We report the creation of ultracold samples of ^{39}K^{133}Cs molecules in their rovibrational ground state. By investigating potentially suitable excited states using one- and two-photon spectroscopy, we have identified a pathway to the ground state via an exceptionally narrow intermediate state. Using stimulated Raman adiabatic passage, we create trapped samples of up to 3500 molecules at temperatures of 1 μK with one-way efficiencies of 71%. The lifetime of these samples is limited by a near-universal two-body loss process, which could shed new light on similar loss mechanisms in other molecular species. Our results are a step toward establishing an alternative molecular species as a platform for the study of bosonic and fermionic quantum matter with strong dipolar interactions.
We report the creation of ultracold samples of ^{39}K^{133}Cs molecules in their rovibrational ground state. By investigating potentially suitable excited states using one- and two-photon spectroscopy, we have identified a pathway to the ground state via an exceptionally narrow intermediate state. Using stimulated Raman adiabatic passage, we create trapped samples of up to 3500 molecules at temperatures of 1 μK with one-way efficiencies of 71%. The lifetime of these samples is limited by a near-universal two-body loss process, which could shed new light on similar loss mechanisms in other molecular species. Our results are a step toward establishing an alternative molecular species as a platform for the study of bosonic and fermionic quantum matter with strong dipolar interactions.
One possibility for the creation of ultracold, high phase space density quantum gases of molecules in the rovibronic ground state relies on first associating weakly-bound molecules from quantum-degenerate atomic gases on a Feshbach resonance and then transferring the molecules via several steps of coherent two-photon stimulated Raman adiabatic passage (STIRAP) into the rovibronic ground state. Here, in ultracold samples of Cs2 Feshbach molecules produced out of ultracold samples of Cs atoms, we observe several optical transitions to deeply-bound rovibrational levels of the excited 0(u)+ molecular potentials with high resolution. At least one of these transitions, although rather weak, allows efficient STIRAP transfer into the deeply-bound vibrational level (see text for symbols)v = 73 > of the singlet X1 sigma(g)+ ground state potential, as recently demonstrated (J. G. Danzl, E. Haller, M. Gustavsson, M. J. Mark, R. Hart, N. Bouloufa, O. Dulieu, H. Ritsch, and H.-C. Nägerl, Science, 2008, 321, 1062). From this level, the rovibrational ground state (see text for symbols)v = 0, J = 0 > can be reached with one more transfer step. In total, our results show that coherent ground state transfer for Cs2 is possible using a maximum of two successive two-photon STIRAP processes or one single four-photon STIRAP process.
We report the creation of ultracold bosonic dipolar ^{23}Na^{39}K molecules in their absolute rovibrational ground state. Starting from weakly bound molecules immersed in an ultracold atomic mixture, we coherently transfer the dimers to the rovibrational ground state using an adiabatic Raman passage. We analyze the two-body decay in a pure molecular sample and in molecule-atom mixtures and find an unexpectedly low two-body decay coefficient for collisions between molecules and ^{39}K atoms in a selected hyperfine state. The preparation of bosonic ^{23}Na^{39}K molecules opens the way for future comparisons between fermionic and bosonic ultracold ground-state molecules of the same chemical species.
Ultracold atomic gases have proven to be a versatile tool for the study of many-body quantum physics thanks to the high degree of control that they offer. However, interactions between neutral atoms are usually short-ranged and isotropic, which limits their applicability as a simulator of real condensed matter systems. One of the most common methods of introducing stronger interactions into an ultracold gas is associating pairs of atoms of two different elements into molecules. Such molecules possess a body-fixed electric dipole moment, which can be addressed by an external electric field or microwave radiation to engineer long-ranged, tunable dipolar interactions. In addition to constituting an analogue quantum simulator, ultracold dipolar molecules show potential as a platform for digital quantum computation, precision measurements, and the study of quantum chemistry. The main result presented in this thesis is the first successful synthesis of ultracold 39K133Cs molecules in their vibrational and rotational ground state. Using a novel simultaneous cooling scheme for potassium and caesium, our team has been able to produce mixtures that are cold enough for efficient magnetoassociation into weakly bound molecules to take place. By performing high-resolution laser spectroscopy of these molecules, I have identified a two-photon pathway to the ground state via an exceptionally narrow intermediate excited state. I was then able to perform Stimulated Raman Adiabatic Passage (STIRAP) along that pathway with one-way efficiencies up to 71%, producing trapped samples of up to 3500 ground-state molecules at temperatures of 1 µK. The lifetime of these samples is currently limited by a near-universal two-body loss process, a further study of which could shed light on similar loss mechanisms in other molecular species. I have also carried out preliminary rotational spectroscopy, which paves the way towards using KCs molecules as a controlled interacting spin system and a refinement of their hyperfine coupling constants. The thesis ends with a detailed outline of further steps that should be taken to improve the efficiency of STIRAP, as well as the population and coherence lifetimes in KCs samples. It is followed by a list of suggested experiments that could be carried out using our system after it undergoes the described technical upgrades. Some of these experiments are directly inspired by the results I have obtained during my six-month research stay in the KRb laboratory at JILA.
An Ultracold Gas of Bosonic 23Na39K Ground-State Molecules
We report the creation of ultracold bosonic dipolar 23Na39K molecules in their absolute rovibrational ground state. Starting from weakly bound molecules immersed in an ultracold atomic mixture, we coherently transfer the dimers to the rovibrational ground state using an adiabatic Raman passage. We analyze the two-body decay in a pure molecular sample and in molecule-atom mixtures and find an unexpectedly low two-body decay coefficient for collisions between molecules and 39K atoms in a selected hyperfine state. The preparation of bosonic 23Na39K molecules opens the way for future comparisons between fermionic and bosonic ultracold ground-state molecules of the same chemical species.
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 ground-state 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 40K87Rb molecules. Since then, only a few more molecular species from different alkali atoms have been created, namely 6Li23Na, 23Na40K, 23Na87Rb and 87Rb133Cs. In this thesis, I report the successful creation of a new species of ultracold polar ground-state molecules: 23Na39K. Starting from an ultracold mixture of bosonic 23Na and 39K 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 23Na39K+39K reveals an unexpectedly low loss rate coefficient although sticky three-body collisions are assumed to occur. This behavior demands further investigations regarding different hyperfine collisional chan-nels and magnetic field regions.
We perform one- and two-photon high resolution spectroscopy on ultracold samples of RbCs Feshbach molecules with the aim to identify a suitable route for efficient ground-state transfer in the quantum-gas regime to produce quantum gases of dipolar RbCs ground-state molecules. One-photon loss spectroscopy allows us to probe deeply bound rovibrational levels of the mixed excited (A(1)Σ(+)-b(3)Π)0(+) molecular states. Two-photon dark state spectroscopy connects the initial Feshbach state to the rovibronic ground state. We determine the binding energy of the lowest rovibrational level |v'' = 0, J'' = 0> of the X(1)Σ(+) ground state to be D = 3811.5755(16) cm(-1), a 300-fold improvement in accuracy with respect to previous data. We are now in the position to perform stimulated two-photon Raman transfer to the rovibronic ground state.