Laser Cooling Molecules

Laser cooling of atoms has been a fantastically successful technique. Not only is it possible to reduce the temperature of a gas of atoms to the nanokelvin range, refinements of the technique allow the production of atomic Bose-Einstein condensates, the coldest matter allowed by the laws of physics. There is now enormous interest in exploring the physics of ultracold molecules, for example to study chemical reactions at ultralow temperatures, for spectroscopy and high-precision measurement, for quantum information processing, and for the study of the basic physics of quantum degenerate matter with long range anisotropic interactions. Until now however, the key technique used to produce ultracold atoms, laser cooling, has not been applied to molecules - even simple diatomic molecules - because the electronic ground state has many vibrational and rotational levels. The problem is that laser cooling requires the absorption and emission of many thousands of photons. Most molecules are quickly pumped into levels where they no longer interact with the laser light. The key to laser cooling molecules is to plug all of these leaks using additional laser wavelengths, a formidable task for most molecular species.In this research, we will explore the laser cooling of SrF and BaF. These rather simple molecules have the property that electronic transitions (which are used to extract energy from the molecules and thereby cool them) tend not to change the molecule's vibrational quantum number. This means that only a few laser wavelengths are required to plug the leaks into the other rotational and vibrational states. We plan to generate this light using four diode lasers, which are very reliable and relatively cheap. We will produce the molecules using a supersonic expansion, a technique that our research group is already very familiar with. Molecules emerge from the expansion with a temperature of a few Kelvin, very cold compared to room temperature but still far too warm to investigate interesting new physics and applications. Laser cooling will make the molecules thousands of times colder. Our plan is first to demonstrate cooling in one dimension, then to extend the techniques to 2d and 3d. The ultimate goal of the programme is to trap the molecules using a combination of magnetic and electric fields and to use laser light to cool them to microkelvin temperatures. We believe such a sample of cold trapped molecules will be a fascinating tool with which to explore new areas of science.

In the long term we believe that ultracold molecules could be useful in novel applications, e.g. for interferometric sensors with control over the internal states as well as the centre of mass motion, or for realising quantum information schemes using cold molecules. A more immediate impact from this research will be the training provided for the PDRA and postgraduate student. The research techniques will be applicable not only to experimental cold atom and molecule research, but also in the much wider fields of quantum optics, laser technology, and photonics. The technologial impact of this research will be first felt by other academic research groups. The immediate commercial impact will likely not be through the specific techniques we pioneer, but rather through the provision of highly skilled and trained personnel. The results of the research will be disseminated through the CCM website http://www.imperial.ac.uk/ccm/, the preprint archive, the popular press, refereed journals and talks at conferences. Our website has pages specifically devoted to popular understanding of our research. These outputs will help to raise the profile of the College and of UK research. The collaboration with Los Alamos will help to boost the international profile of the research. The application of cold molecules in basic physics - testing supersymmetric theories and searching for a time variation of fundamental constants - is an excellent example of science that appeals to the curiosity and fascination of the general public and increases the engagement of the public with science. Examples of our past activities in public engagement include schools talks, public lectures, an exhibition at the Royal Society, and radio and television presentations for the BBC. The physics department at Imperial College London has recently been selected to run a doctoral training centre on controlled quantum dynamics. The courses presented by this centre will be available for the postgraduate student and will provide an excellent formal introduction to many aspects of the research programme. This will be in addition to the standard training and professional skills development provided by the department.