Entanglement of the Hermite-Gaussian modes states of photons
  We found that the Hermit-Gaussian(HG) modes of the down converted beams from the spontaneous parametric down conversion are quasi-conserved and the generated photon pairs are HG modes entangled for some special cases. This is valuable for either the investigation of fundament properties of multi-dimensional entanglement or quantum information applications. 
Xi-Feng Ren, Guo-Ping Guo, Jian Li and Guang-Can Guo