Superradiance of an ensemble of nuclei excited by a free electron laser

In 1954 Dicke predicted the accelerated initial decay of multiple atomic excitations1, laying the foundation for the concept of superradiance. Further studies(2-4) 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(4-12), yet the acceleration of the initial decay has not been experimentally demonstrated. Here we use resonant diffraction of X-rays from the Mossbauer transition(13) of Fe-57 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 theory(1) 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.