Session P1: Degenerate Quantum Gases and Alkaline Earth Atoms

Observation of Thermodynamics due to Optical Feshbach Resonances

Ultracold alkaline earth atoms have enabled great advances in precision measurement science, yielding the world's most precise experimentally-agreed-upon optical frequency [1]. Recent proposals have also argued that these atoms can be used for new quantum computing schemes and for the realization of novel quantum many-body systems [2,3]. For better many-body control of these systems, Feshbach resonances are desired, but there are no magnetic Feshbach resonances in alkaline earths due to their spinless electronic ground state configurations. However, optical Feshbach resonances have been observed to modify the mean-field energy of a BEC of ytterbium, an alkaline earth-like atom [4]. We present the first systematic study of the optical Feshbach resonance effect, including a detailed study of photoassociative inelastic collisions and the direct observation of light-induced thermodynamics. \\[4pt] [1] S. Blatt et al., Phys. Rev. Lett. \textbf{100}, 140801 (2008)\\[0pt] [2] A.J. Daley et al., Phys. Rev. Lett. \textbf{101}, 170504 (2008)\\[0pt] [3] A.V. Gorshkov et al., Nature Phys \textbf{6}, 289 (2010)\\[0pt] [4] K. Enomoto et al., Phys. Rev. Lett. \textbf{101}, 203201 (2008)   

Dressed Two-Body Bound States in Bose-Fermi Mixture Near Broad Interspecies Feshbach Resonances

Since the observations of molecules of Fermi atoms near Feshbach resonances, fascinating pairing correlations in cold Fermi gases have been successfully investigated both experimentally and theoretically. Interspecies Feshbach resonances in Fermi-Bose mixtures of 6Li -23Na , 40K -87Rb and 6Li -87Rb have been experimentally observed. Previous theoretical studies on Fermi-Bose mixtures have been mainly focused on narrow resonances or when the atom-molecule coupling is very weak; phase boundaries in this limit depend on atom-molecule coupling strengths. Experimentally, creating and probing correlations in Fermi-Bose mixtures near narrow resonances are more challenging than near broad resonances. So some of well studied resonances are quite broad. We have studied the evolution of dressed two-body bound states near broad interspecies Feshbach resonances which can be potentially probed in experiments. We further discuss the effect of Fermi surface dynamics due to fluctuating particle-hole pairs known as GMB (Gorkov-Melik-Barkhudarov) corrections.   

Triply degenerate quantum mixture of 41K, 40K and 6Li

We report the observation of a triply quantum degenerate mixture of $^{41}$K, $^{40}$K and $^{6}$Li atoms. It is demonstrated that bosonic $^{41}$K atom is an efficient coolant for sympathetic cooling of fermionic $^{40}$K and $^6$Li atoms. We also present our investigation of $^{41}$K and $^{40}$K, a Bose-Fermi mixture where a 12 G s-wave resonance is observed. Negligible differential gravitational sag between potassium isotopes makes this resonance an excellent candidate for studying unexplored properties of Bose- Fermi mixtures. The $^{40}$K and $^6$Li mixture provides access to strongly correlated Fermi-Fermi mixtures with imbalanced masses.   

Finite temperature properties of alkaline earth atoms in a lattice

We show that alkaline earth atoms, possessing a large spin degeneracy $N=2I+1$ up to $N=10$, when loaded into optical lattice are capable of creating significantly colder Mott insulators as $N$ increases. The recent experimental achievement of quantum degeneracy in alkaline earths opens a new vista of quantum many- body physics when these are loaded into optical lattices. These atoms possess an SU(N) symmetric interaction that can stabilize frequently exotic phases, such as antiferromagnets, valence bond solids, and spin liquids. A crucial challenge for alkalis has been reaching sufficiently cold temperatures to stabilize these phases. As a first result, we examine the temperatures achieved after standard adiabatic lattice loading protocols, for temperature in the Mott regime but above the temperature where spin physics is relevant. Our results can be stated in two illuminating ways: (1) For current initial gas temperatures, the temperatures after adiabatically loading alkaline earths with $N>2$ can be much colder than for $N=2$, even relative to the temperatures of interesting physics. (2) The ``critical" entropy of the Mott insulator grows much more rapidly with $N$ than does the initial gas entropy.   

An intercombination-line optical Feshbach resonance in a $^{88}$Sr BEC

We demonstrate control of the scattering properties of strontium atoms in a $^{88}$Sr BEC with an optical Feshbach resonance (OFR) near the $^{1}$S$_{0}-^{3}$P$_{1}$ intercombination transition at 689 nm. Significant changes in scattering length on the order of 30 a$_{0}$ can be achieved within 5 MHz detuning of several different photoassociative lines with extremely small atom losses. We observe changes of the size of the expanded BEC due to the OFR beam. The background scattering length of $^{88}$Sr is very close to zero (a = -2 a$_{0})$, so a red-detuned OFR laser leads to more negative scattering length and condensate collapse, while blue-detuned laser leads to more expansion energy, as expected if the scattering length is made more positive.   

Quantum Phases of Atom-Molecule Mixtures of Fermionic Atoms

Cold atom experiments have realized a variety of multicomponent quantum mixtures, including Bose-Fermi atomic mixtures. Mixtures of fermionic atoms and diatomic molecules, which are boson, have also been obtained by tuning of the interactions with external fields [1]. We study many-body correlations in such a system where the molecules are weakly bound and therefore pairs of fermionic atoms easily convert into and dissociate from the bound molecule state and this exchange mediates a long-range interaction between the fermions. We consider a simple many-body Hamiltonian that includes the destruction of fermionic atom pairs to form single bosonic molecules and vice versa [2]. We employ a functional renomalization-group approach and calculate the renormalized frequency-dependent interaction vertices and fermion self-energies. We find an instability from the disordered quantum liquid phase to a BCS phase and calculate the energy scale for the transition. The unusual frequency-dependence of this mediated interaction leads to strong renormalization of the self-energy, and also affects the couplings in the BCS channel. [1] M. Greiner, C. A. Regal, J. T. Stewart, and D. S. Jin, Phys. Rev. Lett. {\bf 94}, 110401 (2005) [2] E. Timmermans, K. Furuya, P. W. Milonni, and A. K. Kerman, Phys. Lett. A {\bf 285}, 228 (2001)   

Many body effects in a widely tunable Bose-Fermi mixture

A Bose-Einstein condensate immersed in the Fermi sea provides a rich platform for the study of many body effects such as polaron physics, boson-induced superfluidity and models of high-tc superconductivity. Few bosonic impurities in a Fermi sea form bosonic polarons, dressed quasi-particles that can condense, while few fermionic impurities in a Bose condensate might dress into heavy fermions with an immense increase of the effective mass. In an atom trap, both extremes of boson-fermion imbalance can in principle be realized in one and the same sample. Recently we have realized a Bose Einstein condensate of $^{41}$K immersed in a Fermi sea of $^{40}$K at $T/T_F$=0.3 and detected a wide Feshbach resonance between them. The mixture's lifetime is long enough so that bosonic polarons should form at an expected binding energy of about 0.6 $T_F$. In this talk I will summarize our observations and the progress we have made to detect polaron physics in Bose-Fermi mixtures.   

1D SU(N) spin physics with ultracold alkaline earth atoms

We treat SU(N) Hubbard chains using the density matrix renormalization group (DMRG) and an approximate Bethe ansatz. We characterize the accuracy of the Bethe ansatz for $N>2$ and find it agrees with the DMRG results for the ground state energy, with relative errors $\varepsilon < 5\%$ for $N \leq 4$. Based on this, we determine the regimes of validity of perturbation theory for all values of $N$. In particular in the strong coupling regime at unit filling, we identify the parameter range in which the ground state is described in terms of SU(N) Heisenberg models and find that it depends only weakly on N, supported by comparing correlation functions computed for the SU(N) Hubbard and Heisenberg models. This provides a prediction for the parameter range in which quantum simulators with ultracold alkaline earth atoms can uncover unconventional SU(N) spin physics. We complement the study by discussing the fidelity susceptibility, which appears to possess a minimum at the critical point $U_c$, and the Luttinger parameter in the gapless phases.   

Ultracold Interactions between Li Feshbach molecules and Yb atoms

We report on collisional studies in an ultracold three-component system of $\mathrm{^{174}Yb}$ and two spin states of $\mathrm{^{6}Li}$ with tunable interactions. The Li-Li \emph{s}-wave scattering length can be varied through a Feshbach resonance at 834 G. We study the effect of the non-resonant Yb cloud on the formation rate and stability of Li Feshbach dimers and compare to theoretical prediction. We discuss the potential usefulness of a third non-resonant component for the efficient production and cooling of Feshbach molecules.   

Novel spontaneous decay driven scheme of Sisyphus cooling in optical dipole traps

We present a spontaneous decay driven Sisyphus cooling scheme for atoms trapped in far-off-resonant optical dipole traps (ODT). The cooling is achieved by exploiting the difference in the potential energy for atoms trapped in different electronic states. The cooling cycle consists of optical pumping of atoms into the excited state near the bottom of the trap followed by spontaneous decay into the ground state at random position in the ODT. There are no inherent atomic losses in this method. Our cooling scheme shares similarities with one proposed in J. Janis et al. Phys. Rev. A $\textbf{71}$, 013422 (2005) for cooling atoms in a magnetic trap. The presented cooling method sets constraints on the wavelength of the ODT and requires a narrow cooling transition with linewidth on order of the trapping frequencies. Therefore narrow transitions are preferred. However hyperfine splitting of the ground state is not required and optical cooling is combined with a tight spatial confinement. We investigate this problem theoretically and perform numerical simulations for the particular case of $^{88}$Sr. The presented scheme is promising for cooling atomic species with an available narrow transition for example alkali-earth elements such as strontium and calcium.