Measuring individual gap closings in a Floquet-Creutz ladder

Ultracold atoms in optical lattices serve as a formidable platform to study the physics of interacting quantum particles in lattices. The interplay between strong interactions and the geometry, dimensionality and topology of the bandstructure promises the exploration of phenomena ranging from frustrated magnetism to fractional Chern insulators. In particular, Floquet band engineering has been used to create topological band structures.

Strong phase modulation of a lattice potential hybridizes ground and excited bands and enables dynamic control over the topology of the coupled bands. For single-frequency modulation, these couplings appear symmetrically in quasi-momentum. Adding higher harmonic components to the drive breaks time-reversal symmetry and, therefore, individual bandgaps can be controlled. We probe experimentally the Floquet bandstructure of a driven one dimensional optical lattice and demonstrate the closing of individual gaps. In previous work this scheme has been used to engineer a Creutz ladder model. The individual gap closing indicates a topological phase transition in this model.