Time crystals were first predicted in 2012. Now researchers have created time crystals for the first time and say they could one day be used as quantum memories.
Crystals are extraordinary objects, not least because of their symmetry
Crystals form repeating patterns that are the same in some directions but not all directions. That’s something of a surprise given that the laws of physics, which govern their formation, are the same in all directions.
That the laws of physics are spatially symmetrical but crystals are not is a phenomenon known as symmetry breaking. It comes about not by adding energy to a system, but by taking it away. Indeed, crystals are a manifestation of systems in their lowest energy states.
But the laws of physics are not only symmetrical in space but also in time. And that raises the interesting question of whether it is possible to break temporal symmetry in the same way. In other words, is it possible to create time crystals?
The basic process for making time crystals is straightforward. The idea is to create a quantum system, such as a group of ions arranged in a ring, and cool them until they are in their lowest energy state. In these circumstances, the laws of physics would suggest that the ring should be perfectly stationary.
But if time symmetry were broken, then the ring could vary periodically in time. In other words, it would rotate. Of course, it would never be possible to extract energy from this motion—that would violate the conservation of energy. But the temporal symmetry-breaking would manifest itself in this repeating motion in time, just as spatial symmetry-breaking manifests itself as repeating patterns in space.
That’s the theory, but in the real world, things are not quite as simple. The main problem is that the quantum world is not governed by time-dependent variables, so time symmetry cannot be broken on this scale. So in ordinary circumstances, cooling a ring of ions to their lowest energy state would leave them stationary.
But there are circumstances in which quantum systems do evolve over time. Munro and co have focused on these: quantum systems that are not in equilibrium. Their quantum system is a line of ytterbium ions with spins that interact with each other.
That interaction leads to a special kind of behavior. One of the strange features of quantum particles is that they do not usually exist in specific locations. Instead they are smeared out in space with the chances of them appearing anywhere governed by the laws of probability.
But in some circumstances this can change. For example, a single electron inside a material can interfere with itself in a way that forces it to appear in a single location. This is known as Anderson localization, after the physicist who predicted it in the 1950s.
More recently, physicists have investigated groups of quantum particles that interact with each other in a way that causes them all to become localized. This so-called many body localization is a delicate state that maintains the quantum particles in an out-of-equilibrium state. In other words, it forces them to be localized. And that’s exactly how this chain of ytterbium ions behaves.
One of the key properties of these ions is their magnetization or spin, which can be flipped up or down using a laser. Flipping the spin of one ion causes the next to flip, and so on. These spin interactions then oscillate at a rate that depends on how regularly the laser flips the original spin. In other words, the driving frequency determines the rate of oscillation.
But when Monroe and co measured this, they found another effect. These guys discovered that after allowing the system to evolve, the interactions occurred at a rate that was twice the original period. Since there is no driving force with that period, the only explanation is that the time symmetry must have been broken, thereby allowing these longer periods. In other words, Monroe and co had created a time crystal.
The team went on to measure some of the properties of these crystals. They found, for example, that changing the driving frequency did not change the frequency of the time crystal. “This represents the ‘rigidity’ of the discrete time crystal,” they say.
And they found that other perturbations could eventually destroy the time crystal. “When the perturbations are too large, the crystal ‘melts,’” say Monroe and co.
That’s interesting, if esoteric, work. It shows that the time crystals can actually exist, as predicted in 2012 by the Nobel-prize winning physicist Frank Wilczek at MIT and Al Shapere at the University of Kentucky.
As for applications, Monroe and co make a couple of suggestions. They say, for example, that time crystals could be used for quantum information tasks, such as implementing a robust quantum memory.
But the exotic nature of many body localization and the fact that it is still poorly understood may mean that other physicists will want to check out the nature of this effect carefully before confirming that it really shows the existence of time crystals.
So there’s more exciting work to be done.