Tunable Subluminal Propagation of Narrow-band X-Ray Pulses

Abstract

Group velocity control is demonstrated for x-ray photons of 14.4 keV energy via a direct measurement of the temporal delay imposed on spectrally narrow x-ray pulses. Sub-luminal light propagation is achieved by inducing a steep positive linear dispersion in the optical response of 57 Fe Mössbauer nuclei embedded in a thin film planar x-ray cavity. The direct detection of the temporal pulse delay is enabled by generating frequency-tunable spectrally narrow x-ray pulses from broadband pulsed synchrotron radiation. Our theoretical model is in good agreement with the experimental data.Strong nonlinear interaction of light with matter is a key requirement for fundamental and applied quantum optical technologies alike. Since conventional materials typically exhibit weak nonlinearities, the ultimate quest for strong nonlinear interactions of individual quanta has led to the development of a number of methods to significantly enhance nonlinear light-matter interactions. Among the most prominent ones are coherently prepared media based on electromagnetically induced transparency, sub-luminal light and related effects [1,2], as well as cavity-enhanced light matter interactions [3].Recently, nuclear quantum optics featuring the interaction of x-ray light with Mössbauer nuclei in the few keV transition energy range has gained considerable momentum, both theoretically [4-9] and experimentally [10][11][12][13][14][15][16][17][18][19]. Interestingly, these experiments operate with less than one resonant x-ray photon per pulse on average due to restrictions in the available x-ray light sources. This raises the question, whether coherent or cavity-based enhancement techniques could be utilized to realize nonlinear light-matter interactions in nuclear quantum optics despite the low number of resonant photons.Here, we report a first step towards this goal, and demonstrate group velocity control of spectrally narrow x-ray pulses (SNXP). Sub-luminal light propagation is achieved by inducing a steep positive linear material dispersion, and verified by direct measurements of the temporal delay imposed on the SNXP. For this, we suitably manipulate the optical response of the ω 0 = 14.4 keV Mössbauer resonance (single nucleus linewidth γ = 4.7 neV) of a large ensemble of 57 Fe nuclei embedded in a thin film planar x-ray cavity. Our approach thereby combines coherent control, as well as cooperative and cavity enhancements of light-matter interaction in a single setup. To enable the direct detection of the temporal pulse delay, we further propose and implement a flexible scheme to generate frequency-tunable SNXP from broadband synchrotron radiation for applications in xray quantum optics. Our theoretical model is in good agreement with the experimental data.Sub-luminal light was first demonstrated in the visible frequency range [20][21][22], and by now has been implemented in a number of platforms [2], particularly also in cavity settings [23,24]. Manipulation of light propagation has also been reported in the x-ray regime. In Ref.[12], a...

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