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Highlights of the Year

Cooking Up a Time Crystal

Time crystals are quirky states of matter whose structure repeats both in time and in space. The idea, theorized five years ago by Frank Wilczek (see Viewpoint: Crystals of Time), was initially discarded because theorists proved that time crystals cannot exist in thermal equilibrium. But this year, a quartet of US-based scientists exploited an open loophole in the argument against time crystals: such states can exist in non equilibrium systems that are driven periodically by an external force. The researchers presented a recipe for cooking up a time crystal using a string of cold, trapped ions (see Viewpoint: How to Create a Time Crystal). In their scheme, the ions are subjected to periodic spin-aligning pulses. The team predicted that the ions would evolve to form time crystals, whose signatures would be periodic oscillations in the spins’ magnetizations. Within three months of the proposal, time crystals were realized in two different systems: a chain of trapped atoms and spin impurities in diamond.

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