Time is one of nature’s greatest mysteries, yet an unexpected new way to turn a ring of ions into a time crystal may shed light on the nature of time. It may also help the ongoing quest for building a better quantum computer.
Earlier this year, Frank Wilczek of Massachusetts Institute of Technology suggested that it is possible to create a time crystal. Shortly afterwards, a team of physicists at Berkeley Lab proposed a scheme to realise this theoretical concept in the real world.
Xiang Zhang, a faculty scientist with Berkeley Lab's Materials Sciences Division and director of the Nano-scale Science and Engineering Center, is leading the experimental team. "The idea of creating a crystal with dimensions higher than that of conventional 3D crystals is an important conceptual breakthrough in physics and it is very exciting for us to be the first to devise a way to realize a space-time crystal," says Tongcang Li, a post-doc in Zhang's research group.
The novel concept of a time crystal is analogous to the more familiar space crystals we see in everyday substances like table salt and snowflakes. The atoms in these substances repeat themselves periodically in space which makes their arrangement highly symmetrical. Likewise, time crystals repeat themselves in time periodically which makes them perfect clocks.
But it turns out that time crystals are much more than perfect clocks. The curious conditions time crystals are created in means that they automatically come with a set of truly unique properties. It seems Wilczek and the Berkeley Lab team are just scratching the surface when it comes to the imaginative applications these new toys might have.
Wilczek’s starting point was to ask whether the familiar symmetry of space crystals can be achieved in the time dimension. In a space crystal, atoms are naturally arranged periodically as points on a gird. This configuration is stable and abundant in natural substances like table salt and snowflakes because the crystal form is the lowest energy state “most comfortable” form for these substances.
Everyone thought it was impossible to even talk about time crystals. The problem was that everyone thought that the lowest energy state was the state of rest where nothing moves. But quantum mechanics proves otherwise.
Wilczek realised that in quantum mechanics a particle can be moving and be at the same time at its lowest energy state. He took the next step and asked: what happens if this motion was periodic?
This, he concluded, meant that after a certain time the particle will return to its previous position, and the system repeats itself in time naturally. What you have then is a periodic arrangement in time at the lowest possible energy state. In other words, you have a time crystal.
From the Chalkboard to the Lab
|Once the ions get going, quantum mechanics takes over and the rotation will continue after removing the extra electric and magnetic fields.|
In July, Wilczek published an article about quantum time crystals and gave a talk about the topic in Berkeley. The question of what is a time crystal soon transformed into how to make one when Zhang from Berkeley Lab heard the talk.
The Berkeley physicist realised that the time crystal phenomena is similar to that of currents that run in superconductors without resistance or as they call them supercurrents.
The proposed setup his team published is simple: they want to create a ring-shaped space-time crystal by trapping ions at a very low temperature by using a weak magnetic field.
Here is how it works: First you trap the ions by applying an electric field which holds the charged particles in place. At very low temperatures, when the ions are close to each other in this ring shape, they will start repelling each other since they all carry the same charge. Finally, you add a weak magnetic field to make the ions start rotating around the ring.
Once the ions get going, quantum mechanics takes over and the rotation will continue after removing the extra electric and magnetic fields. As the ions are super cool, they are in the lowest energy state which quantum physics tells us must be above zero energy, and this ground state is precisely this rotation. If undisturbed, they don’t speed up or slow down. The ions arrangement keeps on repeating in space and time, and we have a space-time crystal.
An Eternal Clock not a Perpetual Motion Machine
A ring of ions that goes on rotating without any external help might seem like a perpetual motion machine. Wilczek points out in his theoretical article that this is not exactly so. “[A time crystal] must have some form of motion in its ground state and is therefore perilously close to fitting the definition of a perpetual motion machine,” he explains.
This ring can’t be used to create an engine that never stops. One reason is once a time crystal is disturbed, it is no longer in its ground state and it is no longer a time crystal. Another reason is that it is lowest energy state, so it can be only gain but not give out energy.
While not a perpetual motion machine, a time crystal is in face an eternal clock. This is so because it is in lowest energy state which means that it can’t lose energy and stop rotating. Besides keeping perfect time, this time crystal/clock will run eternally. Even the heat death of the universe won’t be able to stop its infinite rotation.
While Wilczek admits that the heart-death of the universe is “very user friendly” to time crystals, Li has an experimentalist take on this observation: “So you need to figure out a method to make a laboratory that can survive in the heat-death of the universe,” says Li.
Quantum Computer Ring
|Computer theorists predict that quantum computers will be much faster than regular computers.|
Both Wilczek and the Berkeley lab team point towards one outstanding application the space-time crystals can have: quantum computing. Research in quantum computing has been increasingly successful since the turn of the century. Computer theorists predict that quantum computers will be much faster than regular computers yet the most powerful model created to date is only 84 quantum-bits which is too small to compare to the average laptop.
Peng Zhang, co-author and member of Zhang's research group, also notes that a space-time crystal might also be used to store and transfer quantum information across different rotational states. With different rotational states standing for the 0s and 1s of a conventional computer, Wilczek postulates that it should be possible to create a quantum computer out of the ion ring.
“To make it interesting you want to have different kinds of ions, maybe several rings that affect each other,” he says. “You can start to think about machines that run on this principle.”
It would be quite interesting if time crystals that began as a theoretical curiosity helped us create the first workable quantum computer which will start another communication revolution. It says something about the importance of pure fundamental science and the unexpected applications it inspires.
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