Observation of Correlated Particle-Hole Pairs and String Order in   Low-Dimensional Mott Insulators
  Quantum phases of matter are characterized by the underlying correlations of the many-body system. Although this is typically captured by a local order parameter, it has been shown that a broad class of many-body systems possesses a hidden non-local order. In the case of bosonic Mott insulators, the ground state properties are governed by quantum fluctuations in the form of correlated particle-hole pairs that lead to the emergence of a non-local string order in one dimension. Using high-resolution imaging of low-dimensional quantum gases in an optical lattice, we directly detect these pairs with single-site and single-particle sensitivity and observe string order in the one-dimensional case. 
M. Endres, M. Cheneau, T. Fukuhara, C. Weitenberg, P. Schauss, C.   Gross, L. Mazza, M.C. Banuls, L. Pollet, I. Bloch, S. Kuhr
cond-mat.quant-gas quant-ph