Dipolar collisions of polar molecules in the quantum regime

Quantum collisions Ultracold polar molecules offer the possibility of exploring quantum gases with inter-particle interactions that are strong, long-range and spatially anisotropic. Here, Ni et al . report the experimental observation of dipolar collisions in an ultracold gas of fermionic 40 K 87 Rb molecules. For modest values of an applied electric field, they observe a dramatic increase in the loss rate due to ultracold chemical reactions. Ultracold polar molecules offer the possibility of exploring quantum gases with interparticle interactions that are strong, long-range and spatially anisotropic. Here, dipolar collisions in an ultracold gas of fermionic potassium–rubidium molecules have been experimentally observed. The results show how the long-range dipolar interaction can be used for electric-field control of chemical reaction rates in an ultracold gas of polar molecules. Ultracold polar molecules offer the possibility of exploring quantum gases with interparticle interactions that are strong, long-range and spatially anisotropic. This is in stark contrast to the much studied dilute gases of ultracold atoms, which have isotropic and extremely short-range (or ‘contact’) interactions. Furthermore, the large electric dipole moment of polar molecules can be tuned using an external electric field; this has a range of applications such as the control of ultracold chemical reactions 1 , the design of a platform for quantum information processing 2 , 3 , 4 and the realization of novel quantum many-body systems 5 , 6 , 7 , 8 . Despite intense experimental efforts aimed at observing the influence of dipoles on ultracold molecules 9 , only recently have sufficiently high densities been achieved 10 . Here we report the experimental observation of dipolar collisions in an ultracold molecular gas prepared close to quantum degeneracy. For modest values of an applied electric field, we observe a pronounced increase in the loss rate of fermionic potassium–rubidium molecules due to ultracold chemical reactions. We find that the loss rate has a steep power-law dependence on the induced electric dipole moment, and we show that this dependence can be understood in a relatively simple model based on quantum threshold laws for the scattering of fermionic polar molecules. In addition, we directly observe the spatial anisotropy of the dipolar interaction through measurements of the thermodynamics of the dipolar gas. These results demonstrate how the long-range dipolar inte