Observation of dipolar spin-exchange interactions with lattice - confined polar molecules

摘要

With the production of polar molecules in the quantum regime, long-range dipolar interactions are expected to facilitate understanding of strongly interacting many-body quantum systems and to realize lattice spin models for exploring quantum magnetism. In ordinary atomic systems, where contact interactions require wave-function overlap, effective spin interactions on a lattice can be mediated by tunnelling, through a process referred to as super-exchange; however, the coupling is relatively weak and is limited to nearest-neighbour interactions. In contrast, dipolar interactions exist even in the absence of tunnelling and extend beyond nearest neighbours. This allows coherent spin dynamics to persist even for gases with relatively high entropy and low lattice filling. Measured effects of dipolar interactions in ultracold molecular gases have been limited to the modification of inelastic collisions and chemical reactions. Here we use dipolar interactions of polar molecules pinned in a three-dimensional optical lattice to realize a lattice spin model. Spin is encoded in rotational states of molecules that are prepared and probed by microwaves. Resonant exchange of rotational angular momentum between two molecules realizes a spin-exchange interaction. The dipolar interactions are apparent in the evolution of the spin coherence, which shows oscillations in addition to an overall decay of the coherence. The frequency of these oscillations, the strong dependence of the spin coherence time on the lattice filling factor and the effect of a multipulse sequence designed to reverse dynamics due to two-body exchange interactions all provide evidence of dipolar interactions. Furthermore, we demonstrate the suppression of loss in weak lattices due to a continuous quantum Zeno mechanism8. Measurements of these tunnelling-induced losses allow us to determine the lattice filling factor independently. Our work constitutes an initial exploration of the behaviour of many-body spin models with direct, long-range spin interactions and lays the groundwork for future studies of many-body dynamics in spin lattices.%超冷分子气体中的极性分子之间的长距离双极相互作用,允许自旋动态从分子的运动完全解耦,这是实现用于研究量子磁性的晶格自旋模型的一个有吸引力的特点。所测出的这种双极相互作用的效应迄今只限于非弹性碰撞和化学反应的改变。本文作者利用固定在一个三维光晶格中的极性分子的双极相互作用来实现一个晶格自旋模型。他们的结果为用长距离相互作用来实现一系列自旋模型提供了可能性,同时也为未来关于自旋晶格中多体动态的研究工作奠定了基础。