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 system, 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. Tuning of the interaction results in the spectacular observation of collapsing condensates ("Bosenova") and the formation of ultra-cold molecular gases out of a BEC. Due to its long-range character and its anisotropic nature the dipole-dipole interaction in a BEC has generated significant theoretical interest. New interesting phenomena like 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.

Strongly interacting Rydberg 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. After the first observation of these molecular states and further studies of this new binding mechanism, coherent control of the molecules is experimentally investigated in this project.


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.

Rydberg Impurities

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

Novel sources of ultra cold matter

Rydberg Quantum Optics

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