Time crystals

For every action, there’s an equal and opposite reaction, right? Not in the maverick world of this new form of matter.

Written by      

Here in the world we can see, matter mostly behaves with a reassuring predictability. When it’s warm, water molecules are V shaped, two hydrogen atoms bonded to a single oxygen atom. When it’s cold, those molecules form ice crystals, joining in a lattice of hexagons and adopting the six-sided symmetry that we recognise in snowflakes.

Crystals, from diamonds to snowflakes to sugar, routinely rearrange themselves, and this notion of shifting symmetry underpins some of the fundamental mechanisms of particle physics. All this movement happens in the three dimensions of space, but physicists have long wondered if some matter might instead shift form back and forth in another dimension altogether—time.

Turns out it can, and researchers have named such structures ‘time crystals’. This new form of matter, says the journal Nature, is “a collection of quantum particles that constantly changes, and never reaches a steady state”. That might sound like perpetual motion to you, in which case you’re probably already recalling high-school physics, which insisted this does not exist.

Well, out here in the world of macro structures, it can’t, but time crystals are yet more evidence that, in the subatomic world of quantum physics, anything goes. Recent experiments have shown that, with a bit of shoving, certain particles can be goaded into a kind of curious, syncopated two-step.

You will need a some ytterbium ions in a closed system, and a bunch of lasers to disrupt their spins. They will begin to oscillate at a very stable frequency—which is the essence of every good time crystal.

Nobel laureate and Massachusetts Institute of Technology professor Frank Wilczek first proposed the notion of a time crystal in 2012, exciting theoretical physicists, who soon deduced that such a thing could exist only in an unbalanced, non-equilibrium system: think of hammering a nail, or another situation where matter and energy are in irregular flux, shunted about by chemical reactions, temperature or pressure.

In 2016, a University of California team published instructions for creating a time crystal in a lab, something that was duly accomplished by two groups that same year, one from the University of Maryland and the other at Harvard. They published their findings in Nature in March this year.

The Maryland researchers used an electric field to levitate ytterbium ions, then zapped them with pulses of laser energy. The floating atoms began to flip, head over heels, but they did so to a very odd beat: oscillating only once for every two pulses of laser energy. It would be like, said the scientists, tapping a piano key twice, but only producing one note. The excited ions change their spin in the same manner, but strictly to their own time signature, regardless of the energy they’re exposed to. Just as familiar crystals resist spatial pressure, time crystals seem to hold to the beat of their own, otherworldy, drum.

Lift up a ball on a Newton’s Cradle, then let it swing. It strikes the row of balls, and the furthest one receives the same amount of energy you released, swinging into the air. Release two balls, and the furthest two reflect the action. This is because Newton’s law of the conservation of energy dictates that energy can neither be created nor destroyed—only transformed.

Now, imagine a Newton’s Cradle that obeys the laws of a time crystal. You’d release one ball and two would ricochet off the far end of the row. Symmetry has gone to hell.

Take a carton of milk from the fridge and leave it on the bench. The milk will, after a time, reach the same temperature as the bench. They’ll be in thermal equilibrium. A time crystal never does that—its molecules are in perpetual discrepancy, and that energises it. Left undisturbed, it might spin forever, gyrating to some lopsided internal metronome. Nobody knew such a metronome existed—until just the right kind of stimulus was applied.

You’ve got three choices—a quantum annealer (sort of easy), an analogue quantum (sort of hard), or a universal quantum (eye-poppingly difficult). We have the technology to build an annealer today, but it will only do one thing at a time (sort of useless). An analogue quantum can simulate quantum interactions that are impossible for any of today’s computers. But if you really want to impress your neighbours, build a universal quantum computer—it will change the world forever.

So what are time crystals good for? Well, they seem to be a closed system—no energy is lost to the outside world. And it looks as though they may offer an atomic environment much like a superconductor, where electrons can whirl completely unfettered, which is why researchers think we might put time crystals to use in quantum computing.

What’s that? It’s just an idea right now, but in theory it would use quantum mechanical processes to do the job your laptop does at the moment with binary-speaking transistors.

In your computer, the binary digits, or bits, are always in one of two states: 0 or 1. Quantum bits, though, can be in two or more quantum states at once. What’s more, they can be added, or ‘superposed’, to make new, valid, quantum states.

The bottom line is that a quantum computer would make yours look like an abacus. It would process data and solve problems millions of times faster, but it needs paired atoms in constant entanglement: when one changes state, the other automatically does, too.

Until recently, physicists thought that that bondage could happen only at very low temperatures, until Mikhail Lukin at Harvard created a time crystal from a  ‘dirty’ diamond—one with lots of nitrogen atom impurities. When Lukin flipped the spins of those nitrogen atoms with microwave pulses, they shifted in unison, at a constant frequency. In other words, they were in perfect quantum entanglement. Best of all, they did it at room temperature, perhaps solving one of the biggest problems for quantum computing.

Andrew Potter, a professor at the University of Texas at Austin, told media that the time-crystal experiment “opens the door to a whole new world of non-equilibrium phases”. “Hopefully, this is just the first example of these, with many more to come.”

Not everyone’s convinced. In a commentary published in Nature, Chetan Nayak, a professor at the University of California’s Santa Barbara campus, doubted whether time crystals truly exist. While Nayak backed efforts to “spontaneously break the time-translational symmetry of the laws of physics”, he suggested that ions in the crystals may not be able to perform their iconoclastic backflips forever, and called for more research to prove “that the oscillations remain in phase over extended times, and are not washed out by the inevitable fluctuations”.

What we do know is that, deep inside the subatomic world, for every action, some perplexing phenomenon is coiled, waiting to react. We just need to know what kind of actions can set them in motion.

More by