In 1954 Dicke predicted the accelerated initial decay of multiple atomic excitations1, laying the foundation for the concept of superradiance. Further studies2,3,4 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 studies4,5,6,7,8,9,10,11,12, 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.
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The XFEL experiment was performed at the BL3 [EH2] of SACLA with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2014B8025). Single-photon excitations were studied at the Nuclear Resonance beamline ID18 of the ESRF. We thank the SACLA staff for extremely stable operation of the facility that enabled completion of this study within a single 72 hour time slot. A.I.C. thanks V. Kocharovsky for helpful discussion of an application of superradiance to lasers with ultra-narrow line widths. We are grateful for comments received during the reviewing process that helped us clarify the message of the paper for a broader audience.
The authors declare no competing financial interests.
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Supplementary Figures 1-8, Supplementary References
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Physical Review Letters (2019)
Journal of Synchrotron Radiation (2019)
Physical Review A (2018)
SciPost Physics (2018)
Physical Review Letters (2018)