2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/95538We 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.15 pages, 5 figuresQuantum Physicstext