Quantum key distribution using different-frequency photons
  In this paper, we present a scheme for quantum key distribution, in which different-frequency photons are used to encode the key. Thses different-frequency photons are produced by an acoustic-optical modulator and two kinds of narrow filters . This scheme may be implementable in practice. 
Bao-Sen Shi, Yun-Kun Jiang, Guang-Can Guo