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arXiv:1603.08001 · Physical Review Letters

Floquet Time Crystals

Dominic V. Else, Bela Bauer, Chetan Nayak

Published Aug 25, 2016• 969 citations
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Abstract

We define what it means for time translation symmetry to be spontaneously broken in a quantum system and show with analytical arguments and numerical simulations that this occurs in a large class of many-body-localized driven systems with discrete time-translation symmetry.
DOI
10.1103/physrevlett.117.090402

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Abstract Time crystals are physical systems whose time translation symmetry is spontaneously broken. Although the spontaneous breaking of continuous time-translation symmetry in static systems is proved impossible for the equilibrium state, the discrete time-translation symmetry in periodically driven (Floquet) systems is allowed to be spontaneously broken, resulting in the so-called Floquet or discrete time crystals. While most works so far searching for time crystals focus on the symmetry breaking process and the possible stabilising mechanisms, the many-body physics from the interplay of symmetry-broken states, which we call the condensed matter physics in time crystals, is not fully explored yet. This review aims to summarise the very preliminary results in this new research field with an analogous structure of condensed matter theory in solids. The whole theory is built on a hidden symmetry in time crystals, i.e., the phase space lattice symmetry, which allows us to develop the band theory, topology and strongly correlated models in phase space lattice. In the end, we outline the possible topics and directions for the future research.
Time crystals are proposed states of matter that spontaneously break time translation symmetry. Despite much recent interest, there is no settled definition for such states and existing definitions only tangentially refer to the well-established foundations of spontaneous symmetry breaking. We offer a definition of time crystals, which treats time translation much like any other symmetry and follows the traditional recipe for defining Wigner symmetries and order parameters. Using our definition, we find: (i) systems with time independent Hamiltonians should not exhibit time translation symmetry breaking, and (ii) the many-body localized $\ensuremath{\pi}$ spin glass/Floquet time crystal can be viewed as breaking both a global internal symmetry and the time translation symmetry, as befits the two aspects of its name.
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Spontaneous symmetry breaking is a ubiquitous concept and is well described in many textbooks of physics. However, direct observation of spontaneous symmetry breaking is lacking. Here we present the observation and analysis of a spontaneously broken discrete time-translation symmetry in our driven system. We experimentally find a 50-50 split between two stable and temporal-distinct solutions, indicative of the breaking of a Z2{\mathbb{Z}}_{2}Z2​ (Ising-like) time-translation symmetry. The experiment allows for further exploration of the symmetry breaking in our discrete time crystal and for engineering excitations in space and time in the quantum domain.