JILA, National Institute of Standards and Technology and University of Colorado, Boulder, Colorado 80309, USA.
Nature. 2013 Sep 26;501(7468):521-5. doi: 10.1038/nature12483. Epub 2013 Sep 18.
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 wavefunction overlap, effective spin interactions on a lattice can be mediated by tunnelling, through a process referred to as superexchange; 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 mechanism. 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.
通过在量子领域产生极性分子，可以预期长程偶极相互作用将有助于理解强相互作用的多体量子系统，并实现用于探索量子磁学的晶格自旋模型。在普通原子系统中，接触相互作用需要波函数重叠，而晶格上的有效自旋相互作用可以通过隧穿来介导，这一过程称为超交换；然而，这种耦合相对较弱，并且仅限于最近邻相互作用。相比之下，即使没有隧穿，偶极相互作用也存在，并延伸到最近邻之外。这使得即使在具有相对高熵和低晶格填充的气体中，相干自旋动力学也能持续存在。在超冷分子气体中，已经测量到的偶极相互作用的影响仅限于对非弹性碰撞和化学反应的修饰。在这里，我们利用被三维光学晶格固定的极性分子的偶极相互作用来实现晶格自旋模型。分子的旋转状态编码了自旋，通过微波对分子进行制备和探测。两个分子之间的旋转角动量的共振交换实现了自旋交换相互作用。在自旋相干性的演化中，可以观察到偶极相互作用，除了相干性的整体衰减之外，还出现了振荡。这些振荡的频率、自旋相干时间对晶格填充因子的强烈依赖以及设计用于由于两体交换相互作用而反转动力学的多脉冲序列的效果，都提供了偶极相互作用的证据。此外，我们证明了由于连续量子 Zeno 机制，弱晶格中的损耗会被抑制。对这些隧穿诱导损耗的测量允许我们独立确定晶格填充因子。我们的工作初步探索了具有直接、长程自旋相互作用的多体自旋模型的行为，并为未来研究自旋晶格中的多体动力学奠定了基础。
