A long due paper to describe a discovery by Jian, Yuzhuo, with supportive hard work by Xing, is now on arXiv:

Spectroscopic localization of atomic sample plane for precise digital holography

Digital holographic microscopy (DHM) can fundamentally expand the imaging capacity for cold atomic research, if a few unique technical challenges can be addressed. In particular, to retrieve generic atom properties from digital holograms requires precise refocus of the reconstructed images to the sample plane. The difficulty arises from the fact that typical atomic samples are spatially featureless. Previously, the only similar effort appears to be exploiting tiny signals of atomic shot-noise correlation in phase-contrast imaging, with limited refocusing distances. 

We propose and demonstrate that a ``phase angle spectroscopy’’ can be exploited in DHM to localize the atomic samples in 3D with super-resolution. The underlying principle is to exploit the additional diffraction phase picked up by small objects during defocus, known as Gouy phase anomaly, that leads to deviation of apparent spectroscopic responses from theoretical prediction. Comparing with traditional methods, the novel spectroscopic method is more sensitive, accurate, and with a refocusing distance unlimited by transfer-function approximations. Our work removes a major barrier for application of DHM in atomic physics. The highly precise sample plane localization forms an excellent starting point for complex spectroscopic imaging of 3D sparsely distributed samples, an important scenario in ultracold-atom based quantum simulation research today.