We review our experimental progress toward achieving the production of quantum degenerate bosonic polar molecules, 41 K 87 Rb. Our primary goal is to transfer quantum degenerate Feshbach molecules into the absolute ground state via the stimulated Raman adiabatic passage (STIRAP). We have achieved a dual-species Bose–Einstein condensate of 41 K and 87 Rb in a magnetic trap, which consists of N K =1.4 × 10 4 potassium atoms and N Rb =3.5 × 10 4 rubidium atoms. An optimum optical transition for STIRAP is being identified by a separate experiment based on photoassociation in a dual-species magneto-optical trap of 41 K and 87 Rb.
We review our experimental progress toward achieving the production of quantum degenerate bosonic polar molecules, 41 K 87 Rb. Our primary goal is to transfer quantum degenerate Feshbach molecules into the absolute ground state via the stimulated Raman adiabatic passage (STIRAP). We have achieved a dual-species Bose–Einstein condensate of 41 K and 87 Rb in a magnetic trap, which consists of N K =1.4 × 10 4 potassium atoms and N Rb =3.5 × 10 4 rubidium atoms. An optimum optical transition for STIRAP is being identified by a separate experiment based on photoassociation in a dual-species magneto-optical trap of 41 K and 87 Rb.
Microwave-stimulated Raman adiabatic passage (STIRAP) between internal states of a Bose-Einstein condensate (BEC) magnetically trapped in the vicinity of an atom chip is demonstrated by coherently transferring about 90 percent of a 7.000-atom BEC initially in the (F=2,mF=2) hyperfine level of the 5 2S1/2 ground state of 87Rb into (F=2,mF=1), using (F=1,mF=1) as an intermediate (unpopulated) level. The STIRAP protocol used in this experiment is robust to external perturbations as it is an adiabatic transfer, and power efficient as it involves only resonant (or quasi-resonant) processes. Taking into account the effect of losses and collisions in a Bloch equations model, we show that the maximum transfer efficiency is obtained for significantly non-zero values of the one- and two-photon detunings, which is confirmed quantitatively by our experimental measurements (and is shown to come mostly from inelastic collisions within (F=2,mF=1)). This work shows that microwave STIRAP between hyperfine ground states of magnetically trapped atoms is feasible, paving the way for STIRAP-based quantum information or metrology experiments integrated on a chip.
This thesis reports the formation of a dual-species Bose-Einstein condensate of 87Rb and 133Cs in the same trapping potential. Quantum degenerate mixtures exhibit rich physics inaccessible to single species experiments and provide an ideal starting point for the creation of ultracold dipolar molecules. These molecules offer a wealth of new research avenues including precision metrology, quantum simulation and computation. The experimental method exploits the efficient sympathetic cooling of 133Cs via elastic collisions with 87Rb, initially in a magnetic quadrupole trap and subsequently in a levitated optical trap. Evaporative cooling in the dipole trap must compete against a high interspecies three-body inelastic collision rate \mbox{∼10−25−10−26~cm6/s}. The two condensates each contain up to \mbox{2×104} atoms and exhibit a striking phase separation, revealing the mixture to be immiscible due to strong repulsive interspecies interactions. Sacrificing all the 87Rb during the cooling leads to the creation of single-species 133Cs condensates of up to \mbox{6×104} atoms. In addition this thesis reports the observation of an interspecies Feshbach resonance at 181.7(5)~G and the creation of a pure sample of Cs2 molecules via magneto-association on the 4(g)4 resonance at 19.8~G. These results represent important steps towards the creation of ultracold polar RbCs molecules.
We produce 84 Sr 2 molecules using Bose-enhanced Raman photoassociation. We apply the stimulated Raman adiabatic passage (STIRAP) technique on a Bose-Einstein condensate (BEC) to produce more than 8 × 10 3 ultracold molecules. This chemical reaction is only made possible because of the Bose enhancement of the optical transition dipole moment between the initial atomic state and an intermediate molecular state. We study the effect of Bose enhancement by measuring the transition Rabi frequency in a BEC and by comparing it with measurements for two atoms in sites of a Mott insulator. By breaking the dimers' bond and directly observing the separated atoms, we measure the molecular inelastic collision rate parameters. We discuss the possibility of applying Bose-enhanced STIRAP to convert a BEC of atoms into a BEC of molecules, and argue that the required efficiency for STIRAP is within experimental reach.
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 generalize the idea of chainwise stimulated Raman adiabatic passage (STIRAP) [Kuznetsova et al., Phys. Rev. A 78, 021402(R) (2008)] to a photoassociation-based chainwise atom-molecule system, with the goal of directly converting two-species atomic Bose-Einstein condensates (BEC) into a ground polar molecular BEC. We pay particular attention to the intermediate Raman laser fields, a control knob inaccessible to the usual three-level model. We find that an appropriate exploration of both the intermediate laser fields and the stability property of the atom-molecule STIRAP can greatly reduce the power demand on the photoassociation laser, a key concern for STIRAPs starting from free atoms due to the small Franck-Condon factor in the free-bound transition.
Although photoassociation and the Feshbach resonance are feasible means in principle for creating a molecular Bose–Einstein condensate (MBEC) from an already quantum-degenerate gas of atoms, collision-induced mean-field shifts and irreversible decay place practical constraints on the efficient Raman delivery of stable molecules. Focusing on stimulated Raman adiabatic passage, we propose that the efficiency of both mechanisms for producing a stable MBEC can be improved by treating the density of the initial atom condensate as an optimization parameter.