Efficient Measurement of the Bi-photon Spatial Mode Entanglement with Stimulated Emission Tomography

The technique of stimulated emission tomography (SET) provides excellent characterization of SPDC sources of bi-photon states since it increases the average number of photons detected by several orders of magnitude than the traditional coincidence counting method. In a SET experiment, the signal caused by the vacuum fluctuation in SPDC is replaced by a more intense prepared seed with the same mode properties, resulting in an amplification of the corresponding idler. Based on this idea, our experiment uses the difference frequency generation (DFG), a purely classical second-order nonlinear process, to measure the orbital angular momentum (OAM) spectrum of an entangled photon pair produced by a Type-II SPDC crystal. We inject the seed beam at 780 nm with different Laguerre-Gaussian modes together with a pump beam at 405 nm into a Type-II BBO crystal and measure the Laguerre-Gaussian mode distribution of the idler at 842 nm. We observe a strong idler production and good agreement with the theoretical prediction of the OAM spectrum. We expect that this experiment paves the way for the efficient measurement of bi-photon wavefunctions produced by ultra-thin SPDC sources and also the characterization of high-dimensional entangled photon pairs produced in SPDC.