Quantum-optical control of cold polar molecules

This project is a combined experimental (MPQ) and theoretical (LMU) effort to develop quantum-optical methods for control of cold molecules. The MPQ group has developed a technique to produce guided beams of slow polar molecules. The method is based on the efficient extraction of the slowest molecules from a thermal gas (see Fig. B.2.1). In addition, the thermal gas can be precooled by replacing the thermal reservoir by a cryogenic buffer gas cell. This extension of the velocity-filtering technique has been developed within the framework of this project. Using a setup merging the advantages of buffer-gas cooling and velocity filtering, we have achieved an enormous improvement in the purity of the guided beam.

To produce molecules at an even lower internal and external temperature, and achieve at the same time an even higher density, new dissipative methods will be necessary. One method is cavity cooling, which has been demonstrated in our group with Rb atoms. Standard laser cooling, which is successfully used for atomic species, is not generally applicable to molecules due to the lack of cycling transitions. Since cavity cooling is based on coherent scattering, it should be applicable to any polarizable particle, which includes molecules. From the theoretical side we have worked out schemes which cool molecular motion as well as molecular vibration and rotation. From the experimental side, we have investigated the enhancement of far-detuned light scattering (Rayleigh scattering) by an optical resonator, an important step towards cavity-assisted detection of cold molecules.

We have also developed a scheme for cooling a very general class of polar molecules (symmetric rotors) to sub-mK temperature. The method makes use of the huge interaction between polar molecules and external electric fields. Combining suitably tailored electric trapping fields with infrared vibrational excitation, cooling times of a few seconds with only a dozen spontaneous decays are predicted, thereby eliminating decay to states not participating in the cooling cycle. The realization of this technique is the great challenge being pursued now. For this reason we are constructing a new type of electrostatic trap with microstructured electrodes, which will be loaded with slow molecules produced by the electric guide.

The theory of molecule-light interaction is the core expertise of the LMU group. We have, e. g., published ideas on using quantum engineering of molecular states for quantum information processing. From the theoretical side, the effect of cavity enhancement for cooling of the external and internal degrees of freedom will first be investigated on a model system either in the wavepacket picture or in the density matrix formalism. In a next step the optical cooling cycles will be optimised. In a more advanced stage of the project, more complex molecules will be investigated. Once sufficiently cold and dense gases can be produced, ultracold collisions can be studied, opening up the possibility to explore and control cold chemical reactions. Here shaped ultrafast laser pulses can act as photonic reagents to manipulate reaction pathways and quantum states in molecular systems with increasing complexity. In addition, recent ideas of using cold polar molecules for quantum engineering and quantum information processing can be realised.