In the last decade, X-ray quantum optics has become a new research field, owing to significant advances in X-ray sources such as new-generation synchrotron radiations and X-ray free electron lasers, as well as improvements in X-ray methods and sample fabrication. A very successful physical platform is the X-ray planar thin-film cavity, also known as the X-ray cavity QED setup, which represents a significant branch of X-ray quantum optics. So far, most of X-ray cavity quantum optical studies are based on the Mössbauer nuclear resonance. However, the application scope is limited by the scarcity of available nuclear isotope candidates and the lack of universal applicability. Recently, X-ray cavity quantum control in atomic inner-shell transitions has been realized in experiments where the cavity effects simultaneously modify the transition energy and the core-hole lifetime. These pioneer researches indicate that the X-ray cavity quantum optics with inner-shell transitions will become a new and promising platform. In fact, 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 may have potential applications in the field of core-level spectroscopy.
In this review, we introduce the experimental systems for the X-ray cavity quantum optics with inner-shell transitions, including the cavity structure, sample fabrications, and experimental methods. We explain that X-ray thin-film cavity samples require high flux, high energy resolution, small beam divergence, and precise angular control, therefore synchrotron radiation is needed. The grazing reflectivity and fluorescence measurements are shown in
Fig. 1
, resonance inelastic X-ray scattering is briefly introduced. We also describe the theoretical simulation tools, including the classical Parratt's algorithm, semi-classical matrix formalism, quantum optical theory based on the Jaynes-Cummings model, and the quantum Green's function method. By comparing with nuclear resonance, we discuss the similarities and characteristics of the electronic inner-shell transition. Based on the observables, such as reflectivity and fluorescence spectra, we introduce several recent researches on cavity-induced energy shift, Fano interference, core-hole lifetime control, and others. Finally, we review and discuss several future directions. Especially, designing new cavity structures is crucial for resolving current debates on the cavity effects with inner-shell transitions and discovering new quantum optical phenomena. Integrating modern X-ray spectroscopies with X-ray cavity quantum optics is a promising research field that may bring valuable applications. Furthermore, X-ray free-electron lasers provide much higher pulse intensity and much shorter pulse duration, which will drive X-ray cavity quantum optics studies from linear to multiphoton and nonlinear regimes.
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