With the production of polar molecules in the quantum regime1,2, long-range dipolar interactions are expected to facilitate understanding of strongly interacting many-body quantum systems and to realize lattice spin models3 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 interactions4,5. 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 reactions6,7. 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.
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We thank B. Zhu, M. Foss-Feig, G. Quéméner and M. Lukin for discussions. We acknowledge funding for this work from the NIST, NSF, AFOSR-ARO (MURI), ARO, DOE and ARO-DARPA-OLE. S.A.M. is supported by an NDSEG Graduate Fellowship. B.G. and K.R.A.H. are National Research Council postdoctoral fellows. K.R.A.H. and A.M.R. thank the KITP for hospitality.
This file contains Supplementary Figures 1-3. Supplementary Figure 1 depicts the dependence of differential light shifts on lattice depth for several rotationally excited states, Supplementary Figure 2 shows the insensitivity of Ramsey contrast oscillations to the confining lattice depth and Supplementary Figure 3 shows the determination of the two-body loss coefficient for a rotational-state mixture.
About this article
Physical Review A (2019)