Manipulating the frequency entangled states by   acoustic-optical-modulator
  In this paper, we describe how to realize conditional frequency entanglement swapping and to produce probabilisticly a three-photon frequency entangled state from two pairs of frequency entangled states by using an Acoustic-Optical-Modulator. Both schemes are very simple and may be implementable in practice. 
Bao-Sen Shi, Yun-Kun Jiang, and Guang-Can Guo