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 Waals-Casimir-Polder interaction between atoms and a macroscopic surface. We have formulated a quantum field theory of these long-range electromagnetic interactions that provides a microscopic picture where many-body corrections and the impact of the trap confinement can be studied.
• Ph.D. thesis of J. Schiefele: "Casimir-Polder interaction in second quantization"

In experiments, an ultracold (condensed) phase often coexists with thermally excited particles. In particular in low-dimensional systems, thermal fluctuations are significant and can broaden the Bose-Einstein phase transition into a cross-over. We are exploring the limits of mean-field techniques to handle this situation and compare numerical simulation schemes.

We have investigated the phase diagram of mixtures of ultracold bosons and fermions. Due to the Pauli principle, the scattering between fermions "freezes out" in the ultracold regime, but bosons can act as a "cooling agent" or mediate a pairing interaction.
• Ph.D. thesis of A. Albus: "Mixtures of Bosonic and Fermionic atoms"

Participants and cooperations

• Alexander Albus
• Carsten Henkel
• Jürgen Schiefele
• Martin Wilkens
• Jens Eisert (Freie Universität Berlin, Germany)
• Antonio Negretti (Universität Ulm, Germany)
• Stuart P. Cockburn and Nick Proukakis (Newcastle University, United Kingdom)
• Fabrizio Illuminati (INFN & Universita Salerno, Italy)
• Klaus Sengstock and Kai Bongs (Universität Heidelberg, Germany)
• Stefano Giorgini and Luciano Viverit (Universita Trento, Italy)

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 Bose-Einstein condensate near a surface - quantum field theory of the Casimir-Polder 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
  Quantum-limited position measurements of a dark matter-wave soliton
  Phys. Rev. A 77 (2008) 043606
• F. Illuminati and A. P. Albus
  High-temperature atomic superfluidity in lattice boson-fermion 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
  Ground-state properties of trapped Bose-Fermi mixtures: role of exchange-correlation
  Phys. Rev. A 67 (2003) 063606.
• A. Albus, S. Giorgini, F. Illuminati, and L. Viverit
  Critical temperature of Bose-Einstein condensation in trapped atomic Bose-Fermi mixtures
  J. Phys. 35 L511 (2002).
• A. Albus, S. Gardiner, F. Illuminati, and M. Wilkens
  Quantum field theory of dilute homogeneous Bose-Fermi-mixtures at zero temperature: general formalism and beyond mean-field corrections
  Phys. Rev. A 65 (2002) 053607.

Financial support

• Deutsche Forschungsgemeinschaft DFG (German Research Council)
  Schwerpunktprogramm 1116 (Priority programme) "Interaction in ultra-cold atomic and molecular gases''
• European Science Foundation ESF
  "Casimir" research network
  "BEC2000+" programme