Optoelectrical Cooling of Formaldehyde to Sub-Millikelvin Temperatures

Due to their strong long-range dipole-dipole interactions and large number of internal states, polar molecules cooled to ultracold temperatures enable fascinating applications ranging from ultracold chemistry to investigation of dipolar quantum gases. However, realizing a simple and general technique to cool molecules to ultracold temperatures, akin to laser cooling of atoms, has been a formidable challenge.\\ We present results for opto-electrical Sisyphus cooling applied to formaldehyde (H$_2$CO). In this generally applicable cooling scheme, molecules repeatedly move up and down electric field gradients of a trapping potential in different rotational states to efficiently extract kinetic energy\footnote{M. Zeppenfeld et al., Phys. Rev. A {\bf 80}, 041401(R) (2009).}. A total of about 300,000 molecules are thereby cooled by a factor of 1000 to 400uK, resulting in a record-large ensemble of ultracold molecules\footnote{A. Prehn et al., Phys. Rev. Lett. {\bf 116}, 063005 (2016).}. In addition to cooling of the motional degrees of freedom, optical pumping via a vibrational transition allows us to control the internal rotational state\footnote{R. Gl\"ockner et al., Phys. Rev. Lett. {\bf 115}, 233001 (2015).}. We thereby achieve a purity of over 80\% of formaldehyde molecules in a single rotational M-sublevel. Our experiment provides an excellent starting point for precision spectroscopy and investigation of ultracold collisions.