1202.5607 (Hongyi Yu et al.)

Fraunhofer diffraction of Stokes photons from cold atomic ensembles:
entanglement witness, and collectively enhanced metrology    [PDF]

Hongyi Yu, Wang Yao

We exploit the Fraunhofer diffraction of Stokes photons for detecting spin entanglement and for probing spin and motional dynamics in cold atomic ensembles. For many-body states where the spin-excitation number has small or maximum uncertainty, entanglement is witnessed by a qualitative diffraction feature, namely a sharp peak or dip in a chosen direction. For general states, the spin pair-correlation sum can be read out from the relative strength of the peak or dip over its background which, together with the expectation value and uncertainty of spin-excitation number, detect entanglement through the spin squeezing inequalities. Spin precessions in Zeeman field gradient lead to displacement of the diffraction peak or dip while atomic motions lead to decay of its strength. These can serve as principles for vector gradiometer of fields and for non-demolition measurement of atomic temperature and collisional dynamics. Using a spin-coherent-state of N unentangled atoms as the probe, the information of interest are encoded by a collectively-enhanced diffraction peak of strength ~N^2, making possible gradiometer with 1/N sensitivity and thermometer by a single shot.

View original: http://arxiv.org/abs/1202.5607