In 1954 Dicke predicted the accelerated initial decay of multiple atomic excitations¹, laying the foundation for the concept of superradiance. Further studies^2,3,4 suggested that emission of the total energy was similarly accelerated, provided that the system reaches the inversion threshold. Superradiant emission of the total energy has been confirmed by numerous studies^{4,5,6,7,8,9,10,11,12}, yet the acceleration of the initial decay has not been experimentally demonstrated. Here we use resonant diffraction of X-rays from the Mössbauer transition¹³ of ⁵⁷Fe nuclei to investigate superradiant decay, photon by photon, along the entire chain of the de-excitation cascade of up to 68 simultaneous coherent nuclear excitations created by a pulse of an X-ray free-electron laser. We find agreement with Dicke’s theory¹ for the accelerated initial decay as the number of excitations is increased. We also find that our results are in agreement with a simple statistical model, providing a necessary baseline for discussing further properties of superradiance, within and beyond the low-excitation regime.

1954年，迪克（Dicke）预测了多个原子激发¹的加速初始衰减，为超辐射概念奠定了基础。进一步的研究^2、3、4提出，只要系统达到反演阈值，总能量的排放也会类似地加速。大量研究^{4,5,6,7,8,9,10,11,12}证实了总能量的超辐射发射，但尚未通过实验证明加速初始衰减。在这里，我们使用Mössbauer跃迁中的X射线的共振衍射⁵⁷ of ¹³Fe核研究沿着X射线自由电子激光脉冲产生的多达68个同时发生的相干核激发的去激激级联的整个链条上的光子逐个光子的超辐射衰变。我们发现，与迪克理论¹一致的是，随着激励次数的增加，初始衰减加快。我们还发现我们的结果与简单的统计模型相吻合，为在低激发范围内外讨论超辐射的其他特性提供了必要的基线。