Lineshape measurement of an extreme-weak amplitude-fluctuating light   source by the photon-counting-based second-order correlation spectroscopy
  We demonstrate lineshape measurement of an extreme-weak amplitude fluctuating light source by using the photon-counting-based second-order correlation spectroscopy combined with the heterodyne technique. The amplitude fluctuation of a finite bandwidth introduces a low-lying spectral structure in the lineshape and thus its effect can be isolated from that of the phase fluctuation. Our technique provides extreme sensitivity suited for single-atom-level applications. 
Hyun-Gue Hong, Wontaek Seo, Moonjoo Lee, Wonshik Choi, Jai-Hyung Lee,   and Kyungwon An