Abstract: I will present four experiments which show striking properties of structured beams when interacting with a single well-localized atom. I will first show what happens when you place a single atom at the dark center of a vortex/doughnut beam. Surprisingly, they can be excited. Even more, I will show how the chirality of the beam can be transferred to the atom's internal and external motion. This is seen as changes in the the allowed atomic transitions which reveal that both the intrinsic (polarization) and the extrinsic (orbital/structural) angular momentum of the beam can imparted on the atom. In particular we show that two units of angular momentum can be transferred from the beam to the atom and that structured beams can excite motion transversal to the beam's propagation direction. Finally I will show a position-resolved measurement of the azimuthal Doppler shift of a structured beam. These results reveal it’s characteristic divergence at the center and it’s scale-invariant nature. They provide a window to the understanding, and probably constitute the first indirect observation, of the elusive super-kicks predicted by Barnett and Berry in 2013.
The talk will globally review the results of the following four papers:
- "Transfer of optical orbital angular momentum to a bound electron.” Nature Communications 7,12998 (2016).
- "Twisted-light–ion interaction: the role of longitudinal fields." Physical Review Letters 119.25 (2017): 253203.
- "Coherent transfer of the transverse momentum of an optical vortex beam to the motion of a single trapped ion." Physical Review Letters 129.26 (2022): 263603.
- “Observation of Space-Dependent Rotational Doppler Shifts with a Single Ion Probe”, Phys. Rev. Lett. 133, 183601 (2024)

Bio: Christian Schmiegelow is director of the Cold Ion and Atoms Laboratory at the University of Buenos Aires and CONICET, Argentina.  He specializes in atom-light interactions, including structured light beams and standing waves. His current research focuses on quantum information, quantum simulations, and quantum thermodynamics.  He has worked on experimental quantum physics with single photons during his PhD at the University of Buenos Aires and CITEDEF, Argentina and with trapped ions at the Schmidt-Kaler Group at the University of Mainz, Germany. He has received the ICO/ICTP Gallieno Denardo Award for significant contributions in Optics and Photonics as well as the National Academy of Sciences (Argentina) Enrique Gaviola Award for young researchers in physics.