Anatomy of a Phase Shift PFC-supported researchers have identified the characteristics of a key phase transition that is significant for fundamental condensed-matter physics as well as quantum information science. The traditional model used to describe the state of ultracold atoms in an optical lattice – the Bose-Hubbard model – is limited by the fact that it only describes homogeneous systems. But ultracold atomic gases in a lattice are, in fact, “globally” inhomogeneous. That is, they contain different phases at different locations because of the nature of the trap. In 2009, an international team of theorists devised a scheme for describing the state of such systems. Now the PFC team’s experimental measurements of what happens during the phase transition provided evidence confirming the theorists’ model, and identifying the transition point as a function of three critical variables. (See diagram at right.) “Phases of a 2D Bose Gas in an Optical Lattice,” K. Jimenez-Garcia, R.L. Compton, Y.-J. Lin, W.D. Phillips, J.V. Porto, I.B. Spielman, Phys. Rev. Lett., 105, 110401 (2010)