Single-photon frequency conversion and multi-mode entanglement via   constructive interference on Sagnac Loop
  Based on constructive interference in Sagnac waveguide loop, an efficient scheme is proposed for selective frequency conversion and multifrequency modes W entanglement via input-output formalism. We can adjust the probability amplitudes of output photons by choosing parameter values properly. The tunable probability amplitude will lead to the generation of output photon with a selectable frequency and W photonic entanglement of different frequencies modes in a wide range of parameter values. Our calculations show the present scheme is robust to the deviation of parameters and spontaneous decay. 
Anshou Zheng, Guangyong Zhang, Liangwei Gui, and Jibing Liu
quant-ph