X-ray cavity quantum optics of inner-shell transitions
Authors: Wang Shu-Xing , Li Tian-Jun , Huang Xin-Chao , Zhu Lin-Fan ,   Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China,   I. Physikalisches Institut, Justus-Liebig-Universität Gießen and Helmholtz Forschungsakademie Hessen für FAIR (HFHF), Campus Gießen, GSI Helmholtzzentrum für Schwerionenforschung, Gießen 35392, Germany,   FXE Instrument, European XFEL, Schenefeld 22869, Germany
Published: November 2024

<sec>Over the past decade, X-ray quantum optics has emerged as a dynamic research field, driven by significant advancements in X-ray sources such as next-generation synchrotron radiation facilities and X-ray free-electron lasers, as well as improvements in X-ray methodologies and sample fabrication techniques. One of the most successful platforms in this field is the X-ray planar thin-film cavity, also known as the X-ray cavity QED setup. To date, most studies in X-ray cavity quantum optics have focused on Mössbauer nuclear resonances. However, this approach is constrained by the limited availability of suitable nuclear isotopes and the lack of universal applicability. Recently, experimental realizations of X-ray cavity quantum control in atomic inner-shell transitions have demonstrated that cavity effects can simultaneously modify transition energies and core-hole lifetimes. These pioneering studies suggest that X-ray cavity quantum optics based on inner-shell transitions will become a promising new platform. Notably, the core-hole state is a fundamental concept in various modern X-ray spectroscopic techniques. Therefore, integrating X-ray quantum optics with X-ray spectroscopy holds the potential to open new frontiers in the field of core-level spectroscopy.</sec><sec>In this review, we introduce the experimental systems used in X-ray cavity quantum optics with inner-shell transitions, covering cavity structures, sample fabrications, and experimental methodologies. We explain that X-ray thin-film cavity experiments require high flux, high energy resolution, minimal beam divergence, and precise angular control, necessitating the use of synchrotron radiations. Grazing reflectivity and fluorescence measurements are described in detail, along with a brief introduction to resonant inelastic X-ray scattering techniques. The review also outlines simulation tools, including the classical Parratt algorithm, semi-classical matrix formalism, quantum optical theory based on the Jaynes-Cummings model, and the quantum Green’s function method. We discuss the similarities and unique features of electronic inner-shell transitions and highlight recent advancements, focusing on cavity-induced phenomena such as collective Lamb shift, Fano interference, core-hole lifetime control, etc. Observables such as reflectivity and fluorescence spectra play a central role in these studies. Finally, we review and discuss potential future directions for the field. Designing novel cavities is crucial for addressing current debates regarding cavity effects in inner-shell transitions and uncovering new quantum optical phenomena. Integrating modern X-ray spectroscopies with X-ray cavity quantum optics represents a promising research frontier with significant application potential. Furthermore, X-ray free-electron lasers, with much higher pulse intensity and shorter pulse duration, are expected to propel X-ray cavity quantum optics into the nonlinear and multiphoton regimes, opening new avenues for exploration.</sec>