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Tuesday, March 17, 2020 at 12:20pm
Hosted by Carl Frank
Cavity Quantum Electrodynamics in the Hard X-ray Range
The field of cavity quantum electrodynamics (CQED) is a mainstay in research on quantum optics in the visible light and microwave range . When a resonant medium, such as an ensemble of atoms, is inserted into a cavity which has a mode that is energetically close to the atomic resonance, these two oscillators can dramatically influence each other’s spectral properties. A wide range of physical effects thus becomes experimentally accessible, from the enhancement of the rate of spontaneous emission to the formation of superpositions of photons and excited states of matter.
In recent years some of these effects have been observed in the x-ray range [2,3,4]. The cavities in this case are formed by ultrathin multilayers fabricated by sputter deposition. If these multilayers are illuminated at low angles of incidence, total external reflection allows for the formation of standing waves - the cavity modes. When resonant layers are included, quantum optical effects are observed in energy- and time-resolved reflectivity measurements.
I give an overview over several recent experiments in which cavity-induced shifts of the atomic resonance (the so-called collective Lamb shift), as well as strong coupling, i.e. the hybridization of atomic resonances and x-ray photons were observed. These include both atomic-shell resonances in solids, especially the LIII-edge of Tantalum  and Mössbauer [2,3] resonances in nuclei, particularly the famous 14.4 keV transition in the 57Fe isotope. I will also discuss how these results pertain to more practical questions in x-ray spectroscopy, and the way forward for this relatively new sub-field of x-ray optics.
 H. Walther et al. Cavity Quantum Electrodynamics. Reports on Progress in Physics 69 (2006)
 R. Röhlsberger et al. Collective Lamb shift in Single-Photon Superradiance. Science 328 (2010)
 J. Haber et al. Rabi Oscillations of X-ray radiation between two nuclear ensembles. Nat. Phot. 11 (2017)
 J. Haber et al. Spectral Control of an X-ray L-edge transition via a thin-film cavity. Phys. Rev. Lett. 122 (2019)