Optical cavities exhibit two key figures: the rate for scattering into a cavity mode relative to all free-space modes, termed Purcell factor, and the spectral width of the transmission resonances. We have now trapped atoms in a cavity operating in the ultimate quantum regime, when the Purcell factor exceeds unity and the frequency shift associated with scattering of a single photon exceeds the cavity bandwidth. In this regime the atoms can be manipulated by tailored dissipation with sub-recoil resolution. We observe cavity-induced heating of a Bose-Einstein condensate and subsequent cooling at particle densities and temperatures incompatible with conventional laser cooling.

Science (2012), in press 

We have condensed bosons in the 4th band of an optical chequerboard lattice providing a topologically induced avoided band crossing involving the 2nd, 3rd, and 4th bands. When the condensate is slowly tuned through the avoided crossing, accelerated band relaxation arises and the zero momentum approximately C4-invariant condensate wave function acquires finite momentum order and reduced C2 symmetry. For faster tuning Landau-Zener oscillations between different superfluid orders arise, which are used to characterize the avoided crossing.

Physical Review Letters, 108, 075302 (2012)

arXiv:1110.3716v2

We have devised an experimental protocol, supporting a theoretical proposal to realize a topological semimetal in a fermionic optical lattice.

K. Sun, W. Vincent Liu, A. Hemmerich, and S. Das Sarma

Nature Physics 8, 67–70 (2012)

Several branches of modern atomic physics (metrology, precision spectroscopy, quantum information, quantum gas physics) can profit substantially from ultracold samples of two electron atoms. We demonstrated a new route to Bose-Einstein condensation, specifically designed for such two electron atoms, which do not admit practicable cooling schemes via intercombination lines, as for example calcium or magnesium. We present the second Bose-Einstein condensate of calcium atoms worldwide with a data basis unambiguously showing, that condensation was achieved. Our method is versatile and robust, and it can be used to condense other two-electron atoms in any long-lived state (i.e., not necessarily the ground state), which can be loaded via optical pumping.

arxiv.org/abs/1201.2573

Emulating condensed-matter physics with ground-state atoms trapped in optical lattices has come a long way. But excite the atoms into higher orbital states, and a whole new world of exotic states appears.

Maciej Lewenstein & W. Vincent Liu feature our work on orbital superfluidity in a news and views article in Nature Physics.

The successful emulation of the Hubbard model in optical lattices has stimulated extensive efforts to extend their scope to also capture more complex, incompletely understood scenarios of many-body physics. A promising approach is to consider higher bands, where the orbital degree of freedom gives rise to a structural diversity that is directly relevant, for example, for the physics of strongly correlated electronic matter. Here we report evidence for the formation of a superfluid in the P-band of a bipartite optical square lattice with S-orbits and P-orbits arranged in a chequerboard pattern. The observed momentum spectra feature cross-dimensional coherence with a lifetime of nearly 20 ms. Depending on the value of a small adjustable anisotropy of the lattice, our findings are explained either by real-valued striped superfluid order parameters with different orientations P_x ± P_y, or by a complex-valued P_x ± i P_y order parameter, which breaks time-reversal symmetry.

Nature Physics 7, 147-153 (2011)
arXiv:1006.0509v3 (2010)

We report on the first observation of bosons condensed into the energy minima of an F-band of a bipartite square optical lattice. Momentum spectra indicate that a truly complex-valued staggered angular momentum superfluid order is established. The corresponding wave function is composed of alternating local F_2x³-3x + i F_2y³-3y-orbits and local S-orbits residing in the deep and shallow wells of the lattice, which are arranged as the black and white areas of a checkerboard. A pattern of staggered vortical currents arises, which breaks time reversal symmetry and the translational symmetry of the lattice potential. We have measured the populations of higher order Bragg peaks in the momentum spectra for varying relative depths of the shallow and deep lattice wells and find remarkable agreement with band calculations.

Phys. Rev. Lett. 106, 015302 (2011)

arXiv:1008.4752v1