Ultracold molecules have recently attracted much attention because of their envisioned impact on both technology and fundamental science. The physics within them, i.e. their rich internal structure, and between them, i.e. long-range interactions, offer an increased complexity compared to atoms, while allowing for full experimental quantum control of the relevant degrees of freedom. A promising method of production consists of a first stage, whereby the component atoms are trapped and cooled to ultracold temperatures, and a second stage, whereby these are associated into dimers. In particular, the ability of cooling alkaline-earth (AE) elements, besides more traditional alkalis (A), has put production of AE-AE and A-AE molecules within experimental reach. Homonuclear ground-state AE-AE, because of their insensitivity to external electric and magnetic fields, have been proposed for metrology and precision measurements. In this work we investigate efficient production of Sr dimers starting from either an atomic Mott-insulator or Bose-Einstein condensate. Heteronuclear ground-state A-AE would allow for novel few-body and many-body physics experiments. In this thesis we investigate production schemes for RbSr dimers, which oweing to their large electric and magnetic dipole moments and heavy mass, are ideal candidates for the attainment of quantum degeneracy and subsequent experiments. We show our results from optical spectroscopy and combine them with thermal fluorescence data from our collaborators in Warsaw to derive the ground-state potential energy curve. Finally, we report on the experimental observation of magnetic Fano-Feshbach resonances between Rb and Sr and argue for their applicability to efficient molecule production.
Ultracold molecules have recently attracted much attention because of their envisioned impact on both technology and fundamental science. The physics within them, i.e. their rich internal structure, and between them, i.e. long-range interactions, offer an increased complexity compared to atoms, while allowing for full experimental quantum control of the relevant degrees of freedom. A promising method of production consists of a first stage, whereby the component atoms are trapped and cooled to ultracold temperatures, and a second stage, whereby these are associated into dimers. In particular, the ability of cooling alkaline-earth (AE) elements, besides more traditional alkalis (A), has put production of AE-AE and A-AE molecules within experimental reach. Homonuclear ground-state AE-AE, because of their insensitivity to external electric and magnetic fields, have been proposed for metrology and precision measurements. In this work we investigate efficient production of Sr dimers starting from either an atomic Mott-insulator or Bose-Einstein condensate. Heteronuclear ground-state A-AE would allow for novel few-body and many-body physics experiments. In this thesis we investigate production schemes for RbSr dimers, which oweing to their large electric and magnetic dipole moments and heavy mass, are ideal candidates for the attainment of quantum degeneracy and subsequent experiments. We show our results from optical spectroscopy and combine them with thermal fluorescence data from our collaborators in Warsaw to derive the ground-state potential energy curve. Finally, we report on the experimental observation of magnetic Fano-Feshbach resonances between Rb and Sr and argue for their applicability to efficient molecule production.
states. As first outcomes of our analysis, we calculate the s-wave scattering properties for all stable isotopic combinations and corroborate the locations of Fano-Feshbach resonances between alkali Rb and closed-shell Sr atoms recently observed [V. Barbéet al., Nat. Phys., 2018, 14, 881]. These results and more generally our strategy should greatly contribute to the generation of ultracold alkali-alkaline-earth dimers, whose applications range from quantum simulation to state-controlled quantum chemistry.
Heteronuclear polar molecules have recently attracted enormous attention owing to their ground state having a large electric dipole moment. The long range anisotropic dipole-dipole interaction in such systems is the basis for a variety of applications including quantum computing, precision measurements, ultracold chemistry and quantum simulations. Heteronuclear bi-alkali molecules, only a small subset of polar molecules, have received special attention mainly because the constituent alkali atoms are easy to laser cool and can be relatively easily associated to form molecules at ultracold temperatures. Our choice, the LiRb molecule, is motivated by the relatively high dipole moment (4.1 Debye) of the LiRb molecule in its rovibronic ground state. In this thesis, we study the LiRb molecule using laser spectroscopy and report, for the first time, the production of ultracold LiRb molecules by photoassociation (PA). The LiRb molecule is the least studied among all bi-alkali molecules and the first spectroscopic measurements on hot vapor phase LiRb molecules were reported only in 2011. We describe these measurements and their significance in chapter 2, after a brief introduction to the field of ultracold polar molecules in chapter 1. In chapter 3, we discuss our apparatus for simultaneous cooling and trapping of Li and Rb atoms in a dual-species magneto-optical trap (MOT) and report the measurement of interspecies collision-induced losses. In chapter 4, we describe the production of ultracold LiRb molecules in excited electronic states by photoassociation (PA). We report the measurements of the C6 coefficients for the Li (2s 2S1/2) + Rb (5p 2P1/2) and the Li (2s 2S1/2) + Rb (5p 2P3/2) asymptotes. We find a molecule formation rate (PLiRb) as high as 3.5x107 s-1 and a PA rate coefficient (KPA) as high as 1.3x10-10 cm3/s, the highest among heteronuclear bi-alkali molecules. In chapter 5, we discuss results on two-photon PA and we conclude, in chapter 6, with a road roadmap of future experiments for the production and detection of ultracold LiRb molecules in their rovibronic ground state.
In this thesis two experiments with heteronuclear Bose-Bose mixtures are discussed. The goal of the first experiment is a controlled doping of a rubidium condensate with single caesium atoms. These undertake the task of a non-destructive probe to investigate quantum mechanical phenomena time- and spatially resolved. In this thesis the necessary methods to produce, store, and detect both components, single atoms and the condensate, are realized. In a first experiment up to 10 caesium atoms are stored as a probe in contact with a cold rubidium atomic cloud. The interaction parameters are extracted from the dynamics of the single atoms. This is an important step towards the controlled doping of a condensate. The aim of the second experiment is the production and spectroscopy of ultracold heteronuclear potassium-rubidium molecules with universal properties. Close to two magnetic s-wave Feshbach resonances weakly bound molecules in high vibrational states are created, and their binding energy and the position of the associated Feshbach resonance are determined. These results in combination with two narrow d-wave Feshbach resonances provide the basis for a more precise parametrization of the potassium-rubidium molecular potential. The knowledge of this is important to identify a proper scheme to transfer the molecules via coherent coupling into their rovibrational ground state. These molecules offer a permanent dipole moment and thus are of particular interest for e.g. quantum information processing due to their anisotropic, long-range dipole-dipole interaction.
This thesis reports on experiments with quantum degenerate atomic mixtures and molecular gases at ultracold temperatures.The work describes the first realization of a quantum degenerate Fermi-Fermi mixture of two different species with unequal masses.In addition, the first quantum degenerate three-species mixture is realized.Furthermore, the first heteronuclear bosonic molecules with temperatures close to quantum degeneracy are created from a two-species mixture.Within this work a new and very versatile experimental platform to study quantum degenerate two-species Fermi mixtures is presented.The experimental concept relies on sympathetic cooling of two fermionic species ( 40 K and 6 Li) by a large bosonic gas ( 87 Rb).It is shown that large atom numbers and reliable operation are guaranteed by careful choice of the experimental components and parameters, which is essential to deal with the complexity of such an experimental platform with three atomic species.The first important milestone towards quantum degeneracy is realized by simultaneous trapping 87 Rb, 40 K and 6 Li in a magneto-optical trap.This marks the first realization of magneto-optical trapping of two fermionic species and also of three species.To achieve quantum degeneracy, different experimental challenges and difficulties of the three species with very different initial temperatures, scattering cross sections and masses were overcome.A combined compressed MOT and temporal dark MOT phase, a careful state cleaning process during the cooling process, species selective evaporative cooling of rubidium and removing of high energetic lithium atoms turn out to be essential for the successful realization of a quantum degenerate Fermi-Fermi-Bose mixture.Furthermore, at the end of the sympathetic cooling process, the cooling efficiency of lithium is improved by catalytic cooling through potassium.A quantum degenerate Fermi-Fermi mixture can be realized in the strongly interacting regime at a s-wave Feshbach resonance.By an adiabatic magnetic field sweep across the Feshbach resonance ultracold bosonic heteronuclear molecules are created with high efficiencies of up to 50 %.The associated number of molecules are up to 4 × 10 4 molecules, which is the largest number of heteronuclear molecules produced so far.Furthermore, close to resonance an increased molecular lifetime of more than 100 ms is observed.Therefore, this is the first system that may be used to explore many-body physics of a heteronuclear mixture in the strongly interacting regime across a Feshbach resonance.The heteronuclear Fermi mixture adds the mass ratio and the different internal structure as new parameters to quantum degenerate Fermi gases.The molecular cloud being close to quantum degeneracy marks the first starting point for the realization of a heteronuclear molecular BEC and to investigate the BEC-BCS crossover with unequal masses.Moreover, the molecules can be transferred into the rovibrational ground state to create a polar BEC with an anisotropic, long-range interaction.
The field of ultracold matter has developed rapidly over the course of the past twenty to thirty years. One outstanding achievement has been the realisation of Bose-Einstein condensation (BEC) only 23 years ago. Many other fascinating observations of quantum phenomena have been made at an incredibly fast pace using ultracold atomic samples with contact force interactions as a testbed. Due to the rapid successes of all these experiments on atomic ensembles, researchers began to aim for more elaborate inter-particle interactions. A possible additional one is the dipole-dipole interaction (DDI), the long-range character of which stands out in comparison to the contact interactions. Moreover, DDI is anisotropic and the possibility to tune the direction of the dipoles in the experiment as well as the interaction strength promises the possibility to study a plethora of new quantum phenomena. In recent years, several experiments using different combinations of alkali atoms to create ground-state hetero-nuclear molecules in the ultracold regime have been set up, following the pioneering work on KRb molecules. Of all possible combinations of two alkali atoms, NaK combines the advantages of a high induced electric dipole moment of 2.72 Debye, chemical stability of the ground-state molecules, the possibility to realise bosonic as well as fermionic molecules and a large knowledge base for cooling and trapping of the two atomic species. This thesis reports on the development and construction of an experiment apparatus for the production of ultracold bosonic dipolar ground state 23Na39K molecules. This includes the setup of an ultra-high vacuum (UHV) system, several laser light sources, microwave (MW) and radio frequency (RF) antennas and electronics to control the various components of the experiment. The system pre-cools the atomic samples of 23Na and 39K and loads them into a dual-species three-dimensional magneto-optical trap (3D MOT). After a transfer to a magnetic quadrupole trap, the mixture is cooled by forced MW evaporation until three-body losses limit dual-species operation at high densities and zero magnetic field. Using the pure optical trapping potential of a crossed optical dipole trap (cODT) and RF- state-transfer, the scattering properties of the bosonic mixture of 23Na+39K are investigated in extensive atom loss measurements for a magnetic field range from 0 to 750 G. Several Feshbach resonances, zero crossings of the interspecies scattering length and inelastic loss features in many different scattering channels are identified. The results of this study are used to refine the molecular potentials of NaK and to produce a dual-species quantum-degenerate mixture of two BECs in the state (f = 1, mf = −1) of 23Na + (f = 1, mf = −1) of 39K. The measurements and refined molecular potentials will considerably aid in the future realisation of 23Na39K molecules in their absolute ground state.
This thesis reports the investigation of the scattering properties of a mixture of Rb and Cs and the formation of ultracold Feshbach molecules. The production of Feshbach molecules is a crucial step towards the production of ultracold polar molecules, which is of significant interest for a wide range of potential applications. We have investigated the scattering properties of a mixture of 85Rb and 133Cs in their lowest spin channel in a magnetic field range from 0 to 700\,G. Furthermore, we explored the Feshbach spectrum of 85Rb alone in both, the (f=2,mf=−2) and (2,+2) states up to a magnetic field of 1000\,G. Additionally a Feshbach resonance in a (2,+2)+(3,+3) spin mixture was experimentally confirmed. We associated Cs2 Feshbach molecules using a Feshbach resonance at 19.9\,G. \mbox{2.1(1)×104} molecules with a temperature of ∼60\,nK were produced from a sample of 3.28(2)×105 Cs atoms with a PSD of 0.20(1). Due to a magnetic field gradient, the molecules `bounce' at an avoided crossing between two states at 13.5\,G. This gradient field was also used to produce multiple molecular clouds from one atomic sample. A combination of both techniques led to a `collision' between two Cs2 clouds. Furthermore, we associated up to ∼5000 heteronuclear 87RbCs Feshbach molecules using an interspecies resonance at 197.1\,G. Confined in the dipole trap the molecules have a lifetime of 0.21(1)\,s. We have measured the magnetic moment of the molecules in different internal states in a magnetic field range from 181 to 185\,G. Molecular loss spectroscopy on electronically excited states was performed to identify candidates for the intermediate state of a STIRAP transfer of the molecules into their rovibrational ground state. Subsequently, the binding energy of the rovibrational ground state was measured to be 3811.574(1)\,cm−1, using two-photon spectroscopy.