This experimental research project focuses on of creating and exploring dense ultracold gasses of polar molecules. Exploration of dipolar quantum gasses is of intense current interest because of interest in how anisotropic potentials affect collective phenomena. The NaCs molecule to be studied in this project has strong dipole-dipole forces that will exhibit novel behavior. The project will is to extend prior work in creating NaCs polar molecules which are in the electronic ground state but in rovibrationally excited states. The goal of this project is to use a coherent control technique (Adiabatic Passage by Light Induced Potentials) to produce molecules in the rovibrational ground state from a gas of ultracold atoms. In addition methods to trap these molecules in an electric potential that has a rotating saddle point will be developed. A broader impact of this work lies in the ability of experiments with dipolar gasses to investigate quantum phase transitions which are normally associated with condensed matter systems. Cold polar molecules also have a potential to be used as quantum bits for information processing. As a further and significant broader impact, the project has a significant educational component; undergraduate and graduate students will actively contribute to this research. The project is an ideal training ground for the next generation of scientists and engineers. Students trained in these fields are highly sought after in academic, government and industrial jobs and play a key role in the US scientific workforce.
Luminorefrigeration: deep cooling polar molecules by optical pumping The ability to laser cool atoms depends crucially on the availability of a closed cycling transition that allows many successive photons to be scattered by the atom. By comparison, molecules have a richer internal structure that is both an asset and a liability. The rich structure offers new degrees of freedom for exciting experiments, however, it also precludes any simple scheme for molecular laser cooling. Kinetically cold (T<1 mK) polar molecules in the rovibrational ground state are the starting point for investigation of novel quantum states related to high-Tc superconductors, for tests of time reversal symmetry, for quantum computation and for experiments in quantum chemistry [1]. So far, the most successful approaches for creating the sample start by building molecules from ultracold atomic gasses. To create the molecules and populate the rovibrational ground state (labeled X1Σ+(v=0)) current methods involve approaches that are either relatively simple but inefficient or ones that are highly efficient yet technically challenging. The earliest method used "pump-and-dump" in RbCs to transfer a small population into X1Σ+(v=0). Subsequently, direct photoassociation into X1Σ+(v=0) was observed for LiCs and NaCs, however, the sample was contaminated by molecules in other vibrational states. More recently the JILA group demonstrated an elegant method in KRb involving magnetoassociation followed by coherent transfer to X1Σ+(v=0) using a sophisticated frequency comb reference. In an experiment published in July 2012 [2] a group at the University of Rochester demonstrated vibrational cooling of NaCs molecules to X1Σ+(v=0) using a simple diode laser based method via optical pumping. Luminorefrigeration (Luminofrigoriques) is the term for this approach and was conceived by A. Kastler to describe optical pumping (OP) of sodium atoms between hyperfine states [3]. The sample is "cooled" when the final (dark) state is at a lower energy than the initial state. OP plays a key role in laser cooling of atoms and has been adapted to produce samples of ground state Cs2 molecules and for molecular ions, but it has never been realized for polar molecules - until now. Electronic ground state NaCs molecules at T~250 µK were first formed by photoassociation from laser cooled and trapped gasses of Na and Cs atoms. Photoassociation is an optically mediated chemical reaction in which a colliding pair of atoms is bound into an excited state molecule that then decays radiatively forming a bound ground state molecule. The resulting molecules were shown spectroscopically to include a wide range of vibrational states in the singlet and triplet ground state wells. Using a set of inexpensive laser diodes, an OP excitation spectrum was engineered by temperature tuning the lasers. The spectrum was selected such that the pumping light would excite population in the initial distribution of levels allowing it to decay into the X1Σ+(v=0) state, which was dark. The process was shown capable of generating in excess of 105 molecules/s. This luminorefrigeration should be readily generalizable to other bialkali polar molecules such as KCs, LiCs, and RbCs and make experiments with ultracold polar molecules available to many more groups worldwide. [1] See for example the special issue on the "Physics and Chemistry of Cold Molecules," Eds. O. Dulieu, R. Krems, M. Weidemüller, S. Willitsch, Physical Chemistry Chemical Physics (PCCP), 13, 18684 (2011) and references therein [2] A. Wakim, P. Zabawa, M. Haruza and N. P. Bigelow, "Luminorefrigeration: vibrational cooling of NaCs," Optics Express, 20 16083 (2012). [3] A. Kastler, "Quelques suggestions concernant la production optique et la détection optique d’une inégalité de population des niveaux de quantifigation spatiale des atomes: Application a` l’expe ?rience de Stern et Gerlach et a` la resonance magne ?tique," J. Phys. Radium 11, 255–265 (1950).
