

Summary
Laser cooling techniques have realized the "coldest places in the
Universe" where atoms and molecules move at speeds of few cm/s and
temperatures below 1 nK. These dilute gases reach a quantum regime
where their de Broglie wavelength is of the order of 1 micron and
exceeds the interparticle distance. Bosonic particles form a
macroscopic matter wave, fermionic atoms experience the pressure
from Pauli repulsion, and quantum chemistry forms ultracold molecules.
Projects
A cloud of ultracold Bosons in a trap provides a mechanical oscillator
with weak damping. This has been used to measure small forces arising
from the van der WaalsCasimirPolder interaction between atoms and a
macroscopic surface.
We have formulated a quantum field theory of these longrange electromagnetic
interactions that provides a microscopic picture where manybody corrections
and the impact of the trap confinement can be studied.

Ph.D. thesis of J. Schiefele: "CasimirPolder interaction in second quantization"
In experiments, an ultracold (condensed) phase often coexists with thermally
excited particles. In particular in lowdimensional systems, thermal fluctuations
are significant and can broaden the BoseEinstein phase transition into a
crossover. We are exploring the limits of meanfield techniques to handle
this situation and compare numerical simulation schemes.
Participants and cooperations
Papers

S. P. Cockburn, A. Negretti, N. P. Proukakis, and C. Henkel
Comparison between microscopic methods for finite temperature Bose gases
Phys. Rev. A 83 (2011) 043619

J. Schiefele and C. Henkel
Bosonic enhancement of spontaneous emission near an interface
Phys. Lett. A 375 (2011) 680

J. Schiefele and C. Henkel
A BoseEinstein condensate near a surface  quantum field theory of the CasimirPolder interaction
Phys. Rev. A 82 (2010) 023605

J. Schiefele and C. Henkel
Casimir energy of a BEC: from moderate interactions to the ideal gas
J. Phys. A 42 (2009) 045401

A. Negretti, C. Henkel, and K. Mølmer
Quantumlimited position measurements of a dark matterwave soliton
Phys. Rev. A 77 (2008) 043606

F. Illuminati and A. P. Albus
Hightemperature atomic superfluidity in lattice bosonfermion mixtures
Phys. Rev. Lett. 93 (2004) 090406

A. Albus, F. Illuminati, and J. Eisert
Mixtures of Bosonic and Fermionic Atoms in Optical Lattices
Phys. Rev. A 68 (2003) 023606.

A. Albus, F. Illuminati, and M. Wilkens
Groundstate properties of trapped BoseFermi mixtures: role of exchangecorrelation
Phys. Rev. A 67 (2003) 063606.

A. Albus, S. Giorgini, F. Illuminati, and L. Viverit
Critical temperature of BoseEinstein condensation in trapped atomic BoseFermi mixtures
J. Phys. 35 L511 (2002).

A. Albus, S. Gardiner, F. Illuminati, and M. Wilkens
Quantum field theory of dilute homogeneous BoseFermimixtures at zero temperature: general formalism and beyond
meanfield corrections
Phys. Rev. A 65 (2002) 053607.
Financial support
 Deutsche Forschungsgemeinschaft DFG (German Research Council)
Schwerpunktprogramm 1116 (Priority programme) "Interaction in ultracold atomic and molecular gases''
 European Science Foundation ESF
"Casimir" research network
"BEC2000+" programme


