## https://en.wikipedia.org/wiki/Spontaneous_emission

- https://en.wikipedia.org/wiki/Stimulated_emission

- 2016 - Sidney Nagel, Burton [Echoes from Anharmonic Normal Modes in Model Glasses](https://arxiv.org/pdf/1601.05157.pdf)

*We show that these echoes are due to anharmonic, weakly-coupled vibrational modes*, and perhaps **provide an alternative explanation** for the phonon echoes observed in glasses at low temperatures

We offer **an 

2016 [Random Nonlinear Infinite-Level-System Model for **Amorphous Solid Phonon Echo and Saturation Phenomena**](https://arxiv.org/abs/1510.05538)
Di Zhou, Anthony J. Leggett



## https://en.wikipedia.org/wiki/Free_neutron_decay

in free space