X-ray cavity quantum optics with inner-shell transitions

Abstract

In the last decade, X-ray quantum optics has emerged as a new research field, driven by significant advancements in X-ray sources such as new generation synchrotron radiations and X-ray free electron lasers, as well as improvements in X-ray methodologies 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 X-ray cavity quantum optical studies are based on the Mössbauer nuclear resonances. However, the scope of the applications is limited by the few available nuclear isotope candidates and the lack of general 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 works suggest that the X-ray cavity quantum optics with inner-shell transitions will become a new promising platform. Actually, the core-hole state is the fundamental concept in a variety of modern X-ray spectroscopic techniques. Therefore, integrating X-ray quantum optics with X-ray spectroscopies could lead to potential applications in core-level spectroscopies communities.<br>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, necessitating synchrotron radiations. The grazing reflectivity and fluorescence measurements are shown in Fig.1, along with a brief introduction to resonant inelastic X-ray scattering. 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. We discuss the similarities and characteristics of the electronic inner-shell transition compared to the nuclear resonance. Based on the observables, such as reflectivity and fluorescence spectra, we introduce several recent works, including cavity-induced energy shift, Fano interference, and core-hole lifetime control. In conclusion, we summarize the review and discuss several future directions. In particular, designing new cavity structures is essential to addressing 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 area that could lead to valuable applications. Furthermore, X-ray free-electron lasers, which offer much higher pulse intensity and much shorter pulse duration, will advance X-ray cavity quantum optics studies from linear to multiphoton and nonlinear regimes.