@article {2206, title = {Observation of bound state self-interaction in a nano-eV atom collider}, journal = {Nature Communications }, volume = {9}, year = {2018}, month = {2018/11/20}, abstract = {

Quantum mechanical scattering resonances for colliding particles occur when a continuum scattering state couples to a discrete bound state between them. The coupling also causes the bound state to interact with itself via the continuum and leads to a shift in the bound state energy, but, lacking knowledge of the bare bound state energy, measuring this self-energy via the resonance position has remained elusive. Here, we report on the direct observation of self-interaction by using a nano-eV atom collider to track the position of a magnetically-tunable Feshbach resonance through a parameter space spanned by energy and magnetic field. Our system of potassium and rubidium atoms displays a strongly non-monotonic resonance trajectory with an exceptionally large self-interaction energy arising from an interplay between the Feshbach bound state and a different, virtual bound state at a fixed energy near threshold.

}, doi = {https://doi.org/10.1038/s41467-018-07375-8}, url = {https://arxiv.org/abs/1807.01174}, author = {Ryan Thomas and Matthew Chilcott and Eite Tiesinga and Amita B. Deb and Niels Kj{\ae}rgaard} } @article {2000, title = {Above threshold scattering about a Feshbach resonance for ultracold atoms in an optical collider}, journal = {Nature Communications}, volume = {8}, year = {2017}, month = {2017/09/06}, abstract = {

Studies of magnetically tunable Feshbach resonances in ultracold atomic gases have predominantly been carried out in the zero collision-energy limit. Here, we explore above threshold collisions at well-defined energies in the vicinity of a narrow magnetic Feshbach resonance by means of a laser-based collider. Our experiment focuses on collisions between ground-state 87Rb atoms in the |F = 2,mF = 0i and |F = 1,mF = 1i hyperfine states, which have a known s-wave resonance at 9.040(7) G at threshold that strongly couples to inelastic channels, where 1 G = 10\−4 T. Using our collider we can track the magnetic field shift in resonance position as the energy is tuned. This presents a challenge due to the narrow width of the resonance in conjunction with inherent broadening mechanisms of the collider. We find, however, that the narrow Feshbach scattering feature becomes imprinted on the spatial distribution of atoms in a fashion that allows for an accurate determination of resonance position as a function of collision energy through a shift in center-of-mass position of the outgoing clouds. This shift has a dispersive line shape with a zero value at the resonance position. We obtain excellent agreement with theory on the resonance position.

}, doi = {10.1038/s41467-017-00458-y}, url = {https://arxiv.org/abs/1704.07109}, author = {Milena S. J. Horvath and Ryan Thomas and Eite Tiesinga and Amita B. Deb and Niels Kj{\ae}rgaard} } @article {1959, title = {Dispersive optical detection of magnetic Feshbach resonances in ultracold gases}, journal = {Physical Review A}, volume = {96}, year = {2017}, month = {2017/08/18}, pages = {022705}, abstract = {

Magnetically tunable Feshbach resonances in ultracold atomic systems are chiefly identified and characterized through time consuming atom loss spectroscopy. We describe an off-resonant dispersive optical probing technique to rapidly locate Feshbach resonances and demonstrate the method by locating four resonances of\ 87Rb, between the\ |F=1,mF=1\⟩\ and\ |F=2,mF=0\⟩\ states. Despite the loss features being\ 100\ mG wide, we require only 21 experimental runs to explore a magnetic field range \>18 G. The resonances consist of two known s-wave features in the vicinity of 9 G and 18 G and two previously unobserved p-wave features near 5 G and 10 G. We further utilize the dispersive approach to directly characterize the two-body loss dynamics for each Feshbach resonance.

}, doi = {10.1103/PhysRevA.96.022705}, url = {https://arxiv.org/abs/1702.02216}, author = {Bianca J. Sawyer and Milena S. J. Horvath and Eite Tiesinga and Amita B. Deb and Niels Kj{\ae}rgaard} } @article {1914, title = {Multiple scattering dynamics of fermions at an isolated p-wave resonance}, journal = {Nature Communications}, volume = {7}, year = {2016}, month = {2016/07/11}, pages = {12069}, abstract = {

The wavefunction for indistinguishable fermions is anti-symmetric under particle exchange, which directly leads to the Pauli exclusion principle, and hence underlies the structure of atoms and the properties of almost all materials. In the dynamics of collisions between two indistinguishable fermions this requirement strictly prohibits scattering into 90 degree angles. Here we experimentally investigate the collisions of ultracold clouds fermionic\ 40K\ atoms by directly measuring scattering distributions. With increasing collision energy we identify the Wigner threshold for p-wave scattering with its tell-tale dumb-bell shape and no\ 90o\ yield. Above this threshold effects of multiple scattering become manifest as deviations from the underlying binary p-wave shape, adding particles either isotropically or axially. A shape resonance for\ 40K\ facilitates the separate observation of these two processes. The isotropically enhanced multiple scattering mode is a generic p-wave threshold phenomenon, while the axially enhanced mode should occur in any colliding particle system with an elastic scattering resonance.

}, doi = {10.1038/ncomms12069}, url = {http://www.nature.com/articles/ncomms12069}, author = {Ryan Thomas and Kris O. Roberts and Eite Tiesinga and Andrew C.J. Wade and P. Blair Blakie and Amita B. Deb and Niels Kj{\ae}rgaard} }