Inching Toward Equilibrium

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Science  07 May 2010:
Vol. 328, Issue 5979, pp. 671
DOI: 10.1126/science.328.5979.671-c

Ultracold Bose and Fermi gases in optical lattices show promise to simulate quantum phases in condensed-matter systems. The manipulation of atoms in these experiments often involves nonequilibrium processes, such as ramping up the lattice potential; nevertheless, an equilibrium thermodynamics framework is normally used to describe the system, provided that the speed of tunneling between neighboring sites is faster than the ramping rate.

Now, Hung et al. demonstrate that the equilibration process after a ramp can be surprisingly slow. They load an almost pure, flat Bose-Einstein condensate into a two-dimensional optical lattice and use in situ imaging to monitor the density profile evolution after a ramp for various final lattice depths close to the boundary between the superfluid and Mott insulator regimes. The characteristic time for reaching an equilibrium mass distribution increases with the final lattice depth and is two orders of magnitude higher than the tunneling time scale. A radial temperature gradient also appears. The authors suggest that the sluggish mass and heat flow are a consequence of quantum criticality, as well as the low dimensionality of the system.

Phys. Rev. Lett. 104, 160403 (2010).

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