Crystals are structures that regularly repeat in space. A few years ago, scientists wondered if there existed structures that also regularly repeat in time, so-called "time crystals". The answer is yes and we can make them.

This week, two papers by two different groups were published in Nature detailing the creation of time crystals, which were announced back in January. The University of Maryland and Harvard University studies both approached the creation in a different way, although both based on the theoretical background developed at Princeton University by Professor Shivaji Sondhi and Dr Vedika Khemani.

"Our work discovered the essential physics of how time crystals function," said Sondhi in a statement. "What is more, this discovery builds on a set of developments at Princeton that gets at the issue of how we understand complex systems in and out of equilibrium, which is centrally important to how physicists explain the nature of the everyday world."

The Harvard team constructed a time-repeating pattern in a synthetic diamond, while the University of Maryland used a line of ytterbium atoms to produce the regular repetition. The structures are out of equilibrium and move in time, but not in a continuous way. They are locked in persistent oscillations that periodically reach a certain configuration. This makes them time crystals.

"The creation of time crystals has allowed us to add an entry into the catalog of possible orders in space-time, previously thought impossible," said Khemani, a co-author on the Harvard paper.

Sondhi compared these systems to a sponge that is being squeezed regularly. "When you release the sponge, you expect it to resume its shape. Imagine now that it only resumes its shape after every second squeeze even though you are applying the same force each time. That is what our system does," he said.

Time crystals are not just a new fanciful state of matter, they could have technical applications in fields like quantum computing.

"Although any applications for this work are far in the future, these experiments help us learn something about the inner workings of this very complex quantum state," Christopher Monroe, co-author on the University of Maryland paper, added.