Atom chips

Atom chips allow for a great variety of trapping geometries for atomic ensembles by means of magnetic, electric, optical, microwave and radio-frequency potentials. Our research aims at the creation of multiply connected topologies, like rings, tori, and cylinders. These traps are used to investigate the low temperature behaviour of ultracold quantum gases when the dimensionality of the trapping geometry changes. Of particular interest are studies of low dimensional systems (2D and 1D) with periodic boundary conditions.

Two-dimensional quantum gases do not show a true Bose-Einstein condensation (BEC) in homogenous systems, but rather a Berezinskii-Kosterlitz-Thouless (BKT) type transition. A BEC can still occur in harmonic traps though, hence providing a “mixture” of BEC and BKT physics. A quasi-homogenous geometry as provided by the torus trap will enable the study of pure BKT physics.

Our current atom chip generation will produce cylinder-symmetric traps by using a combination of dc and radio-frequency fields. Further to the investigation of low-dimensional systems the chip can be used to dynamically split an elongated cloud of ultracold atoms. This “unzipping” of the cloud is intended to be used as a quantum simulator for effects of the Fulling-Davies-Unruh type.

Quantum optics and light-matter interfaces

We investigate techniques to map the quantum states of light pulses into the spin degrees of freedom of atomic ensembles and vice versa. Using their long coherence times, the atoms serve as a quantum memory, enabling, e.g., long-distance quantum communication.

On the other hand, mapping quantum states of matter onto light makes them accessible to well established quantum optical techniques like photon counting and homodyne detection. These can then be used as analytical tools for a variety of strongly correlated many-body states. We plan to trap very elongated clouds of atoms inside the small hollow core of a photonic crystal fibre, which will lead to strong interactions with the light field even for gases containing only a few hundred atoms.

Toroidal and ring-shaped matterwaves

Another line of experiments will make use of atom chip based traps with non-trivial topologies. By confining an ultracold gas to the surface of a hollow torus, we want to realise a two-dimensional matter wave with periodic boundary conditions.
This will help overcoming certain restrictions to the validity of theoretical models, which are encountered when using harmonic trapping potentials. A major ingredient to our experiments are radio-frequency (rf) dressed potentials.
By exploiting the vector type coupling between atoms and rf-fields we gain control over atomic motion and will be
able to let atoms counterpropagate in two rings. Such a setup can be used as a matter wave Sagnac interferometer that is extremely sensitive to rotation.

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