Macro-coherent two photon and radiative neutrino pair emission
  We discuss a possibility of detecting a coherent photon pair emission and related radiative neutrino pair emission from excited atoms. It is shown that atoms of lambda- and ladder-type three level system placed in a pencil-like cylinder give a back to back emission of two photons of equal energy $\Delta/2$, sharply peaked with a width $\propto $ 1/(target size) and well collimated along the cylinder axis. This process has a measurable rate $\propto$ (target number density) $^2 \times$ target volume, while a broader spectral feature of one-photon distribution separated by (mass sum of a neutrino pair)$^2/(2\Delta)$ from the two photon peak may arise from radiative neutrino pair emission, with a much smaller rate. 
M. Yoshimura, C. Ohae, A. Fukumi, K. Nakajima, I. Nakano, H. Nanjo,   and N. Sasao