Laser Cooling Molecules

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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.