Measuring topology by dynamics: Chern number from linking number

TL;DR

This paper demonstrates experimentally that the Chern number, a topological invariant, can be inferred from the linking number of momentum-space vortex trajectories in a driven quantum system, linking static topology to dynamics.

Contribution

It extends the measurement of topological invariants to far-from-equilibrium dynamics in Floquet systems using fermionic atoms in optical lattices.

Findings

Linking number of vortex trajectories equals the ground-state Chern number

Dynamical Chern number remains zero during unitary evolution

Phase diagram mapped via dynamical topological index

Abstract

Integer-valued topological indices, characterizing nonlocal properties of quantum states of matter, are known to directly predict robust physical properties of equilibrium systems. The Chern number, e.g., determines the quantized Hall conductivity of an insulator. Using fermionic atoms in a periodically driven optical lattice, here we demonstrate experimentally that the Chern number determines also the far-from-equilibrium dynamics of a quantum system. Following the proposal of ref. [Wang et al., Phys. Rev. Lett. 118, 185701 (2017)] and extending it to Floquet systems, we measure the linking number that characterizes the trajectories of momentum-space vortices emerging after a strong quench. We observe that it directly corresponds to the ground-state Chern number. This one-to-one relation between a dynamical and a static topological index allows us to experimentally map out the phase diagram of our system. Furthermore, we measure the instantaneous Chern number and show that it remains zero under the unitary dynamics.