Floquet Engineering Ultracold Polar Molecules to Simulate Topological Insulators

TL;DR

This paper proposes a practical method to simulate three-dimensional topological insulators, specifically Hopf insulators, using ultracold polar molecules with Floquet engineering and AC polarizability tuning, enabling experimental detection of their unique edge states.

Contribution

It introduces a novel experimental blueprint for realizing Hopf insulators in ultracold polar molecule lattices using Floquet control and polarizability tuning techniques.

Findings

Floquet engineering can optimize dipolar hoppings for large band gaps.

AC polarizability tuning allows precise control of rotational state resonances.

Edge state spectra can be used to detect the Hopf insulator topology.

Abstract

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 \emph{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 a companion manuscript [1901.08597] we predict that Hopf insulators can in fact arise naturally in dipolar interacting systems. Here, we investigate a specific such architecture in lattices of polar molecules, 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 bandstructures. 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 $\sigma^+$ light for ${}^{40}$K$^{87}$Rb. 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.