How to Create a Time Crystal

The story of time crystals—whose lowest-energy configurations are periodic in time rather than space—epitomizes the creative ideas, controversy, and vigorous discussion that lie at the core of the scientific process. Originally theorized by Frank Wilczek in 2012 [1] (see 15 October 2012 Viewpoint), time crystals were met with widespread attention, but also a healthy dose of skepticism [2]. This ignited a debate in the literature, culminating in a proof that time crystals cannot exist in thermal equilibrium, as originally imagined by Wilczek [3]. But the tale did not end there. It was later argued that time crystals might still be possible in periodically driven systems, which can never reach thermal equilibrium [4–6]. Three recent papers have now completed the story, one proposing a roadmap for creating a nonequilibrium time crystal in the lab [7], and two describing subsequent experimental demonstrations in systems of trapped ions [8] and spin impurities in diamond [9] (both posted on the physics arXiv preprint server).