Time crystals

A time crystal, space-time crystal, or four-dimensional crystal, is a periodic structure that repeats in time, as well in space. Normal three-dimensional crystals have a repeating pattern in space, but remain unchanged with respect to time; time crystals repeat themselves in time as well, leading the crystal to change from moment to moment. Time crystals extend the idea of a crystal to four dimensions. A time crystal never reaches thermal equilibrium as it is a type of non-equilibrium matter - a form of matter proposed in 2012, and first observed in 2017. This state of matter cannot be isolated from its environment - it is an open system in non-equilibrium. This allows for the crystal to be in perpetual motion. In technical terms, a time crystal spontaneously breaks the symmetry of time translation and thus it is said to exhibit time translation symmetry breaking (TTSB).

The idea of a time crystal was first put forward by Nobel laureate and MIT professor Frank Wilczek in 2012. Subsequent work then developed a more precise definition for time crystals, ultimately leading to a proof that time crystals in equilibrium are not possible. However, these proofs left the door open for time crystals in non-equilibrium systems. In 2016, Norman Yao and his colleagues from the University of California, Berkeley put forward a concrete proposal that would allow time crystals to be created in a laboratory environment. Yao's blueprint was then used by two teams, a group led by Christopher Monroe at the University of Maryland and a group led by Mikhail Lukin at Harvard University, who were both able to successfully create a time crystal. Both experiments were published in the journal Nature in March 2017.

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