Entanglement detection and quantum metrology by Raman photon diffraction imaging

Abstract

We show that far-field diffraction images of spontaneously scattered Raman photons can be used for detection of spin entanglement and for metrology of field gradients in cold atomic ensembles. For many-body states with small or maximum uncertainty in the spin-excitation number, entanglement is simply witnessed by the presence of a sharp diffraction peak or dip. The gradient vector of external fields is measured by the displacement of a diffraction peak due to inhomogeneous spin precession, which suggests a possibility for precision measurements beyond the standard quantum limit without entanglement. Monitoring of the temporal decay of the diffraction peak can also realize a nondemolition probe of the temperature and collisional interactions in trapped cold atomic gases. The approach can be readily generalized to cold molecules, trapped ions, and solid-state spin ensembles. © 2013 American Physical Society.