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 reactions1, the design of a platform for quantum information processing2,3,4 and the realization of novel quantum many-body systems5,6,7,8. Despite intense experimental efforts aimed at observing the influence of dipoles on ultracold molecules9, only recently have sufficiently high densities been achieved10. 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 interaction can be used for electric-field control of chemical reaction rates in an ultracold gas of polar molecules. Furthermore, the large loss rates in an applied electric field suggest that creating a long-lived ensemble of ultracold polar molecules may require confinement in a two-dimensional trap geometry to suppress the influence of the attractive, ‘head-to-tail’, dipolar interactions11,12,13,14.
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This work was supported by the US National Institute of Standards and Technology programme Innovations in Measurement Science–Ultracold Stable Molecules, the US National Science Foundation (NSF) Physics Frontier Center at JILA, the US Department of Energy, Air Force Office of Scientific Research Multidisciplinary Research Initiative on Ultracold Molecules, and a NSF graduate fellowship (B.N.).
The authors declare no competing financial interests.
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Ni, KK., Ospelkaus, S., Wang, D. et al. Dipolar collisions of polar molecules in the quantum regime. Nature 464, 1324–1328 (2010). https://doi.org/10.1038/nature08953
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