Last year, he set about trying to form his atoms into a time crystal.


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

  • 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 harboring 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.


Physicists agree that the two systems spontaneously break a kind of time symmetry and therefore mathematically fulfill the time-crystal criteria.


But there is some debate about whether to call them time crystals.

"This is an intriguing development, but to some extent it's an abuse of the term," says Oshikawa.

Yao says that the new systems are time crystals, but that the definition needs to be narrowed to avoid including phenomena that are already well understood and not nearly so interesting for quantum physicists.


But Monroe and Lukin's creations are exciting for different reasons, too, says Yao.


They seem to be the first, and perhaps simplest, examples of a host of new phases that exist in relatively unexplored out-of-equilibrium states, he says. They could also have several practical applications.


One could be quantum simulation systems that work at high temperatures.


Physicists often use entangled quantum particles at nanokelvin temperatures, close to absolute zero, to simulate complex behaviors of materials that cannot not be modeled on a classical computer.


Time crystals represent a stable quantum system that exists way above these temperatures - in the case of Lukin's diamond, at room temperature - potentially opening the door to quantum simulations without cryogenics.


Time crystals could also find use in super-precise sensors, says Lukin. His lab already uses diamond defects to detect tiny changes in temperature and magnetic fields.


But the approach has limits, because if too many defects are packed in a small space, their interactions destroy their fragile quantum states.


In a time crystal, however, the interactions serve to stabilize, rather than disrupt, so Lukin could harness millions of defects together to produce a strong signal - one that is able to efficiently probe living cells and atom-thick materials.


The same principle of stability from interactions could apply more widely in quantum computing, says Yao.


Quantum computers show huge promise, but have long struggled with the opposing challenges of protecting the fragile quantum bits that perform calculations, yet keeping them accessible for encoding and reading out information.

"You can ask yourself in the future whether one could find phases where interactions stabilize these quantum bits," says Yao.

The story of time crystals is a beautiful example of how progress often happens when different strands of thought come together, says Roderich Moessner, director of the Max Planck Institute for the Physics of Complex Systems in Dresden, Germany.


And it may be, he says, that this particular recipe proves to be just one of many ways to cook up a time crystal.