Scientists Have Confirmed a Brand New Phase of Matter: Time Crystals

For several months, scientists have been abuzz with the idea that time crystals might finally have been realised - an unusual kind of crystal whose atomic pattern repeats not only through space but also through time, leading to continual oscillation without energy being added.

That speculation has now been replaced with a detailed, formal account: researchers have published an explicit method for creating and measuring these odd crystals. On top of that, two separate research teams say they have already produced time crystals in the laboratory by following this recipe, pointing to the discovery of an entirely new phase of matter.

Although the idea can sound highly theoretical, it signals a major shift in physics. For decades, much of the field has focused on materials defined as being “in equilibrium”, including familiar categories such as metals and insulators.

At the same time, physicists have long suspected that the Universe contains many more exotic kinds of matter that do not sit in equilibrium - and that we have scarcely begun to investigate them. Time crystals have been one such predicted example. The latest reports suggest they are not merely hypothetical.

If confirmed, having a concrete instance of non-equilibrium matter could sharpen our understanding of the physical world and may also open routes towards new technologies, including quantum computing.

"This is a new phase of matter, period, but it is also really cool because it is one of the first examples of non-equilibrium matter," said lead researcher Norman Yao from the University of California, Berkeley.

"For the last half-century, we have been exploring equilibrium matter, like metals and insulators. We are just now starting to explore a whole new landscape of non-equilibrium matter."

Time crystals and non-equilibrium matter: the idea behind the “moving” ground state

To make sense of this, it helps to rewind a little, because time crystals have been discussed in physics for several years.

The concept was first proposed in 2012 by Nobel Prize–winning theoretical physicist Frank Wilczek. In his description, time crystals are structures that seem to exhibit motion even at their lowest-energy configuration - the ground state.

In ordinary materials, reaching the ground state (also described as the zero-point energy of a system) implies that motion should, in principle, be ruled out, because movement would require energy to be spent.

Wilczek argued that time crystals could be an exception to that expectation.

Conventional crystals have atomic arrangements that repeat across space - the carbon lattice in diamond is a standard example. In their ground state, however, such crystals are in equilibrium and therefore do not move.

Time crystals, by contrast, would have a structure that repeats in time as well as in space, and they would keep oscillating even in the ground state.

One way to picture it is with jelly: if you tap it, it wobbles again and again. A time crystal behaves in a broadly similar repetitive way, except that the crucial difference is that the oscillation occurs without energy being supplied.

In that sense, a time crystal resembles endlessly oscillating jelly in its natural ground state - which is why it is treated as a distinct phase of matter: non-equilibrium matter that cannot simply remain at rest.

Norman Yao’s time-crystal blueprint in Physical Review Letters

Predicting time crystals is one thing; providing a practical route to producing them is quite another - and that is where the new work comes in.

Yao and colleagues have developed a detailed blueprint laying out how to build a time crystal, how to measure its key properties, and what the different phases around a time-crystal regime ought to look like. In effect, the team has charted the counterpart of solid, liquid, and gas behaviour for this new phase of matter.

The study appeared in Physical Review Letters, and Yao described the article as "the bridge between the theoretical idea and the experimental implementation".

Two independent laboratory time crystals: University of Maryland and Harvard

This is not being presented as theory alone. Using Yao’s blueprint, two independent groups - one at the University of Maryland and the other at Harvard - report that they have followed the outlined steps and created time crystals of their own.

These announcements were posted towards the end of last year on the pre-print platform arXiv.org (here and here) and have been submitted to peer-reviewed journals. Yao is listed as a co-author on both manuscripts.

Until peer review and journal publication are complete, it is sensible to remain sceptical about both claims. Even so, it is encouraging that two different teams used the same blueprint to generate time-crystal behaviour in two very different physical systems.

University of Maryland: a chain of 10 ytterbium ions

The University of Maryland group produced their time crystals by using a “conga line” of 10 ytterbium ions, each with entangled electron spins.

Chris Monroe, University of Maryland

To turn this arrangement into a time crystal, the researchers needed to keep the ions out of equilibrium. They did so by driving the system with two alternating lasers: one produced a magnetic field, and the other partially flipped the atoms’ spins.

Because the atoms’ spins were entangled, the system settled into a stable, repeating pattern of spin flips - the hallmark of a crystal-like order.

That behaviour alone would not be enough to qualify as a time crystal: the system also had to break time symmetry. When the team observed the ytterbium-ion chain, they found a striking feature.

Even though the two lasers periodically “nudged” the ions, the overall system repeated itself at twice the period of those nudges - a response that should not appear in a normal system.

"Wouldn't it be super weird if you jiggled the Jell-O and found that somehow it responded at a different period?" said Yao.

"But that is the essence of the time crystal. You have some periodic driver that has a period 'T', but the system somehow synchronises so that you observe the system oscillating with a period that is larger than 'T'."

By changing the magnetic field and the laser pulse pattern, the researchers could move the time crystal into different phases, analogous to the way ice changes phase as it melts.

Norman Yao, UC Berkeley

Harvard: nitrogen vacancy centres in diamond

Harvard’s time crystal was implemented in a contrasting way. Instead of ions, the team used densely packed nitrogen vacancy centres in diamond - yet they reported the same overall phenomenon.

"Such similar results achieved in two wildly disparate systems underscore that time crystals are a broad new phase of matter, not simply a curiosity relegated to small or narrowly specific systems," explained Phil Richerme from Indiana University, who wasn't involved in the study, in a perspective piece accompanying the paper.

"Observation of the discrete time crystal… confirms that symmetry breaking can occur in essentially all natural realms, and clears the way to several new avenues of research."

Yao’s blueprint is published in Physical Review Letters, and you can see the Harvard time crystal paper here, and the University of Maryland paper here.

Update 31 January 2017: We had previously compared the constant oscillation of the time crystals as being in perpetual motion at ground state, which isn't accurate. We've now corrected this explanation.