Time Crystals. It sounds like something that would only exist in the mind of a science fiction writer, and until a few years ago that would have been true. However recent experimental advances combined with the potential to make a significant impact on the rapidly expanding field of quantum computing have made time crystals a hot topic of discussion right now.

But what are time crystals?

To understand the nature of time crystals, we must first understand what a crystal actually is. Why do some carbon atoms form worthless coal and ash, while others form valuable diamonds. 

The answer is symmetry and order. By definition, a crystal has all (or almost all) of its atoms in a well ordered arrangement. A diamond is actually just a lattice of carbon atoms, each of which is the exact same distance from its nearest neighbours. More exotic gems have combinations of different atoms, but are still simply repeating patterns of atoms, each of which is at the node of a particular type of lattice. 

But again, how does this extend to crystals of time?

In fact the term time crystals is slightly misleading. It is not a crystalline form of time, but rather a system which exhibits the same type of order and symmetry in how it evolves over time. Where a traditional crystal might have a carbon atom located at regular interval in space, a time crystal has a particular atomic state appear at regular intervals in time. That state could be a particular orientation of its spin, or some form of oscillation of the entire atom, but it happens at regular intervals in time.

However time crystals are even more interesting than that, because they also seem to remain in a form of perpetual motion. These states are in the quantum mechanical ground state, meaning that they cannot lose energy to their environment, but they also exhibit some form of periodic motion. In effect, such time crystals display motion without kinetic energy, which was previously thought to be impossible.

(The full history of the development of time crystals, from their first theoretical proposals ten years ago, through the theorems that claimed they could not exist and the counter-proofs that under certain circumstances they can, and the recent experimental verification of their existence are beyond the scope of this article, but I will write about them in the future if there is sufficient interest)

At the start of this article I mentioned that there is an increased interest in time crystals due to their application to quantum computing. Again, this is quite a complex subject, but the basic principle is that an array of "atoms" (which may be other forms of quantum state, but that isn't important here) in both space and time could be used to store qubits for quantum computing. Experiments over the past few months using ultracold atoms were able to store a single qubit that does not decay over time, and then to bring two such qubits close together to allow them to interact with each other. Such methods could be used in the future to build memory chips for quantum computers, or to develop new forms of quantum processing units. It is very new technology, but has the potential to make a significant impact on the field of quantum computing.

So that is a very brief overview of the exciting new field of time crystals. I would encourage those of you who are interested in quantum computing or even in quantum mechanics in general to watch some of the numerous videos or read the articles that explore the subject in greater detail. 

It is a truly fascinating advance for our understanding of the quantum world.