Zeeman Sisyphus Deceleration of Molecules

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The quantum mechanical characteristics of molecules can be used to reveal fundamental properties of our universe. Arrays of interacting molecules can be used to mimic materials and probe quantum systems. The internal structure of molecules can be used as sensitive probes to measure fundamental constants. Reactions and collisions between cold controlled molecules can be used to investigate quantum chemistry. With these applications molecules offer a promising route to advancing our understanding of nature. To realise these opportunities, we must cool molecules down to very low temperatures. This can be done by applying the knowledge gained from ultracold atomic experiments. Work on cooling and controlling atoms started in the last century and advanced rapidly following the invention of the laser. This work led to; the realisation of novel phases of matter (e.g., Bose-Einstein condensates), the trapping of individual atoms, the most precise clocks, and much more. Molecules promise to take us even further due to their strong, long-range interactions, rich internal structure (namely vibrational and rotational degrees of freedom) and efficient coupling to external fields. The complex structure of molecules makes them fundamentally harder to cool as laser slowing, used for atoms, requires closed transitions which can scatter tens of thousands of photons. To overcome this, two complimentary routes to creating ultracold molecules developed. The first is indirect techniques using samples of ultracold atoms and associating them to form molecules. These techniques require molecules made up of laser-coolable atoms, generally forming bialkali molecules. These molecules have a very complicated internal structure. The second route is direct cooling: molecules are created first and then decelerated. Various deceleration techniques have been developed including Stark, Zeeman and centrifuge decelerators. Over the last decade, a subset of molecules has been laser cooled and subsequently trapped in a magneto-optical trap. These species have a small mass, remarkably closed cooling transitions, and simple internal structure. Extending laser cooling to heavy molecules would require many more photons to be scattered. Even if those molecules have equally closed transitions, the slowing will be less efficient and require more lasers. This both increases the complexity and the cost of an experiment. In this project we will test a new type of decelerator which requires only a couple of hundred photons. A molecule travels through a spatially varying magnetic field, it is optically pumped into a weak-field seeking state each time it approaches a high field region and then into a strong-field seeking state as it approaches a low field. The molecule therefore constantly loses energy and slows down. As this decelerator uses magnetic fields to remove energy as opposed to photon recoil, the mass of the molecule irrelevant. This technique is therefore applicable to a much larger class of molecules, all they need is a magnetic moment. We will demonstrate the decelerator using a cryogenic beam of calcium monofluoride molecules. The results of this project will be directly comparable to laser cooling data and would be readily implemented with different molecules.