Observation of the Preservation of the Two-Photon Coherence in   Plasmon-assisted Transmission
  We experimentally study two-photon coherence in plasmon-assisted transmission with a two-photon Mach-Zehnder (MZ) interferometer. Two collinear photons of identical or orthogonal polarization are simultaneously incident on one optically thick metal film, perforated with a periodic array of subwavelength holes. The de Broglie wavelength of plasmon-assisted transmitted photons is measured, which shows that two photons are re-eradiated by the plasmons and the quantum coherence of biphoton is preserved in the conversion process of transforming biphoton to plasmons and then back to biphoton. 
Guo-Ping Guo, Xi-Feng Ren, Yun-Feng Huang, Chuan-Feng Li, Zhe-Yu Ou   and Guang-Can Guo