Quantum Optics Group (LMU) - Quantum Many Body Systems Division (MPQ)

Polar molecules - A Quantum Gas with Long-range Interactions

Dipoles on a 2D layer

 Long-range interactions in ultracold quantum gases offer new and fascinating possibilities for investigating quantum matter. While in many quantum gases, the many-body physics is dominated by contact interaction, dipole-dipole interactions offer a path to systems governed by long-range interactions. Theoretical investigations of this subject have yielded predictions of unique effects, which can only be seen in quantum gases with dipolar behaviour. For example, it is believed that long-range spin-spin interactions play a key role in the magnetic behaviour of electronic materials which may be a key to the understanding of iron pnictide-type superconductors (Bohn et al., Science 357, 2017). In our experiment, we use an ultracold gas of 23Na40K molecules - the dipolar interaction between these molecules is 100 times larger than what can be reached with atoms, which makes them an excellent candidate for the investigation of dipolar quantum gases.





Dual-Species Quantum Gas

MOT chamber with electrodes
Sodium MOT


Since direct cooling of molecules to quantum degeneracy is not yet possible, we first produce a mixture of sodium and potassium atoms in two-species magneto optical trap. To achieve ultracold temperatures in both atomic species, we perform evaporative cooling first in a magnetic and then in an optical dipole trap. We can achieve Bose-Einstein condensation of Sodium with up to 105 atoms. The same number of fermionic Potassium atoms are sympathetically cooled by the sodium cloud to near quantum degeneracy. For making molecules however, we stop cooling slightly above the temperature required for quantum degeneracy.




Molecules in the Ground State

487nm Laser system
Dye laser producing 652nm light



We apply a radiofrequency pulse to the mixture, causing association of molecules in a weakly bound metastable state. These are transferred to the electronic, vibrational and rotational ground state via stimulated Raman adiabatic passage, using a pair of lasers at 487 nm and 652 nm. To ensure mutual coherence of these lasers during the transfer time, both are stabilized to relative linewidths of 10-12 or lower. Only afterwards are the molecules long-lived and can be polarized.



Stimulated Raman Adiabatic Passage
Laser light for cooling and imaging of Sodium

Current Research


In the publication "Modelling the adiabatic creation of ultracold, polar 23Na40K molecules", we demonstrated the production of 5000 molecules in the ground state and developed a theoretical model of the adiabatic transfer. See journals.aps.org/pra/abstract/10.1103/PhysRevA.97.013405.

In our latest paper, "Extending Rotational Coherence of Interacting Polar Molecules in a Spin-Decoupled Magic Trap", we were able to observe, for the first time, the effect of dipolar interaction on a bulk gas of molecules. This became possible with a combination of extremely stable electric fields and utilising magic trapping conditions to extend the coherence between rotational states. See: journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.253401.


Currently, we are working on the association of molecules in a 3D lattice, which will hopefully increase both the association efficiency and molecule lifetime.

This is only the beginning of polar molecule research. If you would like to join this endeavor, let us know! We are always looking for talented new team members.




People (alphabetical)

Former Members

Diana Amaro

Dr. Nikolaus Buchheim

Peter Budweiser

Nathan Evetts

Matthias Gempel

Dr. Christoph Gohle

Ingo Laut

Claus Lindner

Junqiu Liu

Dr. Zhenkai Lu

Dr. Tobias Schneider

Yannick Seis

Andréas Tresmontant

Mohamed Zaghoo