CaF - Ultracold Physics with Diatomic Molecules



Fig 0: CaF Gen1 appratus

In the CaF 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 an optical 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 and quantum computation, or a path finder for precision measurement experiments.

The first step of this experiment is to load a magneto-optical trap (MOT) with the diatomic radical calcium monofluoride (CaF) using a 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  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 (so called RF MOT). Finally, over a million molecules are loaded into the MOT.


Fig 1: CaF Gen2 RF MOT, shot on Google Pixel

We then apply Λ-enhanced gray molasses technique to cool the molecules in the free space to ~4uK, which is far below doppler limit. This sub-doppler cooling mechanism also works in an optical dipole trap. We then turn on a high power 1064nm optical dipole trap beam to overlap with the molecular cloud and keep the cooling light on at the same time, more than 10,000 molecules are loaded into the optical dipole trap.

Optically trapped CaF molecules are then transported into a science glass cell. The transport for laser cooled molecules has several requirements making current available optical transport methods less efficient or infeasible. Room temperature black-body radiation can excite CaF molecules to higher vibrational excited state with opposite parity and limits the lifetime to ~5 seconds. Temperature of the molecules in the optical dipole trap is relatively high compared to neutral atoms and requires a deep trap to ensure a low loss transport. We proposed a hybrid transport scheme based on an optical lattice and a focus tunable optical dipole trap. The optical dipole trap provides a deep trapping potential for efficient loading from the molasses and hold molecules against gravity, while the optical lattice can provide a high acceleration during fast transport. Using this method, we successfully transport CaF molecules over 46cm distance in 50ms with high efficiency.


Fig 2: Fast optical transport setup

The molecules are then loaded into a 1D optical tweezer array projected by the high numerical aperture microscope objective. The loading is achieved by overlapping the optical tweezers and the transported molecular cloud in the 1064nm optical dipole trap, with the cooling provided by the Λ-enhanced gray molasses.

Fig 3: A 60-site 1D optical tweezer array of ultracold CaF molecules

Fig 4: The histrogram of the photon scattered by CaF molecules trapped in optical tweezers

We can then optical pump populations into the N=1, F=0 state, and use microwave transition to transfer the population into other target hyperfine states anywhere within N=1 and N=0 manifolds.

Fig 5: Rabi oscillation of single CaF molecule between N=1 and N=0 rotational states

From here on, sophisticate quantum control can be achieved with complicated microwave pulse sequence. By carefully tuning the tensor ac stark shift originated from the tweezer potential by rotating tweezer polarization to a “magic angle”, we demonstrated record high Ramsey coherence time between rotational states with finite temperature molecules. The future goal will be applying further cooling on the molecules in the tweezer (e.g. Raman sideband cooling) and study dipolar interaction between molecules in closely placed optical tweezers.


Fig 6: CaF Gen2 appratus



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