A Time Crystal Without a Driver

In conventional crystals, a highly ordered arrangement of atoms or molecules repeats periodically in space, unaffected by external perturbations. In 2012, theorist Frank Wilczek imagined this ordering extended to the fourth dimension in a system that moves periodically in time at an internally set rhythm (see 15 October 2012 Viewpoint). These so-called time crystals became a reality five years later, but so far, these quantum systems have all required some external driver. Now, Valerii Kozin from the University of Iceland and Oleksandr Kyriienko from the University of Exeter, UK, have explained how to create a time crystal completely isolated from the environment. Such a stable, closed system could be exploited for precision measurement tools and quantum information storage.