By embedding a thin layer of tantalum in an x-ray cavity, we observe a change in the spectral characteristics of an inner-shell transition of the metal. The interaction between the cavity mode vacuum and the L III-edge transition is enhanced, permitting the observation of the collective Lamb shift, superradiance, and a Fano-like cavity-resonance interference effect. This experiment demonstrates the feasibility of cavity quantum electrodynamics with electronic resonances in the x-ray range with applications to manipulating and probing the electronic structure of condensed matter with high-resolution x-ray spectroscopy in an x-ray cavity setting.
Ferromagnetic resonance of a thin film alloy has been tuned by inducing lateral interfaces between layers differing in their lattice ordering and magnetic properties. By disordering B2 Fe60Al40 thin films to the A2 structure, thereby manifesting planar A2/B2 interfaces at selected depths, we show that the resonance lines at 10 GHz are shifted by 284 mT and 35 mT for fields applied perpendicular-to-plane and in-plane, respectively. The resonance line shift occurs over a broad frequency range and is driven by strain relaxation due to the increasing magnetic layer thickness. A finer anomalous line shift occurs as the A2/B2 interface approaches the film/substrate interface prior to being expelled from the film. The A2 structure can be reannealed to the B2 order, implying that disorder/order interface modification can provide a path for reversibly encoding resonant properties in alloy thin films.
Ultrafast and precise control of quantum systems at x-ray energies involves photons with oscillation periods below 1 as. Coherent dynamic control of quantum systems at these energies is one of the major challenges in hard x-ray quantum optics. Here, we demonstrate that the phase of a quantum system embedded in a solid can be coherently controlled via a quasi-particle with subattosecond accuracy. In particular, we tune the quantum phase of a collectively excited nuclear state via transient magnons with a precision of 1 zs and a timing stability below 50 ys. These small temporal shifts are monitored interferometrically via quantum beats between different hyperfine-split levels. The experiment demonstrates zeptosecond interferometry and shows that transient quasi-particles enable accurate control of quantum systems embedded in condensed matter environments.
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