E4: Cold atomic mixtures & collective Modes of Spinor Gases

Lab room: B123 Birge
Lab phone: (510) 642-7269

Our current efforts focus on the low energy scattering properties of cold atomic mixtures, lithium-7 and rubidium-87, which will guide us to produce ultracold mixtures of both species with the ultimate goal of producing LiRb molecules. Our experiments involve a co-existing Rb-87 molasses and Li-7 gray molasses at a temperature of 40 micro-Kelvin after a dual-species magneto-optical trapping stage, a magnetically co-trapped mixture gases at a collisional energy of hundreds of micro-Kelvin where we experimentally measure the inter-species scatter cross section in this spherical quadruple trap, and an optically co-trapped mixture gases at a collisional energy of tens of micro-Kelvin where we aim to study the spin-dependent interaction between the two. These study will contribute to our understanding of cold collisions, where only few partial waves are involved, leading to a clean comparison between experiment and theory, and our understanding of Li-7 and Rb-87 molecular potential energy curve(PEC), which is crucial for our intended later study of LiRb molecule.

Our previous study involves collective excitations in Rb-87 spinor BEC. This ferromagnetic condensate breaks a total of three symmetries at low temperature. Those broken symmetries lead to two Nambu-Goldstone bosons, phonons with linear dispersion relation, and magnons with quadratic dispersion relation. Specifically, we focus on the latter. In a series of experiments, we create magnons, measure and confirm its quadratic dispersion relation through interferometry, use it as a tool to cool and measure an extremely-low entropy gas, and finally, condense it in a box-like potential.

Here are a few examples of our study. For more details, see our Research Highlights.

Low energy scattering in cold atomic mixture

We load Li-7 and Rb-87 sequentially through a dual species Zeeman slower to form a dual species magneto optical trap. Then Rb-87 goes through a polarization gradient cooling stage. Meanwhile, due to the unresolved hyperfine structure in Li-7 D2 line, we implement a gray molasses stage for Li-7. The the two is passes either into a spherical quadruple trap or a optical dipole trap.

In spherical quadruple trap, we do measurements of equilibration rates for Li-7 in Rb-87 reservoir undergoing cross-dimensional relaxation. Thanks to the high mass imbalance between Li and Rb, as well as the non-ergodicity nature of spherical quadruple trap, we are able to measure the small inter-species cross section at a collisional energy of hundreds of micro-Kelvin. In optical dipole trap, we aim to study the spin-dependent interaction in the cold mixture trapped in this spin-independent optical dipole trap at a collisional energy of tens of micro-Kelvin.

Imaging magnons and phonons

In order to study collective excitations, it helps to be able to image them. We have developed several novel methods of imaging the gas magnetization and density with high signal to noise and a variable level of non-destructivity. These methods allow us to image phonons, as in our work on phonon interferometry (right), as well as magnons and interesting spin textures (below).

Magnon interferometry

Precisely measuring the frequency of a collective excitation allows us to test the underlying physics of a many-body system. Here, we study magnons, collective spin waves, in a spin-1 ferromagnetic superfluid. We create collective excitations of different momenta, which appear as a spin density wave. The contrast of the standing wave oscillates in time as the momentum components evolve, giving a direct interferometric readout of the dispersion relation.

Probing the magnetic dipole interaction with nearly-free magnons

In three dimensions, a spin wave is gapless by Goldstone's Theorem: a global rotation of the spin costs no energy, so long wavelength excitations have a vanishing energy gap. In our two-dimensional system, dipolar interactions introduce a preferred direction and break this symmetry. In the absence of this symmetry breaking, the dispersion of the magnons is unaffected by variations in condensate density. However, in the presence of such symmetry breaking, the shift in spin precession frequency with density and spin direction directly yields the gap.

Magnons as a probe of the superfluid phase

The magnons have the phase of the condensate from which they are created. As counterpropagating magnons flow, the interference pattern shows superfluid phase. Here, we track the motion of a single vortex over four seconds by repeatedly creating and imaging magnons in the same condensate.

Using magnons to measure the lowest entropy atomic gas ever

Measuring temperature can be very challenging using commom methods such as time-of-flight at very low temperatures compared to the condensation temperature. The non-condensed fraction, which carries the entropy of the degenerate gas, can be vanishingly small. However, incoherent magnons within the gas can still have a large non-condensed fraction, and readily reveal the temperature of the gas in their momentum distribution.

Magnons give us the ability to accurately measure temperatures in a previously unexplored regime of low entropy, and have revealed that evaporative cooling, a staple of cold atom physics, can be used to create gases that are far colder than previously imagined, as low as 0.02 times the critical temperature in our experiments.

Magnons for cooling in a deep trap

Magnons enable two types of cooling that work without lowering the trap depth, which might be constrained, for example, because it also controls the tunneling rate in a lattice: cycled-decoherence cooling/demagnetization cooling and magnon-assisted evaporation. In magnon-assisted evaporation, magnons increase cooling power of evaporative cooling by increasing the non-condensed phase space density. In cycled-decoherence cooling/demagnetization cooling, magnons created in a degenerate gas absorb energy from the majority-spin gas as they thermalize. After thermalization, the magnons can be expelled and the process repeated.