Researchers Find Signs of ‘Time Crystal’ in Ammonium Dihydrogen Phosphate

A team of physicists at Yale University has observed a signature of a discrete time crystal (DTC) in an unexpected place: a crystal of ammonium dihydrogen phosphate, better known as monoammonium phosphate (MAP).

Rovny et al looked for a signature of a discrete time crystal in a crystal of monoammonium phosphate. Image credit: Michael Marsland, Yale University.

Ordinary crystals such as salt or quartz are examples of 3D, ordered spatial crystals. Their atoms are arranged in a repeating system, something scientists have known for a century.

Time crystals, first identified in 2016, are different. Their atoms spin periodically, first in one direction and then in another, as a pulsating force is used to flip them.

In addition, the ‘ticking’ in a time crystal is locked at a particular frequency, even when the pulse flips are imperfect.

“Understanding time crystals may lead to improvements in atomic clocks, gyroscopes, and magnetometers, as well as aid in building potential quantum technologies,” Yale physicists said.

Their findings are described in a pair of papers published in journals Physical Review Letters and Physical Review B.

The studies represent the second known experiment observing a telltale signature for a DTC in a solid.

“We decided to try searching for the DTC signature ourselves. We had grown MAP crystals for a completely different experiment, so we happened to have one in our lab,” said Yale Professor Sean Barrett.

“MAP crystals are considered so easy to grow that they are sometimes included in crystal growing kits aimed at youngsters.”

“It would be unusual to find a time crystal signature inside a MAP crystal, because time crystals were thought to form in crystals with more internal ‘disorder’.”

Professor Barrett and colleagues used nuclear magnetic resonance to look for a DTC signature — and quickly found it.

“Our crystal measurements looked quite striking right off the bat,” Professor Barrett said.

“Our work suggests that the signature of a DTC could be found, in principle, by looking in a children’s crystal growing kit.”

“Another unexpected thing happened, as well. We realized that just finding the DTC signature didn’t necessarily prove that the system had a quantum memory of how it came to be,” said Yale graduate student Robert Blum.

“This spurred us to try a time crystal ‘echo,’ which revealed the hidden coherence, or quantum order, within the system,” said Yale graduate student Jared Rovny.

The team’s results, combined with previous experiments, present a puzzle’ for theorists trying to understand how time crystals form.

“It’s too early to tell what the resolution will be for the current theory of discrete time crystals, but people will be working on this question for at least the next few years,” Professor Barrett said.

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Jared Rovny et al. 2018. Observation of Discrete-Time-Crystal Signatures in an Ordered Dipolar Many-Body System. Phys. Rev. Lett 120 (18); doi: 10.1103/PhysRevLett.120.180603

Jared Rovny et al. 2018. 31P NMR study of discrete time-crystalline signatures in an ordered crystal of ammonium dihydrogen phosphate. Phys. Rev. B 97 (18); doi: 10.1103/PhysRevB.97.184301