Ultracold Physics with Diatomic Molecules

Introduction

In the laser cooling lab, we seek to study ultracold molecules by first loading molecules into a magneto-optical trap (MOT) and then transferring these molecules to a magnetic trap for further cooling. Interesting later experiments could involve studying atom-molecule or molecule-molecule collisions, as well as using the ultracold sample of diatomic molecules for quantum simulation or precision measurement experiments.

CaF

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Research Overview

The goal of this experiment is to load a magneto-optical trap (MOT) with the diatomic radical calcium monofluoride (CaF) using a two-stage buffer-gas beam source (for details on buffer-gas cells see [1-3]). We first ablate a solid precursor of atomic Ca with a pulsed Nd:YAG laser. We simultaneously flow sulfur hexafluoride (SF6) into the buffer-gas cell, leading to a chemical reaction which produces CaF. The hot molecular gas then thermalizes with ~1 K Helium buffer-gas and is extracted into a beam. The molecular beam has an average forward velocity of 50-60 m/s out of our two-stage cell. While such velocities are low enough to load conventional atomic MOTs (see our previous work on lanthanide atoms), the estimated capture velocity for a MOT of CaF is less than 10 m/s. A slowing stage is thus required to bring a sufficient number of molecules to below the capture velocity. We use a white-light slowing technique for this beam deceleration, as was demonstrated in our recent paper [14]. An additional challenge to trapping molecules is the existence of magnetic dark states in molecules, which arise due to the fact that we trap the molecules on a transition with "inverted" angular momentum structure. We address this problem by switching the polarization and the magnetic field of the MOT very rapidly (~1 MHz) to depopulate those dark states.

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References

  1. N. R. Hutzler, H.-I Lu, and J. M. Doyle, The Buffer Gas Beam: An Intense, Cold, and Slow Source for Atoms and MoleculesChem. Rev., 112, 4803 (2012)
  2. H.-I Lu, J. Rasmussen, M. J. Wright, D. Patterson, and J. M. Doyle, A cold and slow molecular beamPhys. Chem. Chem. Phys. 13, 18986 (2011)
  3. D. Patterson and J. M. Doyle, Bright, Guided, molecular beam with hydrodynamic enhancementJ. Chem. Phys. 126, 154307 (2007)
  4. E. S. Shuman, J. F. Barry, D. R. Glenn, and D. DeMille, Radiative Force from Optical Cycling on a Diatomic MoleculePhys. Rev. Lett. 103, 223001 (2009)
  5. E. S. Shuman, J. F. Barry, and D. DeMille, Laser cooling of a diatomic moleculeNature 467, 820 (2010)
  6. M. T. Hummon, M. Yeo, B. K. Stuhl, A. L. Collopy, Y. Xia, and J. Ye, 2D Magneto-Optical Trapping of Diatomic MoleculesPhys. Rev. Lett. 110, 143001 (2013)
  7. M. Harvey, A. J. Murray, Cold Atom Trap with Zero Residual Magnetic Field: The ac Magneto-Optical Trap Phys. Rev. Lett. 101, 173201 (2008)
  8. V. Zhelyazkova, A. Cournol, T. E. Wall, A. Matsushima, J. J. Hudson, E. A. Hinds, M. R. Tarbutt, and B. E. Sauer, Laser cooling and slowing of CaF molecules Phys. Rev. A. 89, 053416 (2014)
  9. B. Hemmerling, G. K. Drayna, E. Chae, A. Ravi, J. M. Doyle, Buffer gas loaded magneto-optical traps for Yb, Tm, Er and HoNew J. Phys. 16, 063070 (2014)
  10. J. F. Barry, D. J. McCarron, E. B. Norrgard, M. H. Steinecker, and D. DeMille, Magneto-optical trapping of a diatomic moleculeNature 512, 286 (2014)
  11. D. J. McCarron, E. B. Norrgard, M. H. Steinecker, D. DeMille, Improved magneto-optical trapping of a diatomic moleculeNew J. Phys. 17, 035014 (2015)
  12. M. Yeo, M. T. Hummon, A. L. Collopy, B. Yan, B. Hemmerling, E. Chae, J. M. Doyle, J. Ye, Rotational state microwave mixing for laser cooling of complex diatomic moleculesPhys. Rev. Lett. 114, 223003 (2015)
  13. E. B. Norrgard, D. J. McCarron, M. H. Steinecker, M. R. Tarbutt, and D. DeMille, Submillikelvin Dipolar Molecules in a Radio-Frequency Magneto-Optical TrapPhys. Rev. Lett. 116, 063004 (2016)
  14. B. Hemmerling, E. Chae, A. Ravi, L. Anderegg, G. K. Drayna, N. R. Hutzler, A. L. Collopy, J. Ye, W. Ketterle and J. M. Doyle, Laser slowing of CaF molecules to near the capture velocity of a molecular MOTJ. Phys. B 49, 17 (2016)