Session P6: Ultracold Gases with Dipolar Interactions

Ultracold polar molecules

Polar molecules - molecules exhibiting a permanent electric dipole moment - have bright perspectives as systems with long-range and anisotropic interaction. These interactions have been the basis for numerous exciting theoretical proposals ranging from ultra-cold chemistry, precision measurements, quantum phase transitions to novel systems for quantum information processing and quantum control with external magnetic and electric fields. We will present our recent work on the creation of a near quantum degenerate gas of rovibrational ground state polar $^{40}$K$^{87}$Rb molecules. Using a single step of two photon coherent transfer, we transfer weakly bound KRb molecules to the rovibrational ground state of the singlet electronic ground molecular potential. The polar molecules have a permanent electric dipole moment, which we measure with Stark spectroscopy to be 0.566(17) Debye.   

Effects of magnetic dipolar interactions on collective modes and instabilities of alkali BECs

In this talk I will review the phenomenon of roton softening in systems with dipolar interactions. Special emphasis will be on magnetic dipolar interactions in Bose condensates of alkali atoms, when fast Larmor precession and spin dynamics strongly modify the character of unstable modes. I will also discuss the enhancement of roton softening in multilayer stacks of two dimensional condensates. Implications of these theoretical results for recent experiments with Rb-87 and K-39 atoms will be discussed.   

Evidence for supersolid behaviour in a spin-1 rubidium gas

I will present experimental evidence of the coexistence of two types of symmetry-breaking long-range order in a quantum degenerate gas of spin-1 $^{87}$Rb atoms: crystalline magnetic ordering, which spontaneously breaks translational symmetry, and long-range phase coherence of a macroscopic wavefunction, which spontaneously breaks the $U(1)$ gauge symmetry associated with the particle number. Such a gas was prepared by gradually cooling non-degenerate, unpolarized, optically trapped gases into the regime of quantum degeneracy. Using a high-resolution magnetization-sensitive imaging method, we observe a phase transition below which the quantum gas forms a crystalline array of magnetic domains. Based on our previous experiments on the evolution of helical spin textures in such a gas, we ascribe this crystalline order to the competition between a short-range isotropic ferromagnetic interaction and a long-range anisotropic dipolar interaction. We confirm the phase coherence of this gaseous crystal by atom interferometry. Specifically, we use a form of Bragg spectroscopy to measure the first-order correlation function of the superfluid order parameter at variable distance. The coexistence of translational symmetry breaking, characteristic of a solid, and long-range phase coherence, characteristic of a superfluid, are hallmarks of the sought-after supersolid phase.   

Dynamics and coherence in a collapsing dipolar BEC

Chromium atoms in a Bose-Einstein condensate (BEC) interact - in addition to s-wave scattering - via magnetic dipole-dipole interaction. Although the magnetic forces between the atoms which carry a large magnetic moment of 6 Bohr magnetons are still rather weak, they can become the dominant interaction when a Feshbach resonance is used to reduce the contact interaction to zero [1]. In this regime, the stability of a chromium Bose-Einstein condensate depends on the geometry of the trap. This is an intrinsic and unique effect of an anisotropic interaction. We have measured the stability diagram of such a dipolar BEC by exploring the border between stable and unstable regions [2]. When we cross this border with an initially stable condensate by a sudden change of the scattering length into the unstable regime, we observe the collapse and subsequent explosion due to dipole-dipole interaction [3]. The anisotropy of the underlying interaction reveals in the formation of a non-trivial structure during the collapse. I will discuss the dynamics of the collapse depending on the geometry of the trapping potential which we have studied experimentally and compare the results to three dimensional numerical simulations of the Gross-Pitaevskii equation. By interfering several condensates collapsing at the same time, we also study the coherence properties of the collapsing clouds. \\[4pt] [1] T. Lahaye et al. {\it Nature} {\bf 448}, 672 (2007)\\[0pt] [2] T. Koch et al. {\it Nature Physics} {\bf 4}, 218 (2008)\\[0pt] [3] T. Lahaye et al. {\it Phys. Rev. Lett.} {\bf 101}, 080401 (2008)   

Condensed Matter Physics and Quantum Simulations with Cold Polar Molecules and Rydberg Atoms

Polar molecules prepared in their electronic and vibrational ground state are characterized by large dipole moments associated with rotational excitations. This gives rise to large dipole-dipole interactions between molecules, which can be manipulated with external DC and AC microwave fields. The possibility to tune these strong, long-range and anisotropic interactions, combined with the trapping in reduced dimensions, raises interesting prospects for cold ensembles of polar molecules as \textit{strongly correlated} condensed matter system, i.e. provides \textit{analog quantum simulation} of strongly interacting systems and quantum phases. Specific examples to be discussed include the engineering of various spin and Hubbard models for polar molecules in optical lattices, e.g. the Kitaev model and three body interactions, and the tailoring effective molecular interaction potentials based on ``blue-shields'' with microwave fields. Furthermore, dipolar gases in 1D and 2D trapping geometries can form self-assembled lattice structures in single layer and bilayer systems. In addition, a mixture of cold atoms and polar molecules forming a self-assembled lattice gives rise to a new realization of Hubbard models with widely tunable lattice parameters and small lattice spacing, which represents a systems with both strong correlation and phonon dynamics. Extremely strong dipolar or Van der Waals interactions can also be obtained in cold atomic gases excited to Rydberg states. We will briefly discuss new ideas of developing a \textit{digital quantum simulator} for spin models. It is based on performing a stroboscopic sequence of many particle quantum gates based on manipulating dipolar interactions between groups of Rydberg atoms in large spacing optical lattices.