With the recent production of polar molecules in the quantum regime [1,2], long-range dipolar interactions are expected to facilitate the understanding of strongly interacting many-body quantum systems and to realize lattice spin models [3] for exploring quantum magnetism. In atomic systems, where interactions require wave function overlap, effective spin interactions on a lattice can be realized through superexchange; however, the coupling is relatively weak and limited to nearest-neighbor interactions [4][5][6]. In contrast, dipolar interactions exist even in the absence of tunneling and extend beyond nearest neighbors. This allows coherent spin dynamics to persist even for gases with relatively high entropy and low lattice filling. While measured effects of dipolar interactions in ultracold molecular gases have thus far been limited to the modification of inelastic collisions and chemical reactions [7,8], we now report the first observation of dipolar interactions of polar molecules pinned in a three-dimensional optical lattice. We realize a lattice spin model where spin is encoded in rotational states of molecules that are prepared and probed by microwaves. This interaction arises from the resonant exchange of rotational angular momentum between two molecules and realizes a spin-exchange interaction. The dipolar interactions are apparent in the evolution of the spin coherence, where we observe clear 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 multi-pulse sequence designed to reverse dynamics due to two-body exchange interactions all provide clear evidence of dipolar interactions. Furthermore, we demonstrate the suppression of loss in weak lattices due to a quantum Zeno mechanism [9]. Measurements of these tunneling-induced losses allow us to independently determine the lattice filling factor. The results reported here comprise an initial exploration of the behavior of many-body spin models with direct, long-range spin interactions and lay the groundwork for future studies of many-body dynamics in spin lattices.

We have realized long-lived ground-state polar molecules in a 3D optical lattice, with a lifetime of up to 25 s, which is limited only by off-resonant scattering of the trapping light. Starting from a 2D optical lattice, we observe that the lifetime increases dramatically as a small lattice potential is added along the tube-shaped lattice traps. The 3D optical lattice also dramatically increases the lifetime for weakly bound Feshbach molecules. For a pure gas of Feshbach molecules, we observe a lifetime of >20 s in a 3D optical lattice; this represents a 100-fold improvement over previous results. This lifetime is also limited by off-resonant scattering, the rate of which is related to the size of the Feshbach molecule. Individually trapped Feshbach molecules in the 3D lattice can be converted to pairs of K and Rb atoms and back with nearly 100% efficiency.PACS numbers: 37.10.Pq, Controllable long-range and anisotropic dipole-dipole interactions can enable novel applications of quantum gases in investigating strongly correlated many-body systems [1][2][3][4][5][6]. Recent experiments have realized an ultracold gas of polar molecules in the ro-vibrational ground state [7] with high-resolution, single-state control at the level of hyperfine structure [8]. However, an obstacle to creating long-lived quantum gases of polar molecules was encountered with the observation of bimolecular chemical reactions in the quantum regime [9]. Even with the demonstrated strong suppression of the reaction rate for spin-polarized fermionic KRb molecules, the lifetime of a 300 nK sample with a peak density of 10 12 /cm 3 was limited to ∼1 s. Furthermore, when an external electric field is applied to polarize the molecules in the lab frame, the attractive part of the dipole-dipole interaction dramatically increases the ultracold chemical reaction rate, reducing the lifetime of the dipolar gas to a few ms when the lab-frame molecular dipole moment reaches 0.2 Debye [10]. A promising recent development was the demonstration that confining fermionic polar molecules in a 1D optical lattice suppresses the rate of chemical reactions even in the presence of dipolar interactions. Here, the spatial anisotropy of the dipolar interaction was exploited by confining a gas of oriented KRb molecules in a two-dimensional geometry to suppress the attractive part of the dipolar interaction and thus achieve control of the stereodynamics of the bimolecular reactions [11]. In this regime, the lifetime of a trapped gas of polar molecules with a lab-frame dipole moment of ∼0.2 Debye, a temperature of 800 nK, and a number density of 10 7 cm −2 , was ∼1 s.In this letter, we study KRb molecules confined in 2D and 3D optical lattice traps, where we explore the effects of the lattice confinement on the lifetime of the ultracold gas. We note that a lifetime of 8 s has been achieved for homonuclear Cs 2 molecules in a 3D lattice [12]. In our work, we find that long lifetimes are achieved for the molecules in a strong 3D lattice trap, even when there is a signific...

No abstract