# Rubidium Experiment: Quantum Simulation

### Ultracold Atoms in Lattice Potentials

Ultracold atoms can be trapped in periodic optical potentials. The achieved systems, so called optical lattices, much resemble an artificial solid. In earlier work, we have investigated the band structure of optical lattices of variable inversion symmetry, as a step towards simulating the diversity of potentials that nature provides us in the system of electrons in natural crystals. In our setup, optical potentials for atoms can be generated by means of Fourier-synthesis. For a nearly sawtooth-like potential, we use the superposition of a usual standing wave lattice of spatial periodicity λ/2 with a novel multiphoton lattice of periodicity λ/4. In this way, a dissipationless quantum ratchet could be realised, where the rectification of quantum fluctuations leads to a directed atomic motion. Effectively, this demonstrates the operational principle of microscopic quantum motors, as a prototype of microscopic machinery in the quantum world.

In other work, collaborating with the theory group of A. Rosch, we have used an optical lattice with a spatially chirped amplitude to realize an edge state between two spatial regions of different topological order. Atoms confined in the topological edge state were directly observed in real-space with an optical microscope More recently, collaborating with the theory group of E. Solano and E. Rico, we have studied ultra-strong coupling of two mechanical modes in the quantum Rabi regime und the combined action of atoms being confined by a lattice and an optical dipole trapping potential. With this novel experimental scheme, a record high coupling of 6.5 times the field mode frequency was obtained, so that the coupling term dominates over all other energy scales.

Artist's view of a Bose-Einstein Condensate in an optical lattice. The periodic lattice can be symmetric or asymetric, with the latter leading to a non-trivial band structure.

### Some insights

The rubidium chamber. As any experiment on BECs, multiple laser beams have to interact with the atoms inside a vacuum chamber. To allow access, we employ a custom-made hedgehog-like chamber.

The rubidium atoms in the optical lattice perform oscillations, realizing the mechanical mode in the quantum Rabi model. The image is stitched from multiple time-of-flight measurements, from which the atom's momentum can be extracted.

## Recent Publications

Quantum Rabi dynamics of trapped atoms far in the deep strong coupling regime

Johannes Koch, Geram Hunanyan, Till Ockenfels, Enrique Rico, Enrique Solano, Martin Weitz

arXiv:2112.12488