A detailed theoretical recipe for making time crystals has been unveiled and swiftly implemented by two groups using vastly different experimental systems.
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  (see 15 October 2012 Viewpoint), time crystals were met with widespread attention, but also a healthy dose of skepticism . This ignited a debate in the literature, culminating in a proof that time crystals cannot exist in thermal equilibrium, as originally imagined by Wilczek . 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 , and two describing subsequent experimental demonstrations in systems of trapped ions  and spin impurities in diamond  (both posted on the physics arXiv preprint server).
Empty space exhibits continuous translation symmetry: nothing distinguishes one point from any other. Yet ordinary crystals break this symmetry because atoms are periodically arranged in specific locations and display long-range spatial correlations. Given that we live in four-dimensional spacetime, it is natural to wonder if an analogous process of crystallization and symmetry breaking can arise along the time dimension as well . If it does, then any such time crystal should return back to its initial state at specific times, while spontaneously locking to an oscillation period that differs from that of any external time-dependent forces. Hence this definition excludes all known classical oscillatory systems such as waves or driven pendulums.
APS Physics Viewpoint: How to Create a Time Crystal, Phil Richerme
#P4TC: Time Crystals, October 13, 2016