  We demonstrate an efficient cross-phase modulation (XPM) based on a
closed-loop double-{\Lambda} system. The property of the double-{\Lambda}
medium can be controlled by changing the phases of the applied optical fields.
This phase-dependent XPM scheme can achieve large phase modulations at
low-light intensities without requiring cavities or tightly focusing of laser
beams. With this scheme, we observe a {\pi}-level phase shift with two pulses
both consisting of 8 photons in cold rubidium atoms. Such novel scheme provides
a simple route to generate strong interactions between photons and may have
potential applications in all-optical quantum signal processing.
