Ultracold bialkali polar molecules play a leading part at the frontline of quantum physics. They recently attract a lot of attention in the field of ultracold quantum chemistry, quantum many-body physics and quantum simulations. The key for their success is the rich internal level structure with rotational and vibrational degrees of freedom and their large electric dipole moments. Still, only a handful of molecular species are available at ultracold temperatures until now, although it is highly desirable to produce new molecular species to further expand the range of applications. Besides direct laser cooling methods for molecules, the assembly of heteronuclear groundstate molecules from ultracold atomic mixtures is the most promising approach for the creation of polar molecules. It includes the formation of weakly bound Feshbach molecules from the diatomic mixture and the subsequent two-photon stimulated Raman adiabatic passage (STIRAP) transfer to the rovibrational ground state. This creation strategy has been successfully demonstrated for the first time in the pioneering experiments at JILA with ultracold 40 K 87 Rb molecules. Since then, only a few more molecular species from different alkali atoms have been created, namely 6 Li 23 Na, 23 Na 40 K, 23 Na 87 Rb and 87 Rb 133 Cs.In this thesis, I report the successful creation of a new species of ultracold polar ground-state molecules: 23 Na 39 K. Starting from an ultracold mixture of bosonic 23 Na and 39 K atoms, weakly bound molecules are created. For this purpose, a Feshbach resonance in a high angular momentum scattering channel is chosen, experimentally identified and characterized. Close to this resonance the weakly bound Feshbach molecules are formed using resonant radio frequency radiation. For the two-photon ground-state transfer, a unique, highly specialized two-color laser system is designed and realized. It is used for one-and two-photon spectroscopy to identify the relevant transitions for the ground-state transfer. Based on the obtained data, a local model of the singlet-triplet mixed excited state manifolds is developed, with which the hyperfine structure and the magnetic field dependence is predicted with high accuracy. According to these findings, a suitable pathway to a single hyperfine ground state is chosen considering selection rules and experimental conditions such as laser polarization and beam alignment. To precisely determine the two-photon resonance condition for STIRAP, electromagnetically induced transparency measurements are performed. The ground-state transfer is then performed using STIRAP. The experimental findings regarding the STIRAP are successfully supported theoretically by a model based on a five-level master equation. The pure molecular gas shows evidence for two-body dominated loss mechanisms, such as sticky four-body collisions. The molecule-atom mixture of 23 Na 39 K+ 39 K reveals an unexpectedly low loss rate coefficient although sticky three-body collisions are assumed to occur. This behavior demands further investigat...

We present measurements of interspecies Feshbach resonances and subsequent creation of dualspecies Bose-Einstein condensates of 23 Na and 39 K. We prepare both optically trapped ensembles in the spin state |f = 1, m f = −1 and perform atom loss spectroscopy in a magnetic field range from 0 to 700 G. The observed features include several s-wave poles and a zero crossing of the interspecies scattering length as well as inelastic two-body contributions in the M = mNa+mK = −2 submanifold. We identify and discuss the suitability of different magnetic field regions for the purposes of sympathetic cooling of 39 K and achieving dual-species degeneracy. Two condensates are created simultaneously by evaporation at a magnetic field of about 150 G, which provides sizable intra-and interspecies scattering rates needed for fast thermalization. The impact of the differential gravitational sag on the miscibility criterion for the mixture is discussed. Our results serve as a promising starting point for the magnetoassociation into quantum degenerate 23 Na 39 K Feshbach molecules.arXiv:1709.03796v2 [cond-mat.quant-gas]

We create weakly bound bosonic 23 Na 39 K molecules in a mixture of ultracold 23 Na and 39 K. The creation is done in the vicinity of a so far undetected Feshbach resonance at about 196 G which we identify in this work by atom-loss spectroscopy. We investigate the involved molecular state by performing destructive radio frequency binding energy measurements. For the constructive molecule creation we use radio frequency pulses with which we assemble up to 6000 molecules. We analyze the molecule creation efficiency as a function of the radio frequency pulse duration and the atom number ratio between 23 Na and 39 K. We find an overall optimal efficiency of 6 % referring to the 39 K atom number. The measured lifetime of the molecules in the bath of trapped atoms is about 0.3 ms.

We spectroscopically investigate a pathway for the conversion of 23 Na 39 K Feshbach molecules into rovibronic ground state molecules via stimulated Raman adiabatic passage. Using photoassociation spectroscopy from the diatomic scattering threshold in the a 3 Σ + potential, we locate the resonantly mixed electronically excited intermediate stateswhich, due to their singlet-triplet admixture, serve as an ideal bridge between predominantly a 3 Σ + Feshbach molecules and pure X 1 Σ + ground state molecules. We investigate their hyperfine structure and present a simple model to determine the singlet-triplet coupling of these states. Using Autler-Townes spectroscopy, we locate the rovibronic ground state of the 23 Na 39 K molecule (| S = = ñ + X v N , 0, 0 1 ) and the second rotationally excited state N=2 to unambiguously identify the ground state. We also extract the effective transition dipole moment from the excited to the ground state. Our investigations result in a fully characterized scheme for the creation of ultracold bosonic 23 Na 39 K ground state molecules. 23 Na 40 K have been created using excited states in energetically higher D 1 Π and d 3 Π potentials [13]; not shown in figure 1(a).

In this paper, we present an electrode geometry for the manipulation of ultracold, rovibrational ground state NaK molecules. The electrode system allows to induce a dipole moment in trapped diatomic NaK molecules with a magnitude up to 68% of their internal dipole moment along any direction in a given two-dimensional plane. The strength, the sign and the direction of the induced dipole moment is therefore fully tunable. The maximal relative variation of the electric field over the trapping volume is below 10 −6 . At the desired electric field value of 10 kV cm −1 this corresponds to a deviation of 0.01 V cm −1 . Furthermore, the possibility to create strong electric field gradients provides the opportunity to address molecules in single layers of an optical lattice. The electrode structure is made of transparent indium tin oxide and combines large optical access for sophisticated optical dipole traps and optical lattice configurations with the possibility to create versatile electric field configurations.