The seminal papers in this field: Quantum Time Crystals (Wilczek) http://arxiv.org/abs/1202.2539 http://arxiv.org/abs/1202.2539 Classical Time Crystals (Shape

The seminal papers in this field: Quantum Time Crystals (Wilczek) http://arxiv.org/abs/1202.2539 http://arxiv.org/abs/1202.2539 Classical Time Crystals (Shapere, Wilczek) http://arxiv.org/abs/1202.2537 http://arxiv.org/abs/1202.2537 Space-time Crystals of Trapped Ions ( Li, Gong, Yin, Quan, Yin, Zhang, Duan,Zhang ) http://arxiv.org/abs/1206.4772 http://arxiv.org/abs/1206.4772 All three papers appeared in a single issue of Physical Review Letters (The fancy Physics journal). PRL also issued this Physics Viewpoint, a popular science article: http://physics.aps.org/articles/v5/116 http://physics.aps.org/articles/v5/116