![]() Recently, a quantum time crystal that breaks time-translation symmetry continuously has been observed in an atomic Bose–Einstein condensate inside an optical cavity, manifested in the emergence of spontaneous oscillations of the intracavity photon number under optical pumping 14. Key signatures of the many-body-localized discrete time crystal state have also been observed on a quantum simulation platform based on individually controllable carbon-13 nuclear spins in diamond 10, and on arrays of superconducting qubits in a quantum processor 11, 12, whereas a prethermal discrete time crystal (a non-equilibrium driven phase without disorder) has been realized in a trapped-ion quantum simulator 13. Although it has been shown that such closed systems, breaking continuous time-translation symmetry by exhibiting oscillatory dynamics, are prohibited by nature 2, discrete time crystals that show time-translation symmetry breaking imposed by an external modulated parametric drive have been realized on various platforms, including an interacting spin chain of trapped atomic ions 3 a disordered ensemble of spin impurities in diamond 4 a crystal lattice of nuclear spins in an ammonium dihydrogen phosphate crystal 5 an ensemble of ultracold atoms, in which the periodic structure in both space and time is observed 6 a Bose–Einstein condensate of magnons, in which an oscillating magnetic field induces coherent spin precession at a frequency incommensurate with the drive frequency 7 a Kerr non-linear optical microcavity pumped by two lasers, with self-injection locking 8 and a curved array of interacting microwave waveguides 9. A time crystal, as originally proposed by Wilczek 1, is a quantum many-body system whose lowest-energy state is one in which the particles are in continuous oscillatory motion. ![]() In recent years, the physics community has been captivated by the newly described phase of matter known as a ‘time crystal’, with broken time-translation symmetry, analogous to conventional crystals in which space-translation symmetry is broken. In nature, these symmetry-breaking crystalline states with long-range order are achieved spontaneously through a phase transition (for example, from water to ice). Isotropic homogeneous matter is invariant under space-translation symmetry while in crystals with periodic atomic lattices that symmetry is broken. The phenomenon is of interest to the study of dynamic classical many-body states in the strongly correlated regime and applications in all-optical modulation, frequency conversion and timing. Here we report that a classical metamaterial nanostructure, a two-dimensional array of plasmonic metamolecules supported on flexible nanowires, can be driven to a state possessing all of the key features of a continuous time crystal: continuous coherent illumination by light resonant with the metamolecules’ plasmonic mode triggers a spontaneous phase transition to a superradiant-like state of transmissivity oscillations, resulting from many-body interactions among the metamolecules, characterized by long-range order in space and time. Quantum time crystals with discretely broken time-translation symmetry have been demonstrated in trapped ions, atoms and spins whereas continuously broken time-translation symmetry has been observed in an atomic condensate inside an optical cavity. ![]() ![]() Time crystals are an eagerly sought phase of matter with broken time-translation symmetry. ![]()
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