In recent years, physicists have made great strides toward better understanding the equilibration and thermalization of isolated, many-body quantum systems. Already, two distinct phases are well known: there exist systems that thermalize completely, such that for an arbitrary initial pure state any sufficiently small subregion will eventually approach the Gibbs ensemble; and, by contrast, there are systems that exhibit many-body localization (MBL) due to a strong disorder potential, failing to thermalize at any time despite weak interactions. Here we address the question of whether phases of matter could exist between the extremes of full thermalization and MBL in a generic (i.e. non-integrable), isolated, many-body quantum system. I will describe a new diagnostic for thermal many-body wavefunctions, involving making a partial (projective) measurement on a subset of the degrees of freedom and analyzing the spatial entanglement properties of the unmeasured degrees of freedom. In a Quantum Disentangled state, the post-measurement entanglement entropy is an area law, revealing a hidden locality. Eigenstates of such form would indicate a subtle breakdown of thermalization due to quantum effects. Numerics searching for such Quantum Disentangled eigenstates in 1d Hubbard-type models will be discussed in detail.

Matthew Fisher is a theoretical physicist determined to bring quantum mechanics down to earth. After earning his PhD in1986 from the University of Illinois, Matthew was a Research Staff member at IBM's Watson Research Center. Since 1993 he has been a Professor of Physics at UC Santa Barbara, except for a brief stint at Caltech in 2009-2010. In 1995, Matthew received the Alan T. Waterman Award bestowed by the National Science Foundation, and has also been the recipient of the National Academy of Sciences Award for Initiatives in Research (1997). He was elected as a member of the American Academy of Arts and Sciences in 2003 and to the National Academy of Sciences in 2012. Matthew was a co-recipient of the 2015 Oliver E. Buckley Prize in Condensed Matter Physics given by the American Physical Society.