5th Institute of Physics


Dipolar quantum gases

In this project we experimentally investigate the dipole-dipole interactions in Bose-Einstein condensates (BECs). Even though an atomic Bose-Einstein-Condensate (BEC) is a very dilute gas, the most fascinating experimental results arise from the weak interactions between the particles. In most experiments the dominating interaction in a BEC is the isotropic and short-range contact interaction that can be characterized by the s-wave scattering length a. Magnitude and sign of a can be modified using Feshbach resonances. Dipolar gases provide a second anisotropic and long-range interaction. The interplay of these two interactions give rise to interesting many-body phenomena and collective behaviour. Novel quantum phase transitions, dipolar order and spin tunnelling in the condensate have been predicted. New questions concerning stability and shape of such a condensate arise.

Recently we have made the surprising discovery of a very dilute liquid state of matter that is superfluid and can even form selfbound droplets in three dimensions (like water). This novel liquid is very exotic as it is stabilized by quantum fluctuations and about eight orders of magnitude less dense than ordinary liquids. 

Hot atoms in microcells

The strong interaction among Rydberg states leads to a blockade mechanism which only allows for one excitation into a Rydberg state within the blockade volume. This effect has been shown e.g. by our Rydberg experiments with ultracold atoms.

The key idea is to confine thermal vapors of Alkalis in spectroscopy cells smaller than the blockade radius, which is on the order of a few microns. By this only one Rydberg excitation is allowed within in one cell, or several close by cells. This system opens the way to a large nonlinearity on the single photon level with many possible applications in quantum information technology and quantum optics.

We are also working on gas and microwave sensors based on Rydberg atoms in vapor cells. Besides this we are integrating atomic vapor cells with electronic and photonic circuits to build high performance sensing devices..

Giant Rydberg Atoms in ultracold quantum gases

Rydberg atoms are highly excited atoms with one valence electron of principal quantum number n>>1. Because of their huge size of the order n2a0 they are very susceptible to external electric fields and interact strongly. These unusually strong interactions lead to a blockade of excitation in the so called blockade radius of the Rydberg atoms. If the system is driven coherently a single excitation can be shared by several atoms within the blockade radius, forming a 'super atom', a collective quantum state. In this project Rydberg excitation of a magnetically trapped dense cloud is performed. Goals of this experiment, besides the study of interactions between Rydberg atoms (dipole-dipole and van der Waals interaction) and the already shown excitation of Rydberg atoms in a Bose-Einstein condensate, are studies of the coherent collective quantum states. Furthermore a novel type of molecular bond can be formed by Rydberg atoms.

We are currently developing an imaging technique that will provide a picture of a single electron in a defined orbital state. We are also investigating in the transition from molecular physics to many-body physics. Also we are interested in the properties of a single ionic impurity in a quantum gas.

Cold Molecules

Full quantum control of molecules has been an outstanding goal for decades. Cooling molecules provides a most promising answer to address this challenge. With recent progress in experimental quantum physics, such cooling is finally within reach. The aim of this project is to realize laser cooling of diatomic molecules. The resulting cold molecular gas will pave the way for a large number of novel and interdisciplinary applications ranging from few- and many-body physics to cold chemistry and tests of fundamental symmetries.

Former Projects

Strongly interacting Rydberg gases

Novel sources of ultra cold matter

Rydberg Quantum Optics

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