“It’s still fricking weird”: Physicist on the creation of time crystals.

Nature describes how “dirty diamonds” (and other delights) can be used to make crystals that “pulse” without using any energy:

The name sounds like a prop from Doctor Who, but it has roots in actual physics. Time crystals are hypothetical structures that pulse without requiring any energy — like a ticking clock that never needs winding. The pattern repeats in time in much the same way that the atoms of a crystal repeat in space. The idea was so challenging that when Nobel prizewinning physicist Frank Wilczek proposed the provocative concept in 2012, other researchers quickly proved there was no way to create time crystals.

But there was a loophole — and researchers in a separate branch of physics found a way to exploit the gap. [Christopher] Monroe, a physicist at the University of Maryland in College Park, and his team used chains of atoms they had constructed for other purposes to make a version of a time crystal…. “I would say it sort of fell in our laps,” says Monroe.

And a group led by researchers at Harvard University in Cambridge, Massachusetts, independently fashioned time crystals out of ‘dirty’ diamonds. Both versions, which are published this week in Nature, are considered time crystals, but not how Wilczek originally imagined. “It’s less weird than the first idea, but it’s still fricking weird,” says Norman Yao, a physicist at the University of California, Berkeley, and an author on both papers.

They are also the first examples of a remarkable type of matter — a collection of quantum particles that constantly changes, and never reaches a steady state. These systems draw stability from random interactions that would normally disrupt other kinds of matter.

Experimentalists imagined a quantum version of this entity as perhaps a ring of atoms that would rotate endlessly, cycling and returning to its initial configuration. Its properties would be endlessly synchronized in time, just as atom positions are correlated in a crystal. The system would be in its lowest energy state, but its movement would require no external force. It would, in essence, be a perpetual-motion machine, although not one that produces usable energy.

The recipe was incredibly complex, but just three ingredients were essential: a force repeatedly disturbing the particles, a way to make the atoms interact with each other and an element of random disorder. The combination of these, Monroe says, ensures that particles are limited in how much energy they can absorb, allowing them to maintain a steady, ordered state.

In his experiment, this meant repeatedly firing alternating lasers at a chain of ten ytterbium ions: the first laser flips their spins and the second makes the spins interact with each other in random ways. That combination caused the atomic spins to oscillate, but at twice the period they were being flipped. More than that, the researchers found that even if they started to flip the system in an imperfect way, such as by slightly changing the frequency of the kicks, the oscillation remained the same. “The system still locked at a very stable frequency,” says Monroe. Spatial crystals are similarly resistant to any attempt to nudge their atoms from their set spacing, he says. “This time crystal has the same thing.”

At Harvard, physicist Mikhail Lukin tried to do something similar, but in a very different system — a 3D chunk of diamond. The mineral was riddled with around 1 million defects, each harbouring a spin. And the diamond’s impurities provided a natural disorder. When Lukin and his team used microwave pulses to flip the spins, they saw the system respond at a fraction of the frequency with which it was being disturbed.

Yeah, that description gets complicated near the end there, but there’s a graphic showing how it works at the link.

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