Absence of Quantum Time Crystals.

@article{Watanabe2015AbsenceOQ,
  title={Absence of Quantum Time Crystals.},
  author={Haruki Watanabe and Masaki Oshikawa},
  journal={Physical review letters},
  year={2015},
  volume={114 25},
  pages={
          251603
        }
}
In analogy with crystalline solids around us, Wilczek recently proposed the idea of "time crystals" as phases that spontaneously break the continuous time translation into a discrete subgroup. The proposal stimulated further studies and vigorous debates whether it can be realized in a physical system. However, a precise definition of the time crystal is needed to resolve the issue. Here we first present a definition of time crystals based on the time-dependent correlation functions of the order… 

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