Large Anomalous Shifts of Potassium-39 Feshbach Resonances

This study investigates the origin of large, anomalous shifts observed in the positions of potassium-39 (39K) Feshbach resonances when the atoms are confined in an optical dipole trap (ODT). While Feshbach resonances are essential tools for tuning inter-atomic interactions in ultracold gases, their precise positions can be affected by the trapping environment, potentially complicating experimental control and precision measurements.

The researchers performed a systematic study of several 39K Feshbach resonances, including the widely used 33.6 G and 39.9 G resonances, using an all-optical multi-stage cooling setup. They measured the resonance positions across a range of ODT laser powers (and thus trap depths) and temperatures. To explain the observed shifts, they employed a two-body coupled-channel scattering model to calculate the interatomic potentials and Zeeman energy levels of the K2 molecules involved in the resonances.

The study reveals that the 33.6 G and 39.9 G resonances exhibit significant shifts—up to +7.5 G—that scale linearly with the ODT trap depth. These shifts are attributed to a large differential ac Stark shift between the free atom pair and the weakly bound Feshbach molecule. The underlying cause is an unusually high dynamic polarizability of the Feshbach molecule, which is four to seven times larger than the sum of the polarizabilities of the two constituent atoms. This enhancement is identified as a resonance effect: the frequency of the 1063.9 nm trapping laser is near-resonant with a transition from the last vibrational level of the triplet a3Σ+u potential to a vibrational level of the excited b3Σ+g potential. Other resonances studied did not exhibit such large shifts, confirming that this is a specific molecular property rather than a universal feature of the trap.

Understanding these shifts is critical for researchers using 39K in optical traps, as they can significantly alter the expected magnetic field required to achieve a desired scattering length. Failure to account for these trap-induced shifts can lead to errors in experiments involving Bose-Einstein condensates or precision atom interferometry. Conversely, the ability to tune resonance positions via trap intensity offers a potential method for the ultrafast optical control of inter-atomic interactions.