Quantum Optics seminar by Gabriel Araneda, University of Innsbruck, Austria
Atomic ions confined in Paul traps present features that allow the study of quantum emitters with a unique degree of control. These features include the possibility of working with a well-defined and conserved number of atomic emitters, each of them with sub-wavelength localization, and with high degree of control of their internal and external quantum states, including the on-demand creation of entanglement.
In this talk I will present two experiments using Barium ions, where we take advantage of these features to study some properties of the emission of single atoms. In the first experiment, we entangle two effectively-separated atoms and couple their emission to a single optical mode . By varying the relative distance of the atoms, we observe single photon interference in the selected optical mode. The visibility of the observed fringes, as theoretically predicted, corresponds directly to the amount of entanglement. Since the created entangled state is largely sensitive to differences in the local magnetic field at the position of each atom, we use this “single-mode superradiance” to precisely measure the field gradient.
In a second experiment, we study the spin-orbit coupling of single photons spontaneously emitted by a single atom . The presence of orbital angular momentum is evidenced by a slight tilting of the average direction of the emission, which depends on the change of angular momentum experienced by the atom during the emission. The images formed by photons emitted by dipole atomic transitions with opposite change of angular momentum are shifted 158(4) nm with respect to each other, in good agreement with the expected theoretical value = 157.1 nm. This spin-orbit induced “localization error” appears in optical imaging techniques, but also in localization of objects using any type of wave that carries orbital angular momentum relative to the emitters position with a component orthogonal to the direction of observation, including radar and sonar waves, as well as for the future localization of stellar objects using gravitations waves.
Finally, I will discuss our progress toward the realization of the paradigmatic situation of positioning an atom at the center of hemispherical mirror. In this situation, due to QED effects, we expect to achieve almost complete inhibition of the spontaneous emission of a single atom [3,4].
 “Interference of single photons emitted by entangled atoms in free space”, G. Araneda, D.B. Higginbottom, L. Slodicka, Y. Colombe and R. Blatt, Physical Review Letters 120, 193603 (2018)
 “Wavelength-scale errors in optical localization due to spin-orbit coupling of light”, G. Araneda, S. Walser, Y. Colombe, D. B. Higginbottom, J. Volz, R. Blatt and A. Rauschenbeutel,Nature Physics 15, 17 (2019)
 “QED with a spherical mirror”, G. Hetet, L. Slodicka, A. Glatzle, M. Hennrich and R. Blatt, Physical Review A 470, 06381 (2010)
 “Fabrication of precision hemispherical mirrors for quantum optics applications”, D. Higginbottom, G. Campbell, G. Araneda, F. Fang, Y. Colombe, B. C. Buchler, P. K. Lam, Scientific Reports 8, 221 (2018)