Time crystals really do exist, say physicists*

*If we change the definition of 'time crystal', so scratch that Doctor Who screenplay, okay?


A new quantum state of matter has been experimentally observed for the first time, according to two papers published in Nature.

In 2012, Frank Wilczek, a Nobel-prize winning physicist proposed the idea of "time crystals": an open, ground state system that breaks time translational symmetry.

The name is a little confusing as time crystals aren’t real crystals - they don’t melt. But they vaguely follow the same principle of a having a periodically repeated arrangement of atoms in a lattice.

Real crystals break translational symmetry; the atoms are positioned in a rigid regular pattern and they don’t look the same from all directions. But time crystals break time translational symmetry (TTS) - the system looks different in space and time.

Two research groups led by the University of Maryland, USA and Harvard University, USA, independently demonstrated the idea of time crystals. The Maryland team shone a laser on a diamond riddled with nitrogen atom defects and the Harvard group used a chain of ytterbium ions.

The nucleus of the nitrogen atoms and ytterbium ions has a magnetic moment or spin that can rotate. The laser gives the system a kick of energy and the nuclear magnetic moments align, interact with one another, and then precesses like a spinning top - but only in periods of 180 degrees.

A second pulse of the laser causes the spins to flip 180 degrees again, bringing it to a full circle and it returns back to its original position.

This periodic angle of rotation is key to a time crystal, Chetan Nayak, who was not involved with either study and a professor at University of California, Santa Barbara, USA, explained to The Register.

Nayak and his colleagues previously teamed up with Microsoft to theoretically prove how time crystals might exist.

“The systems are rotating at a different frequency from the one that is being applied, like they are working at some set clock. Normally a system would just settle in the state with the same periodicity that’s being applied, but time crystals self organise into a different one.”

“Even if the energy of the laser is off by a bit - say ten per cent - the spins will still flip 180 degrees.”

Normal crystals have random disorder, become unstable and eventually settle in equilibrium, but time crystals are in a state of “quasi-equilibrium”, cycling back and forth between the two orientations.

It’s not exactly what Wilczek had in mind when he first proposed the idea, and the definition has been fudged to accept both systems as time crystals.

Wilczek imagined a spontaneous breaking of time translational symmetry; atoms that moved automatically, forever in a state of perpetual motion. The systems created by both teams, however, require an external laser to get the spins rotating and aren’t in equilibrium.

The problem is that if something is rotating then it should be radiating energy, but something in the ground state - also known as the lowest energy state - has no energy. This completely violates the idea of a time crystal.

It doesn’t matter if the original idea is wrong, Nayak said. “It’s worth pointing out that in physics that we often takes ideas and generalise them when we notice some other common conceptual features even if they’re applied in different context.”

It may be a modification of the original idea, but it still has the potential to be useful, researchers believe.

Having a new quantum state of matter opens up the possibilities of modelling new quantum-body interactions. The rigid rotation and strong interactions between the spins in a time crystal might even be useful for quantum computers, where researchers are looking for new ways to stabilise the fragile nature of qubits to encode information. ®

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