# Time crystals: a review

## TL;DR

This review discusses the development, experimental verification, and theoretical generalizations of time crystals, focusing on discrete and continuous time translation symmetry breaking in condensed matter physics.

## Contribution

It provides a comprehensive overview of the progress in understanding and realizing time crystals, including experimental challenges and new theoretical proposals.

## Key findings

- Experimental observation of discrete time crystals
- Theoretical models for spontaneous symmetry breaking in time
- Generalizations to continuous time translation symmetry

## Abstract

Time crystals are time-periodic self-organized structures postulated by Frank Wilczek in 2012. While the original concept was strongly criticized, it stimulated at the same time an intensive research leading to propositions and experimental verifications of discrete (or Floquet) time crystals -- the structures that appear in the time domain due to spontaneous breaking of discrete time translation symmetry. The struggle to observe discrete time crystals is reviewed here together with propositions that generalize this concept introducing condensed matter like physics in the time domain. We shall also revisit the original Wilczek's idea and review strategies aimed at spontaneous breaking of continuous time translation symmetry.