Persistent Currents in a Molecular BEC of Fermions

We have just shown that we can clearly distinguishing between a ring with a persistent current flowing around it and a stationary superfluid. We relaxing the confinement of the trapping potential in a prescribed way, then releasing the cloud and allow it to expand for several milliseconds. If there is (quantized) circulation around the ring then we see a hole in the middle of the expanded cloud, because of the phase singularity that must be present in the superfluid.

For comparison, if there is no circulation in the superfluid, then the expanded cloud looks like this:

Detecting the vortex signature in this expanding fermionic superfluid is a little more challenging to do well than in previous experiments with atomic Bose condensates (of e.g. rubidium or sodium). The cloud expands quite rapidly even for small numbers of atoms, because the momentum of the atoms is comparatively high due to degeneracy pressure. We’ve got a handle on this, though and it’s clearly not going to be an obstacle to the experiments we have planned.

 

Fermionic Atoms in Ring Lattices

The DMD we’ve been using to create patterned optical potentials can certainly do more than just a simple ring. We’ve just had a first successful attempt at loading ultracold lithium atoms into multi-site ring traps. We probably shouldn’t call them lattices yet since the lattice spacing is still a couple of microns and we don’t expect there to be much tunneling of atoms between the potential wells. Once we have the beam projection system fully optimized and get the lattice spacing down closer to 1 micron, we’ll see about conducting the first experiments with ultracold atoms in ring lattices.

Evaporation in the “Sheet” Trap

With the “Feshbach” magnetic coils now working, we can control the interactions between lithium atoms when they are trapped in the glass “science” cell, and we have observed evaporative cooling of atoms in that trap. The density and temperature already look favorable enough to add in the beam that will shape the ultracold Fermi gas into a ring shape. One big step closer to our experimental goals.