Laser Cooling Molecules

Laser Cooling and Magneto-Optical Trapping


Laser cooling scheme for CaF
How do you cool a molecule like calcium fluoride?

Laser cooling is more difficult for molecules than for atoms because molecules have a more complicated energy level structure. In particular, a molecule in a single vibrational level of an electronically excited state can usually decay to many vibrational levels in the ground electronic state. This means that many lasers are needed to ensure that the molecule keeps scattering photons. For some molecules the vibrational state is unlikely to change in the electronic decay, and then laser cooling is feasible using just a few laser frequencies. We are working on laser cooling of CaF and YbF molecules. The figure shows the cooling scheme used for laser cooling and magneto-optical trapping of CaF. Only 5 lasers are needed!

Slowing and cooling a beam of CaF

CaF molecules slowing down

Before we can trap the molecules in a magneto-optical trap, they have to be slowed down. We make a beam of CaF molecules moving at 150m/s using cryogenic buffer gas cooling. The radiation pressure of counter-propagating laser light slows down the molecular beam. The frequency of the light is chirped to follow the changing Doppler shift as the molecules decelerate.

The graph shows the speed distributions that we measure. The black curves are obtained when no laser slowing is applied and the coloured ones when the slowing is applied with various frequency chirps. The radiation pressure slows down the molecules and also bunches up their velocity distribution. The dashed lines are the results of our numerical modelling. Using this method, we are able to slow molecules down to the capture velocity of a magneto-optical trap. The next step is to trap them.


Things we would like to do with our ultracold molecules:A better slowing method?

  • Trap in magnetic, microwave and optical dipole traps
  • Improve molecule number using a better slowing method
  • Study ultracold collisions between atoms and molecules
  • Trap single molecules in optical tweezer traps
  • Make small arrays for quantum simulation
  • Bring molecules into chip-scale traps
  • Search for time-varying fundamental constants 
  • Measure the electron electric dipole moment with a fountain of ultracold YbF


The teamAnne, Valentina, Mike, Jony, Ben, Ed and Aki

Further reading

Molecules cooled below the Doppler limit, S. Truppe, H. J. Williams, M. Hambach, L. Caldwell, N. J. Fitch, E. A. Hinds, B. E. Sauer and M. R. Tarbutt, arXiv:1703.00580 (2017).

An intense, cold, velocity-controlled molecular beam by frequency-chirped laser slowing, S. Truppe, H. J. Williams, N. J. Fitch, T. E. Wall, E. A. Hinds, B. E. Sauer and M. R. Tarbutt, New Journal of Physics, 19, 022001 (2017)

Three-dimensional Doppler, polarization-gradient, and magneto-optical forces for atoms and molecules with dark states, J. A. Devlin & M. R. Tarbutt, New Journal of Physics, 18, 123017 (2016)

Principles and Design of a Zeeman-Sisyphus Decelerator for Molecular Beams, N. J. Fitch & M. R. Tarbutt, Chem. Phys. Chem., 17, 3609 (2016)

Modeling sympathetic cooling of molecules by ultracold atoms, J. Lim, M. D. Frye, J. M. Hutson, and M. R. Tarbutt, Phys. Rev. A 92, 053419 (2015)

Modeling magneto-optical trapping of CaF molecules, M. R. Tarbutt and T. C. Steimle, Phys. Rev. A, 92, 053401 (2015)

Magneto-optical trapping forces for atoms and molecules with complex level structures, M. R. Tarbutt, New J. Phys., 17, 015007 (2015) 

Laser cooling and slowing of CaF molecules, V. Zhelyazkova, A. Cournol, T.E. Wall, A. Matsushima, J.J. Hudson, E.A. Hinds, M.R. Tarbutt, B. E. Sauer, Phys. Rev. A 89, 053416 (2014)

Design for a fountain of YbF molecules to measure the electron’s electric dipole moment, M.R. Tarbutt, B.E. Sauer, J.J. Hudson and E.A. Hinds, New J. Phys.15, 053034 (2013)