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Physicists make 'time crystal' in lab: flipped ions demonstrate new form of symmetry breaking.

It may sound like science fiction, but it's not: Scientists have created the first time crystal, using a chain of ions. Just as a standard crystal repeats in a regular spatial pattern, a time crystal repeats in time, returning to a similar configuration at regular intervals.

"This is a remarkable experiment," says physicist Chetan Nayak of Microsoft's Station Q Santa Barbara in California.

Scientists at the University of Maryland and the University of California, Berkeley created a chain of 10 ytterbium ions, each with a spin (the quantum version of angular momentum) pointing up or down. A laser flipped the spins halfway around, from up to down; the ions then interacted so each spin influenced the others.

At regular intervals, the researchers repeated this sequence, flipping the ions halfway each time and letting them interact. When the scientists measured the ions' spins, on average the ions went full circle, returning to their original states, in twice the time interval at which they were flipped halfway.

This behavior is sensible: If each flip turns something halfway around, it takes two flips to return to the original position. But the ions' spins returned to their initial orientation at that rate even if they weren't flipped perfectly halfway. That indicates that the ion system responds at a certain regular period, the hallmark of a time crystal, just as atoms in a crystal prefer regular spacing. These time crystals are "one of the first examples of a new phase of matter," says UC Berkeley's Norman Yao, a coauthor of the study, posted online September 27 at

Time crystals take an important unifying concept in physics--the idea of symmetry breaking--and extend it to time. Physical laws typically treat all points in space equally. In a liquid, atoms are equally likely to be found at any point in space. This is a continuous symmetry, as the conditions are the same at any point along the spatial continuum. If the liquid solidifies into a crystal, that symmetry is broken: Atoms are found only at certain regularly spaced positions, with voids in between. If you rotate a crystal, on a microscopic level it would look different from different angles. But liquid will look the same however it's rotated.

In 2012, MIT theoretical physicist Frank Wilczek proposed that symmetry breaking in time might produce time crystals (SN: 3/24/12, p. 8). Follow-up work indicated that time crystals couldn't emerge in a system in a state of equilibrium. But driven systems, which are periodically perturbed by an external force--like the laser flipping the ions--could create such crystals.

Unlike the continuous symmetry that is broken in the transition from a liquid to a solid crystal, in the driven systems of the time crystals, the symmetry is discrete, appearing at time intervals corresponding to the time between perturbations. If the system repeats at a longer time interval than the one it's driven at, as the time crystal does, that symmetry is broken.

Physicists don't yet have a handle on time crystals' potential applications. But, Wilczek says, "I don't think we've heard the last of this by a long shot."

Editor's note: Frank Wilczek is on the Board of Trustees of Society for Science & the Public, which publishes Science News.

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Title Annotation:MATTER ENERGY
Author:Conover, Emily
Publication:Science News
Geographic Code:1USA
Date:Nov 12, 2016
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