Two groups of researchers, based at Harvard University and University of Maryland, reported in Nature on March 9 that they had created ‘time crystals’: something seemingly straight out of science fiction, featuring structures in which atoms and molecules are arranged across space and time.
The theoretical basis for the existence of time crystals was established in 2015 by Prof Shivaji Sondhi, theoretical physicist at Princeton, his then graduate student Vedika Khemani (who was also part of the Harvard team that published in Nature), and Achilleas Lazarides and Roderich Moessner of the Max Planck Institute in Dresden, Germany.
In an email interview, Prof Shivaji Sondhi explained to Anuradha Mascarenhas the significance of building a class of crystals that were once thought impossible to create.
What are time crystals? Why is it important that they have been created?
Time crystals are systems where the role of space in ordinary crystals is replaced by time. In ordinary crystals, a very large number of atoms — in principle an infinite number — show periodicity in space; in time crystals they show periodicity in time. Their importance lies in this replacement of space by time — which is new, realising an idea of the Nobelist Frank Wilczek. The concept of time crystals originated with Wilczek at the Massachusetts Institute of Technology in 1975. In an ordinary crystal, like table salt, atoms are arranged in a repeating pattern. Interactions between neighbouring atoms keeps the crystal rigid and prevents it from dissolving into nothing at the slightest shake. Shouldn’t then there also be crystals that break translational symmetry in time? Atoms move in time, but instead of moving in a fluid or continuous way, they move in a periodic way. It was an idea that led to debates whether such crystals could exist.
How does the new discovery change ways of thinking about matter?
The work addresses some of the most fundamental questions about the nature of matter. Normally one thinks of matter in equilibrium — how it settles when left untouched. The ones which have been created involve constant interaction with the outside world and yet we find that they can show very distinctive behaviour which is mathematically precise. This is new for a quantum system.
How were the time crystals created? What after this discovery?
Our work discovered the essential physics of how time crystals function. The discovery builds on a set of developments that gets at the issue of how we understand complex systems in and out of equilibrium — which is centrally important to how physicists explain the nature of the everyday world. Both experiments build on a lot of work over the past decade and more on controlling small quantum systems. So while they made it look easy, there was a lot of work behind it. Both papers have 26 authors between them! As a result of these theoretical studies, two groups of experimenters began attempting to build time crystals in the laboratory. The Harvard based team used an experimental set-up that involved creating an artificial lattice in a synthetic diamond. A different approach at the University of Maryland used a chain of charged particles called ytterbium ions.
I think we will see more new phases of matter produced by this kind of experimental work. They won’t all be time crystals but they will share the feature that they do not exist if matter is left alone. They also eventually may help protect information in futuristic devices known as quantum computers.
(Edited excerpts from interview)