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Rubidium BEC in Variable Optical Lattices



Beugung von BECs an
Diffraction of BECs on optical lattices of variable spatial periodicity


The directed motion of atoms in a quantum ratchet is due to contributions of transporting Floquet eigenstates
(illustrated here as conveyor belts) with different mean velocities


edge state_eng.png

Left: Illustration of the spatial variation of the band structure of the lattice along position z. At z=0 a topological edge state is expected. Right: Series of vertically binned images for different relative phases of the Raman state preparation beams. Maximum loading into the edge state is observed for a relative phase of π/2.

Ultracold atoms can be trapped in periodic optical potentials. The achieved systems, so called optical lattices, much resemble an artificial solid. We investigate 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 are 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.
We have investigated the transport of atoms in temporally driven ratchet potential. 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.
The variable lattice was also used to modify the atomic dispersion relation, which can be tuned to become linear, i.e. relativistic, similar as in graphene material for electrons, despite operating with particle velocities many orders of magnitude below the speed of light. With a quasirelativistic Bose-Einstein condensate in an optical lattice potential, we have demonstrated Klein-tunneling, and in other work also Veselago lensing for matter waves. More recently, collaborating with the Cologne 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.


Some references:

Fourier Synthesis of Conservative Atom Potentials
G. Ritt, C. Geckeler, T. Salger, G. Cennini, and M. Weitz
Phys. Rev. A 74, 063622 (2006)

Atomic Landau-Zener tunneling in Fourier-synthesized optical lattices
T. Salger, C. Geckeler, S. Kling, and M. Weitz
Phys. Rev. Lett. 99, 190405 (2007) 
Directed Transport of Atoms in a Hamiltonian Quantum Ratchet
T. Salger, S. Kling, T. Hecking, C. Geckeler, L. Morales-Molina, and M. Weitz
Science 326, 1241 (2009), arXiv:0912.0102
Klein-Tunneling of a quasirelativistic Bose-Einstein condensate in an optical lattice
T. Salger, C. Grossert, S. Kling, and M. Weitz
Phys. Rev. Lett. 107, 240401 (2011), arXiv:1108.4447 
Veselago lensing with ultracold atoms in an optical lattice
M. Leder, C. Grossert, and M. Weitz
Nature Communications 5, 3327 (2014), arXiv:1402:3132 
C. Grossert, M. Leder, S. Denisov, P. Hänggi, and M. Weitz
Nature Communications 7, 10440 (2016), arXiv:1407.0605
M. Leder, C. Grossert, L. Sitta, M. Genske, A. Rosch, and M. Weitz
Nature Communications 7, 13112 (2016), arXiv:1604.02060
Einführender Artikel, gut für Studenten geeignet:
Quantenratsche für ultrakalte Atome
T. Salger and M. Weitz
Phys. Unserer Zeit 41, 110 (2010)


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