Phase controlled light switching at low power levels
  We report experimental observations of interference between three-photon and one-photon excitations, and phase control of light attenuation/transmission in a four-level system. Either constructive interference or destructive interference can be obtained by varying the phase and/or frequency of a weak control laser. The interference enables absorptive switching of one field by another field at different frequencies and ultra-low light levels. 
Hoonsoo Kang, Gessler Hernandez, Jiepeng Zhang, and Yifu Zhu