Efficient Measurement of Bi-photon OAM Spectrum 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-I BBO SPDC crystal. We inject the CW seed beam at 780 nm with different Laguerre-Gauss modes together with a CW pump beam at 405nm into the Type-I BBO crystal and measure the Laguerre-Gauss mode distribution of the idler at 842nm. We observe an increase of 9 orders of magnitude in the idler production and good agreement with the theoretical prediction of the OAM spectrum. We expect that this experiment paves way for the efficient measurement of bi-photon wavefunctions produced by ultra-thin and weak SPDC sources and also the characterization of high-dimensional entangled photon pairs produced in SPDC.