$^{31}$P NMR study of discrete time-crystalline signatures in an ordered crystal of ammonium dihydrogen phosphate. (arXiv:1802.00457v1 [quant-ph])

The rich dynamics and phase structure of driven systems includes the
recently-described phenomenon of the "discrete time crystal" (DTC), a robust
phase which spontaneously breaks the discrete time translation symmetry of its
driving Hamiltonian. Experiments in trapped ions and diamond NV centers have
recently shown evidence for this DTC order. Here we show NMR data of DTC
behavior in a third, strikingly different system: a highly ordered spatial
crystal in three dimensions. We devise a novel DTC echo experiment to probe the
coherence of the driven system. We examine potential decay mechanisms for the
DTC oscillations, and demonstrate the important effect of the internal
Hamiltonian during nonzero duration pulses.

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