Session B3: Dipolar Molecular Gases

Many-body spin systems with ultracold polar KRb molecules

Long-range dipolar interactions are expected to facilitate understanding of strongly correlated many-body quantum systems such as quantum magnetism. We have used dipolar interactions of polar molecules pinned in a three-dimensional optical lattice to realize a lattice spin model where spin is encoded in rotational states of molecules that are prepared and probed by microwaves. The many-body dipolar interactions are apparent in the evolution of the spin coherence, which shows oscillations in addition to an overall decay of the coherence. The frequency of these oscillations depends on the strength of the dipolar interaction, which we can vary, and agrees quantitatively with a dipolar spin-exchange model. However, the absence of an external electric field precludes the study of the full spin-1/2 Hamiltonian that includes the Ising interaction. We are now building a novel apparatus that will allow us to reach large electric fields with full tunability of the relative strength of the Ising and exchange terms. Moreover, we anticipate imaging our sample with a high-NA microscope objective, allowing microscopic studies of a many-body spin system of polar molecules.   

Suppressing the loss of ultracold molecules via the continuous quantum Zeno effect

We develop theoretical methods to explain the recently observed suppression of chemical reactions between two rotational states of fermionic KRb molecules confined in 1D tubes with a superimposed optical lattice along them [Yan {\it et al}., Nature 501, 521 (2013)]. The loss suppression is a consequence of both lattice confinement and the continuous quantum Zeno effect, which in this case takes place in the regime where the two-body loss is larger than other energy scales in the lattice. To quantitatively analyze the experiment, we derive a renormalized single-band model which accounts for 3D multi-band effects, and formulate from it a rate equation and mean-field theory validated by comparing with numerically exact t-DMRG. We demonstrate that the renormalized model captures the measured dependence of the loss rate on all lattice parameters, allowing us to determine the filling fraction.   

Dipolar effects in collisions of magnetic ultracold molecules

Formation of Er$_2$ Feshbach molecules has been recently achieved in Innsbruck from magneto-associated ultracold Er atoms. Such molecules can have strong magnetic dipole moments (up to 14 $\mu_B$) and long-range dipole-dipole effects can be observed in the ultracold regime. Using a time-independent quantum formalism, we compute the Er$_2$ + Er$_2$ loss rate coefficient as a function of a tilt angle of an applied magnetic field with respect to the axis of a pancake-shaped two-dimensional confinement. We will present the loss rate coefficient for two different configurations of magnetic fields, one corresponding to a head-to-tail collision and the other corresponding to a side-by-side collision, and compare with recent experimental results for different magnetic dipole moments.   

Statistical product-state distributions for ultracold exoergic reactions in external fields

The first ultracold chemistry experiments were recently performed at JILA, Colorado. Using an ultracold gas of KRb molecules, the group demonstrated strong effects on reaction rates due to quantum statistics, external electric fields, and reduced dimensionality/orientation. While Qu\'em\'ener and Bohn provided the theoretical interpretation of the observed loss rates, Idziaszek and coworkers, and Gao have developed simple quantum models for reaction rates and identified different universality classes. The most important open question is that of product-state distributions. These are very sensitive to the details of the reaction dynamics and could lead to a deeper understanding of the underlying physics. A priori, a rigorous description of these reactions can be derived from the quantum-mechanical formalism of Tscherbul and Krems. Yet, as argued by Mayle et al., the huge number of rovibrational states involved makes such approach impractical for most cases of current experimental interest. I will discuss our efforts in deriving statistical product-state distributions for ultracold exoergic reactions in external fields. These can be used as benchmarks for the funding assumptions of the theory and provide tests for the statistical arguments of Mayle et al.   

Electronic structure of the paramagnetic and polar molecule RbSr

We determine the electronic structure of RbSr, a molecule possessing both a permanent magnetic and electric dipole moment in its own frame allowing its manipulation with external fields at ultracold temperature. Two complementary ab-initio approaches are used: an approach relying based on optimized effective core potentials with core polarization potentials based on a full configuration interaction involving three valence electrons, and an approach using a small-size effective core potential with 19 explicitly correlated electrons in the framework of coupled-cluster theory. We have found excellent agreement between these two approaches for the ground state properties including the permanent dipole moment. We have focused on studies of excited states correlated to the two lowest asymptotes Rb(5p $^{2}$P)$+$Sr(5s$^{2} \quad ^{1}$S) and Rb(5s $^{2}$S)$+$Sr(5s5p $^{3}$P) relevant for ongoing experiments on quantum degenerate gases. We present also the Hund c case potential curves obtained using atomic spin-orbit constants. These potential curves are an excellent starting point for spectroscopic studies of RbSr and of ultracold molecule formation processes.   

Towards Creating Fermionic Ground State Molecules of $^{23}$Na$^{40}$K with Strong Dipolar Interactions

The realization of interesting interactions beyond the simple contact interaction introduces a new paradigm in the field of ultracold quantum gases. Fermionic ground state molecules with long-range and anisotropic dipolar interaction serve as an ideal system to explore the rich physics of dipolar quantum gases, opening new avenues for the creation of supersolid and novel topological phases. In our experiment, we work towards creating fermionic ground state molecules of $^{23}$Na$^{40}$K that are chemically stable and have a large dipole moment of 2.72 Debye. In the quantum regime, these molecules can have dipolar interaction energy that is a substantial fraction of the Fermi energy. Building on our previous work [1, 2], we have demonstrated efficient transfer of loosely bound Feshbach molecules of $^{23}$Na$^{40}$K into a deeper bound state (v=-2) of the ground triplet potential via STIRAP. In addition, our recent progress in exploring the triplet and singlet molecular ground state potentials is presented. \\[4pt][1] J. W. Park et al., Phys. Rev. A 85, 051602(R) (2012) \\[0pt] [2] C.-H. Wu et al., Phys. Rev. Lett. 109, 085301 (2012)   

Production and detection of ultracold LiRb molecules

We report on photoassociation (PA) of ultracold $^{7}$Li and $^{85}$Rb atoms and the production of ultracold heteronuclear $^{7}$Li$^{85}$Rb molecules in the electronic excited and ground states. The PA resonances are detected either using trap loss spectroscopy or by Resonance Enhanced Multi Photon Iononization (REMPI) of the ground-state LiRb molecules formed by spontaneous decay of the photoassociated excited state molecules. We identify several strong PA resonances below the Li (2s $^{2}$S$_{1/2}) + $ Rb (5p $^{2}$P$_{1/2})$ and the Li (2s $^{2}$S$_{1/2}) + $ Rb (5p $^{2}$P$_{3/2})$ asymptotes and experimentally determine the long range C$_{6}$ dispersion coefficients. We find an excited-state molecule formation rate of 3.5x10$^{7}$ s$^{-1}$ and a PA rate coefficient of 1.3x10$^{-10}$ cm$^{3}$/s, the highest among heteronuclear bi-alkali molecules. At large PA laser intensity, we observe the saturation of the PA rate coefficient close to the theoretical value at the unitarity limit. We will also present results on two-photon PA in LiRb.   

Towards STIRAP transfer of $^{6}$Li-$^{40}$K to the ground state using a frequency comb based Raman laser system

$^{6}$Li-$^{40}$K molecules in its absolute ground state have a large dipole moment of 3.6 debye, which makes them a suitable candidate for investigating long range dipole-dipole interactions. Starting from $^{6}$Li-$^{40}$K Feshbach molecules we plan to transfer them to the ground state using stimulated Raman adiabatic passage (STIRAP). A Raman laser system comprising of two lasers at 767 nm and 522 nm respectively, has been developed for spectroscopy and for the STIRAP transfer of $^{6}$Li-$^{40}$K. To ensure high relative phase coherence necessary for STIRAP, the two lasers have been locked to a common high finesse cavity. To nullify slow cavity drifts, a single feedback loop provides frequency corrections to both the lasers. The feedback signal is obtained by measuring the repetition rate of a frequency comb, optically locked to one of the Raman lasers and comparing it to a GPS-disciplined RF oscillator. In this talk, we present our results on the short and long-term stability of the Raman laser system. Additionally, we summarize our calculations of Franck-Condon factors for the selection of states suitable for STIRAP, and provide updates on the status of the experiment.   

Low-field Feshbach resonances in dysprosium

We report the observation [1] of resonance-like loss in the trap population of ultracold dysprosium as a function of magnetic field, which we attribute to anisotropy-induced Feshbach resonances arising from Dy's large magnetic dipole moment and nonzero electronic orbital angular momentum. We recorded these resonances for four different isotopes, three bosonic and one fermionic, over a field range of 0-6 G and show that the number of resonances changes significantly as a function of temperature, even in the nK regime. Most of the observed resonances are of very narrow width. The fermionic isotope, unlike its bosonic counterparts, possesses nonzero nuclear spin and exhibits a much higher density of resonances. \\[4pt] [1] K. Baumann, N. Q. Burdick, M. Lu, and B. L. Lev, to appear in Phys. Rev. A, Rapid Communications (2013). arXiv:1312.6401   

Spectroscopy of ultracold LiRb molecules

Using resonantly enhanced multiphoton ionization (REMPI), we detect ultracold LiRb molecules created via photoassociation (PA) in a $^7$Li/$^{85}$Rb dual species MOT. PA resonances below the D$_1$ and D$_2$ lines of Rb have been observed through both trap loss and REMPI spectroscopy. Decay pathways from different PA resonances allow us to map several different electronic potentials. To study the a$^3\Sigma^+$ potential, we create molecules in the $2(0^-)$ potential. These molecules mainly decay to the triplet electronic ground state. REMPI spectra show the vibrational levels of the a$^3\Sigma^+$ potential, as well as deeply bound levels belonging to the (3)$^3\Pi$ state through which the molecules are ionized. The vibrational structure of the X$^1\Sigma^+$ potential is observed via PA to other electronic potentials, such as the 4(1) which corresponds to the B$^1\Pi$ at close range. Molecules in the X$^1\Sigma^+$ state are likely ionized via the (4)$^1\Sigma^+$ electronic potential. Continuing these studies will help identify possible pathways from free atoms to LiRb molecules in the rovibronic ground state.