What are time crystals and why did they seem impossible?

The answer

Time crystals are a phase of matter whose structure repeats periodically in time rather than in space, like conventional crystals. First theorized by Nobel laureate Frank Wilczek in 2012, they were initially deemed impossible because they seemed to violate thermodynamic equilibrium. In 2021, Google's Sycamore quantum processor created a discrete time crystal using 20 superconducting qubits that oscillated between two states indefinitely without absorbing energy. These systems are periodically driven but respond at a different frequency - a spontaneous breaking of time-translation symmetry. Time crystals have since been created in various platforms including trapped ions and nitrogen-vacancy centers in diamonds. They may have applications in quantum memory and precision sensing technologies.