Quantum Optics Seminar: Han Bao, University of Freiburg, Germany

Magnetic Feshbach resonances offer a powerful tool for controlling interparticle interactions, but their realization in atom‑ion hybrid systems has remained challenging due to the difficulty of reaching the ultracold s‑wave regime. Here we will discuss about the experimental progress at our ¹³⁸Ba⁺+⁶Li system at University of Freiburg. The first observation of Feshbach resonance in our system [1] shows an unexpectedly high number of resonances, pointing to strong second‑order SOC as the key mechanism. Subsequent experiments achieved precise collision‑energy control, enabling the identification of s‑wave and higher‑partial‑wave resonances [2]. Recent result shows that the two lowest hyperfine‑Zeeman states of Li—despite having nearly identical electron spin character—produce dramatically different resonance densities [3]. State‑of‑the‑art coupled‑channel calculations including SOC reproduce the dense spectrum for one state but fail for the other, suggesting physics beyond standard two‑body scattering. Together, these studies establish atom‑ion Feshbach resonances as a tunable quantum platform and pose an open puzzle about initial‑state dependence.

 

[1] Nature 600, 429 (2021).

[2] Phys. Rev. X 15, 011051 (2025).

[3] arXiv:2507.12936 (2026).