We have produced ultracold, polar
RbCs∗ molecules via photoassociation in a laser-cooled mixture of Rb and Cs atoms. Using a model of the
RbCs∗ molecular interaction which reproduces the observed rovibrational structure, we infer decay rates in our experiments into deeply bound $X\text{ }^{1}\ensuremath{\Sigma}^{+}$ ground-state RbCs vibrational levels as high as $5\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }{\mathrm{s}}^{\ensuremath{-}1}$ per level. Population in such deeply bound levels could be efficiently transferred to the vibrational ground state using a single stimulated Raman transition, opening the possibility to create large samples of stable, ultracold polar molecules.
We theoretically investigate the interaction of polar molecules with optical lattices and microwave fields. We demonstrate the existence of frequency windows in the optical domain where the complex internal structure of the molecule does not influence the trapping potential of the lattice. In such frequency windows the Franck-Condon factors are so small that near-resonant interaction of vibrational levels of the molecule with the lattice fields have a negligible contribution to the polarizability, and light-induced decoherences are kept to a minimum. In addition, we show that microwave fields can induce a tunable dipole-dipole interaction between ground-state rotationally symmetric
(J=0) molecules. A combination of a carefully chosen lattice frequency and microwave-controlled interaction between molecules will enable trapping of polar molecules in a lattice and possibly realize molecular quantum logic gates. Our results are based on ab initio relativistic electronic structure calculations of the polar KRb and RbCs molecules combined with calculations of their rovibrational motion.
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 Cs
2 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 Cs
2 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.
In recent experiments on ultracold matter, molecules have been produced from ultracold atoms by photoassociation, Feshbach resonances, and three-body recombination. The created molecules are translationally cold, but vibrationally highly excited. This will eventually lead them to be lost from the trap due to collisions. We propose shaped laser pulses to transfer these highly excited molecules to their ground vibrational level. Optimal control theory is employed to find the light field that will carry out this task with minimum intensity. We present results for the sodium dimer. The final target can be reached to within
99 provided the initial guess field is physically motivated. We find that the optimal fields contain the transition frequencies required by a good Franck-Condon pumping scheme. The analysis identifies the ranges of intensity and pulse duration which are able to achieve this task before any other competing processes take place. Such a scheme could produce stable ultracold molecular samples or even stable molecular Bose-Einstein condensates.
Cold polar molecules provide unique opportunities to test fundamental physics and chemistry. Their permanent electric dipole moments and rich internal structure arising from their vibrational and rotational motion, makes them sensitive probes for new physics. These features also make them ideal for studying ultracold chemistry, for simulating the behaviour of strongly interacting many-body quantum systems, and for quantum information science. This thesis describes a number of advances in cold molecule physics. The optimum method for producing an intense, pulsed, supersonic beam of cold CH molecules is investigated, resulting in a beam with 3.5 × 109 ground state CH molecules per steradian per shot. The beam has a translational temperature of 400 mK and a velocity that is tuneable between 400 and 1800 m s−1 . The lowest-lying Λ-doublet transitions of ground state CH, at 3.3 GHz and 0.7 GHz, are exceptionally sensitive to variations in the fine-structure constant α and the electron-to-proton mass ratio µ. Many modern theories predict that these constants may depend on time, position, or local matter density. Using a novel spectroscopic method, the frequencies of these microwave transitions are measured with accuracy down to 3 Hz. By comparing to radio astronomical observations, the hypothesis that fundamental constants may differ between the high and low-density environments of the Earth and the interstellar medium of the Milky Way is tested. These measurements find no variation and set upper limits of |∆α/α| < 2.1 × 10−7 and |∆µ/µ| < 4.3 × 10−7 . The frequency of the lowest millimetre-wave transition of CH, near 533 GHz, is also measured with an accuracy of 0.6 kHz. The development of a novel type of trap for ground-state polar molecules is presented. Trapping polar molecules is a necessary condition to cool them to ultracold temperatures of 1 µK and below. The trap uses a high intensity microwave field in a Fabry-P´erot resonator. Experimental and theoretical investigations are presented that explore the modes of the cavity, how to obtain the highest possible quality factor, and how to optimally couple the microwave power into the cavity. Finally, a cold supersonic beam of BH molecules is developed. This molecule appears to be particularly well-suited to direct laser cooling due to its favourable rotational structure and Franck-Condon factors. The laser cooling concept is described, and a first spectroscopic investigation of the relevant molecular structure is presented.
The exquisite control of diluted atomic vapors that can be achieved using electromagnetic (em) fields triggered the overwhelming development of researches on ultracold (T<<1millikelvin) quantum gases through laser cooling and trapping, opening ways to amazing applications in quantum technologies. For a composite system like a molecule under such ultracold conditions, the additional control of the internal degrees of freedom has also been recently demonstrated [1] with cw visible, microwave, and radio radiation frequencies for electronic, rotational, and hyperfine transitions, respectively. Vibrational transitions could be addressed with THz transitions, as an attractive path for controlling the interatomic interactions and creating ultracold gases of a wide class of diatomic molecules in their absolute ground state. More specifically, THz radiation can induce a coupling between the bound level of a pair of trapped ultracold ground-state atoms and a loosely-bound vibrational level of the related molecular electronic ground state. This coupling, generated by the intrinsic permanent electric dipole moment of a heteronuclear atomic pair (RbSr in the present work), is related to a specific spectroscopic feature of the atomic pair dressed by the THz field, the Laser Assisted Self-Induced Feshbach Resonance (LASIFR) [2] . In analogy with magnetic Feshbach resonance, the scattering length can be controlled by tuning the THz radiation frequency around a LASIFR (see Fig (a)).