Time crystal

A time crystal is a quantum system that has repeating periodic oscillations of one or more physical measurements in its ground state when seen over time.
Just as an ordinary crystal is seen to have its atoms arranged in repeating patterns in space, so a time crystal has a repeating patterns that occur over time. 
In 2012, the American Nobel Prize winner Frank Wilczek proposed the concept of a time crystal.
In 2017 the atomic physicist Chris Monroe and his group found evidence of oscillations in optically excited chains of ytterbium ions with the behavior predicted for time crystals but not in the same way theorized in 2012.
In 2021, "for the first time, physicists have filmed the oscillation of a time crystal", media said.
In terms of practical use, time crystals may one day be used as quantum memories.