Time crystals are an intriguing state of matter in which a system exhibits periodic motion even in its lowest energy state. This challenges conventional expectations in physics. These systems arise when time translation symmetry is broken, a principle that normally ensures physical laws remain unchanged over time. 

Unlike ordinary systems, time crystals can exhibit persistent oscillations without absorbing net energy over time. This makes them a subject of great interest in condensed matter physics and a promising candidate for future technologies such as quantum computing, sensing, superconductivity, and energy storage. 

Time crystals can be classified as either discrete or continuous. An external periodic force drives discrete time crystals, while continuous time crystals emerge from the collective and self-sustained oscillations of particles. 

In this study, the authors demonstrate a method for converting a continuous time crystal into a discrete one using a process known as subharmonic injection locking. This technique synchronizes the system’s oscillations with a fraction of the driving frequency. It enables the first observation of a transition between continuous and discrete time crystal states in a system that is not in equilibrium. 

This research provides new insights into the behaviour of time crystals and introduces a powerful approach for controlling and manipulating these unusual phases of matter. 

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Time crystals: a review by Krzysztof Sacha and Jakub Zakrzewski (2018)