Molecular Sisyphus Cooling – MolSisCool

Over the two last decades of the twentieth century, physicists completed the cooling of atomic samples initiated with the development of laser techniques. It was then possible to cool atoms at temperatures below 1 µK. Beyond the technological breakthrough, it has allowed further development of a wide variety of studies dealing with degenerate gases, quantum simulation of solid state physics, metrology, quantum chaos, etc. The question of creating molecular samples in the same range of temperature emerged at the end of the nineties. This interest can be summarized by the fact that molecules, compared to atoms, are complex objects that possibly feature a long range and anisotropic interaction. Developing such an experimental skill could then offer new perspectives in fundamental research and technology linked to quantum physics: for example, investigation of collisions and chemical reactions in ultracold and quantum regime, creation of molecular samples in correlated regime, improvement of metrological precision, metrology,etc.

Unfortunately, molecular cooling cannot copy the laser techniques used in atomic cooling. Therefore, for more than ten years now, a great variety of methods have been proposed and tested to create cold molecules. Even if there have been impressive successes, an efficient direct molecular cooling is still missing. The main goal of the MolSisCool project is to develop a new technique of molecular cooling by means of a combination of laser techniques and static electromagnetic fields. On the long view, our objective is also to bridge a technological gap, namely cooling molecules at temperatures in the µK range. The heart of our approach is to implement an optical Sisyphus cooling of molecules which, with a single photon, is able to remove much more kinetic energy than standard Doppler cooling does (~1K vs ~1µK). As molecules cannot perform many successive optical transitions, Sisyphus cooling is thus particularly promising. We aim to demonstrate the feasibility of Sisyphus cooling on barium fluoride (BaF) according to the following route:

1- Production and spectroscopic characterization of a molecular beam of BaF;
2- Optical pumping of the molecular sample to a single rotational level;
3- Transverse cooling of the beam/collimation by Sisyphus cooling;
4- Partial beam deceleration by Sisyphus approach.

The first step consists in setting up the experiment framework: a supersonic molecular beam based on the well mastered free expansion technique. The second is a broadband optical pumping we have already demonstrated on ultracold Cs2 molecules. It will both show the versatility of such a pumping and prepare molecules for the subsequent experiments. The first and fourth steps constitute the central part of this project and, beyond the experimental skills, will require theoretical support.
Bearing in mind that this kind of source is a new tool for fundamental research, we will initiate new studies; thus, we plan to investigate collisional interactions in a mixture of cold BaF molecules and Rydberg atoms. In particular we mention that the development of optical frequency combs provided, through optical fibers, by the SYRTE, should make metrology accessible to any group.