5th Institute of Physics

Cold Molecules

Laser cooling of diatomic molecules

Abstract | Research | The Crew | Open Positions | Bachelor- and Master-Topics


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.


Abstract | Research | The Crew | Open Positions | Bachelor- and Master-Topics


Laser cooling is a very well established technique for atoms, which is used extensively at our Institute and other labs around the world. It relies on the fact that atoms can repeatedly absorb and emit a large number of photons with the same wavelength. Even if the momentum of a single photon is very small, the scattering of many photons enables substantial slowing, and hence cooling, of the atoms.

Applications of cold molecules

Intuitively, it seems very challenging to apply this technique to molecules because of their complex level structure, which e.g. includes many rotational and vibrational levels. As a consequence, already after a few cycles of absorption and emission there is a very high probability for the molecules to fall into a state where they cannot scatter any more photons. We address this challenge by “engineering” suitable transitions in molecules that can scatter enough photons. For example, we carefully choose molecules, where all relevant vibrational states can be addressed with a small number of wavelengths. For rotational states we use selection rules and radio-frequency dressing to prevent decay of the molecules into unwanted levels. With these tools at hand, molecules can be brought all the way to microkelvin temperatures - despite being much more complex than atoms!

In fact, it is exactly this increase in complexity that makes cold molecules extremely interesting tools to study quantum mechanics. For example, many molecules feature strong and tunable dipole moments, which can be used to realize new forms of quantum matter (see here for our other work on dipolar gases).  Cold molecular collisions can give new insights into the foundations of chemistry. Finally, certain molecular level configurations can be used to search for new physics beyond the Standard Model of particle physics, using only a small tabletop experiment instead of a full-scale particle accelerator. 

Abstract | Research | The Crew | Open Positions | Bachelor- and Master-Topics

The Crew

Fabian Alten(Bachelor Student)
Tim Langen(Group leader)
Tobias Sixt(Hiwi)

Abstract | Research | The Crew | Open Positions | Bachelor- and Master-Topics

Open Positions

We are always seeking enthusiastic candidates to join our team. This includes

  • Hiwis (see here for details)
  • Bachelor and Master students (see here for details)
  • PhD students (see here for details)
  • Post Docs

You will be able to learn about and apply a wide range of experimental techniques ranging from optics, lasers and electronics to programming, cryogenics and vacuum technology. If you are interested in realizing a new cutting-edge experiment at the forefront of quantum physics, please contact  Tim Langen.

Abstract | Research | The Crew | Open Positions | Bachelor- and Master-Topics

Bachelor- and Master-Topics

We are setting up a new experiment to directly laser cool and study diatomic molecules (see here for details). Current opportunities for Bachelor and Master students include:

  • Master's thesis: Setup of a buffer gas source for cold molecules: The goal of this project will be the production of a cold beam of dipolar molecules using laser ablation in a cryogenic cell. In the cell, collisions with a cold Helium buffer gas will thermalize the molecules to ∼4 K. The molecular beam, which will be formed using an exit aperture in the cell, will provide very good starting conditions for subsequent laser cooling.
  • Bachelor's thesis: Design of a diode laser system for the cooling of diatomic molecules: The main transitions of our molecules are found in the near-infrared part of the electromagnetic spectrum, where laser diodes with ample power are available. The goal of this project will be to set up a diode laser system, stabilize it to the appropriate wavelengths and detect the first molecules.
There are also always opportunities for HiWi positions in our lab (see here for details)!

If you are interested in learning more, please contact  Tim Langen.


Abstract | Research | The Crew | Open Positions | Bachelor- and Master-Topics