Prof. Dr. Tilman Pfau |
|
|
Research Highlights
 | Dipolar Quantum GasesEven though an atomic Bose-Einstein-Condensate (BEC) is a very dilute system, the most fascinating experimental results arise from the 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. Due to its long-range character and its anisotropic nature the dipole-dipole interaction in a BEC new interesting phenomena like novel quantum phase transitions, two dimensional dipolar solitons and a new kind of roton excitation have been predicted. New questions concerning stability and shape of such a condensate arise. We work with chromium atoms which have an unusually large magnetic dipole moment and were able to prepare the first dipolar gas and study its dynamics and its stability.more |
|
 | Strongly interacting Rydberg gasesRydberg atoms are highly excited atoms with one valence electron of principal quantum number n >>1. Because of their huge size of the order of n2a0 they are very susceptible to external electric fields and to distant other Rydberg atoms. Due to the very strong long range interactions Rydberg atoms provide the opportunity to study novel strongly correlated states of matter. A quantum critical point was predicted for this system. After the first Rydberg excitation of a Bose-Einstein condensate we were able to study universal scaling laws around this critical point. Moreover we were able to coherently control mesoscopic ensembles sharing one Rydberg excitation in so called “super atoms”. In this context we were also able to excite Rydberg atoms in thermal vapor cells with a thickness of 1 micrometer, which is smaller than the size of a “super atom”.more |
|
 | Giant Rydberg MoleculesIn 1934, Enrico Fermi introduced a pseudopotential to describe the scattering between an electron and a ground state atom at low kinetic energies. In the year 2000, C. Greene recognized that this scattering process could lead to weakly bound states between a Rydberg atom and a ground state atom and therefore to a novel type of binding mechanism. In this model, the ground state atom is bound by the polarization potential of the Rydberg electron. In 2009 we have discovered those giant molecules and studied their properties. Besides dimers we have also found trimers and yet another novel binding mechanism. Coherent control of those molecules is possible and will lead to new phenomena ultracold chemistry.more |