have to be in their lowest possible energy state as they do so, meaning that they would naturally continue to rotate even after the universe has succumbed to entropy and cooled to a uniform temperature – a state known as heat-death.
Such behaviour would normally violate the laws of thermodynamics, but continuous rotation is allowed in the case of electrons in a superconductor, which flow without resistance. Wilczek originally suggested that a superconductive ring could serve as a time crystal if electrons could be made to flow separately rather than in a continuous stream, ensuring a repeating pattern. But he couldn’t figure out how to do so in practice.
Now Tongcang Li at the University of California, Berkeley, and colleagues at the University of Michigan in Ann Arbor and Tsinghua University in Beijing, China, have an alternative suggestion that may be possible to construct.
First you need an ion trap, a device which holds charged particles in place using an electric field. This causes the ions to form a ring-shaped crystal, as ions trapped at extremely low temperatures repel each. Next, you apply a weak static magnetic field, which causes the ions to rotate.
Quantum mechanics means that the rotational energy of the ions must be greater than zero, even when the ring is cooled to its lowest energy state. In this state, the electric and magnetic fields are no longer needed to maintain the shape of the crystal and the spin of its constituent ions. The result is a time crystal – or indeed a space-time crystal, because the ion ring repeats in both space and time.
“I’m very pleased with it,” says Wilczek. “They’ve really come up with something that looks like a realisable experimental design.”
Building the crystal will be difficult as it needs temperatures close to absolute zero. “The main challenge will be to cool an ion ring to its ground state,” says Xiang Zhang, a member of the team who is also at Berkeley. He says this should be possible in the near future as ion trap technologies improve.
Wilczek has also theorised that a working time crystal could be made into a computer, with different rotational states standing in for the 0s and 1s of a conventional computer. He says this should be possible with the proposed system. “To make it interesting you want to have different kinds of ions, maybe several rings that affect each other,” he says. “You can start to think about machines that run on this principle.”
Don’t expect to see a time crystal computer any time soon, however. While Wilczek points out that the heat-death of the universe is, in principle, “very user friendly” for this kind of experiment because it would be cold and dark, there are other issues to consider. “We focus on a space-time crystal that can be created in a laboratory,” says Li. “So you need to figure out a method to make a laboratory that can survive in the heat-death of the universe.”
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