Floquet engineering ultracold polar molecules to simulate topological insulators

We present a quantitative, near-term experimental blueprint for the quantum simulation of topological insulators using lattice-trapped ultracold polar molecules. In particular, we focus on the so-called Hopf insulator, which represents a three-dimensional topological state of matter existing outside the conventional tenfold way and crystalline-symmetry-based classifications of topological insulators. Its topology is protected by a linking number invariant, which necessitates long-range spin-orbit-coupled hoppings for its realization. While these ingredients have so far precluded its realization in solid-state systems and other quantum simulation architectures, in an accompanying Letter [T. Schuster et al., Phys. Rev. Lett. 127, 015301 (2021)], we predict that Hopf insulators can arise naturally from the dipolar interaction. Here, we investigate a specific polar molecule architecture, where the effective “spin” is formed from sublattice degrees of freedom. We introduce two techniques that allow one to optimize dipolar Hopf insulators with large band gaps, and which should also be readily applicable to the simulation of other exotic band structures. First, we describe the use of Floquet engineering to control the range and functional form of dipolar hoppings and, second, we demonstrate that molecular AC polarizabilities (under circularly polarized light) can be used to precisely tune the resonance condition between different rotational states. To verify that this latter technique is amenable to current-generation experiments, we calculate, from first principles, the AC polarizability for 
σ+ light for 40K 87Rb. Finally, we show that experiments are capable of detecting the unconventional topology of the Hopf insulator by varying the termination of the lattice at its edges, which gives rise to three distinct classes of edge mode spectra.