JILA, NIST and University of Colorado, Department of Physics, University of Colorado, Boulder, Colorado 80309-0440, USA.
Nature. 2010 Apr 29;464(7293):1324-8. doi: 10.1038/nature08953.
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, the design of a platform for quantum information processing and the realization of novel quantum many-body systems. Despite intense experimental efforts aimed at observing the influence of dipoles on ultracold molecules, only recently have sufficiently high densities been achieved. 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 interactions.
超冷极性分子提供了探索具有强相互作用、长程和各向异性的粒子间相互作用的量子气体的可能性。这与广泛研究的超冷原子稀薄气体形成鲜明对比，后者具有各向同性和极其短程（或“接触”）的相互作用。此外，极性分子的大电偶极矩可以通过外部电场进行调节；这在控制超冷化学反应、设计量子信息处理平台和实现新型量子多体系统等方面有多种应用。尽管人们进行了大量实验努力来观察偶极子对超冷分子的影响，但直到最近才实现了足够高的密度。在这里，我们报告了在接近量子简并制备的超冷分子气体中观察到偶极碰撞的实验结果。对于施加电场的适度值，我们观察到由于超冷化学反应，费米钾-铷分子的损失率明显增加。我们发现损失率与感应电偶极矩呈陡峭的幂律依赖关系，并且我们表明，基于费米极性分子散射的量子阈值定律的相对简单模型，可以理解这种依赖性。此外，我们通过测量偶极气体的热力学，直接观察到偶极相互作用的空间各向异性。这些结果表明，长程偶极相互作用如何用于控制超冷极性分子气体中的化学反应速率的电场。此外，在施加电场中较大的损失率表明，要创建长寿命的超冷极性分子集合体，可能需要限制在二维陷阱几何形状中以抑制吸引力“头对头”偶极相互作用的影响。
