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.
Ultracold Feshbach molecules are a crucial intermediate step for the creation of quantum degenerate gases of strongly dipolar molecules. After coherent transfer to the rovibrational ground state, these dimers can realize stable dipolar gases with strong, tunable long-range interactions. Here, we report the creation of bosonic Na 23 K 41 Feshbach molecules by radio-frequency (RF) association. An RF pulse applied on the molecular side of an interspecies Feshbach resonance at 73.6(1) G associates up to 1.1 ( 1 ) × 10 4 molecules from a thermal mixture of Na 23 and K 41 atoms. Measurements of the binding energy reveal a broad resonance width of 5.1(2) G, facilitating robust control over interspecies interactions. The molecule lifetime in the presence of background atoms exceeds 2 ms and increases to approximately 7 ms after removal of Na 23 . These results constitute a key step toward the production of ultracold Na 23 K 41 ground-state molecules and for pursuing shielded evaporative cooling in the dipolar regime.
The discovery and development of methods to cool, trap and manipulate atomic ensembles have generated a revolution in atomic physics, culminating in the ever expanding research field of ultra-cold matter through the realization of Bose-Einstein condensates (BECs). Since the first realization of BEC more than two decades ago, the majority of studies were concerned with systems displaying short-range contact interaction. Following pioneering experiments in ultra-cold KRb, systems involving ultra-cold mixtures have rose to considerable interest through their capability to form diatomic polar molecules. The long-range character of dipolar particle interactions enables the study of a whole new spectrum of quantum many-body phenomena by giving access to the strongly correlated regime. In this context, NaK is a splendid candidate to investigate dipolar effects due to its large electric dipole moment of $2.72 \, $Debye, its chemical stability and the large history of spectroscopic studies devoted to its ground and excited state manifolds. This thesis reports on the first ever realization of a dual-species degenerate mixture of
23Na and
39K. The experimental apparatus combines two pre-cooled atomic sources into a UHV collection region, where a two-color magneto-optical trap is operated. After transferring both species into an optically plugged quadrupole trap, the
23Na ensemble is cooled by selectively removing the hottest atoms from the trap through microwave transitions, while
39K is sympathetically cooled through its thermal contact to
23Na. Following the transfer into an optical dipole trap, the mixture operation suffers from strong losses as the atomic clouds approach the high-density regime. Interspecies interactions are identified as the system parameter that drives the loss mechanism. In order to realize a quantum degenerate mixture, the dual-species collisional properties are investigated both theoretically and experimentally. By preparing both optically trapped ensembles in the spin state
∣f=1,mf=−1⟩, atom loss spectroscopy is performed in a magnetic field range from
0 to
1000G. 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. Different magnetic field regions are identified for the purposes of sympathetic cooling of
39K and achieving a quantum degenerate mixture. Forced evaporation creates two Bose-Einstein condensates simultaneously at a magnetic field that 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. The experimental setup and measurements are complemented by theoretical calculations that form a feasibility study of molecular NaK. Starting from ultra-cold Feshbach molecules, the study demonstrates possible pathways for the creation of ultra-cold polar NaK molecules in their absolute electronic and rovibrational ground state. In particular, a multi-channel analysis of the electronic ground and K(4p)+Na(3s) excited state manifold of NaK is presented. It analyzes the spin character of both the Feshbach molecular state and the electronically excited intermediate states and discusses possible coherent two-photon transfer paths from Feshbach molecules to rovibronic ground state molecules. %The theoretical study is complemented by the demonstration of STIRAP transfer from the X1Σ+(v=0) state to the a3Σ+ manifold on a molecular beam experiment. The experimental results serve as a promising starting point for the magnetoassociation into quantum degenerate
23Na39K Feshbach molecules. The theoretical analysis assures qualitative understanding as well as quantitative statements for the feasibility of the subsequent ground state conversion. In combination, they fill a critical gap towards the creation of chemically stable ultra-cold molecular Bose-Einstein condensates of NaK.
We report a compact experimental setup for producing a quantum degenerate mixture of Bose 23Na and Fermi 40K gases. The atoms are collected in dual dark magneto–optical traps (MOT) with species timesharing loading to reduce the light-induced loss, and then further cooled using the gray molasses technique on the D 2 line for 23Na and D 1 line for 40K. The microwave evaporation cooling is used to cool 23Na in | F = 2,mF = 2〉 in an optically plugged magnetic trap, meanwhile, 40K in | F = 9/2,mF = 9/2〉 is sympathetically cooled. Then the mixture is loaded into a large volume optical dipole trap where 23Na atoms are immediately transferred to |1,1〉 for further effective cooling to avoid the strong three-body loss between 23Na atoms in |2,2〉 and 40K atoms in |9/2,9/2〉. At the end of the evaporation in optical trap, a degenerate Fermi gas of 40K with 1.9 × 105 atoms at T/TF = 0.5 in the |9/2,9/2〉 hyperfine state coexists with a Bose–Einstein condensate (BEC) of 23Na with 8 × 104 atoms in the |1,1〉 hyperfine state at 300 nK. We also can produce the two species mixture with the tunable population imbalance by adjusting the 23Na magneto–optical trap loading time.
We report the creation of dual-species Bose-Einstein condensates (BECs) of
23Na and
41K. Favorable background scattering lengths enable efficient sympathetic cooling of
41K via forced evaporative cooling of
23Na in a plugged magnetic trap and an optical dipole trap. The
1/e lifetime of the thermal mixture in the stretched hyperfine state exceeds 5 s in the presence of background scattering. At the end of evaporation, we create dual BECs in the immiscible phase, with about
3×105 23Na atoms surrounding
5×104 41K atoms. To further enable the tuning of the interspecies interaction strength, we locate multiple Feshbach resonances at magnetic fields up to 100 G. The broadest
s-wave resonance located at 73.4(3) G features a favorable width of 1.8(2) G. This work sets the stage for the creation of ultracold gases of strongly dipolar bosonic
23Na
41K molecules as well as the exploration of many-body physics in bosonic
23Na-
41K mixtures.
In recent years, there has been notable progress in the preparation and control of ultracold gases of diatomic molecules. The next experimental challenge is the production of ultracold polyatomic molecular gases. Here, we report the creation of an ultracold gas of 23 Na 40 K 2 triatomic molecules from a mixture of ground-state sodium-23–potassium-40 ( 23 Na 40 K) molecules and potassium-40 ( 40 K) atoms. The triatomic molecules were created by adiabatic magneto-association through an atom–diatomic molecule Feshbach resonance. We obtained clear evidence for the creation of triatomic molecules by directly detecting them using radio-frequency dissociation. Approximately 4000 triatomic molecules with a high-peak phase-space density of 0.05 could be created. The ultracold triatomic molecules can serve as a launchpad to probe the three-body potential energy surface and may be used to prepare quantum degenerate triatomic molecular gases.