Brillouin scattering and dynamical diffraction of entangled photon pairs

Photons incident on a periodic medium at the Bragg angle are dynamically diffracted and exhibit the pendellösung phenomenon familiar from x-ray and neutron-diffraction experiments. The entangled two-photon state incident from the positive and negative Bragg angle exhibits a pendellösung length that is half that for the single-photon case. In our experiment the photon pairs were produced via parametric down-conversion and the periodic medium was a crystal whose refractive index was periodically altered by an acoustic wave. The predicted Pendellösung-length behavior was experimentally confirmed.