01444nas a2200169 4500008004100000245007500041210006900116260001500185490000600200520093200206100001701138700002201155700001901177700001901196700002201215856003701237 2018 eng d00aObservation of bound state self-interaction in a nano-eV atom collider0 aObservation of bound state selfinteraction in a nanoeV atom coll c2018/11/200 v93 a

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.

1 aThomas, Ryan1 aChilcott, Matthew1 aTiesinga, Eite1 aDeb, Amita, B.1 aKjærgaard, Niels uhttps://arxiv.org/abs/1807.0117401813nas a2200169 4500008004100000245010100041210006900142260001500211490000600226520127000232100002701502700001701529700001901546700001901565700002201584856003701606 2017 eng d00aAbove threshold scattering about a Feshbach resonance for ultracold atoms in an optical collider0 aAbove threshold scattering about a Feshbach resonance for ultrac c2017/09/060 v83 a

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.

1 aHorvath, Milena, S. J.1 aThomas, Ryan1 aTiesinga, Eite1 aDeb, Amita, B.1 aKjærgaard, Niels uhttps://arxiv.org/abs/1704.0710915016nas a2200181 45000080041000002450084000412100069001252600015001943000011002094900007002205201446000227100002314687700002714710700001914737700001914756700002214775856003714797 2017 eng d00aDispersive optical detection of magnetic Feshbach resonances in ultracold gases0 aDispersive optical detection of magnetic Feshbach resonances in  c2017/08/18 a0227050 v963 a

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.

1 aSawyer, Bianca, J.1 aHorvath, Milena, S. J.1 aTiesinga, Eite1 aDeb, Amita, B.1 aKjærgaard, Niels uhttps://arxiv.org/abs/1702.0221609134nas a2200205 4500008004100000245007700041210006900118260001500187300001000202490000600212520852300218100001708741700002208758700001908780700002308799700001808822700001908840700002208859856004708881 2016 eng d00aMultiple scattering dynamics of fermions at an isolated p-wave resonance0 aMultiple scattering dynamics of fermions at an isolated pwave re c2016/07/11 a120690 v73 a

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 90 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.

1 aThomas, Ryan1 aRoberts, Kris, O.1 aTiesinga, Eite1 aWade, Andrew, C.J.1 aBlakie, Blair1 aDeb, Amita, B.1 aKjærgaard, Niels uhttp://www.nature.com/articles/ncomms12069