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We report the first experimental demonstration of electromagnetically induced transparency (EIT) based cross-phase modulation (XPM) at few-hundred-photon levels. A phase shift of 0.005 rad of a probe pulse modulated by a signal pulse with an energy of 100 attojoules, equivalent to ~ 400 photons, was observed in a dark spontaneousforce optical trap. The experimental data show the single-photon-level XPM phase shift is 1 × 10-5 rad, which is in good agreement with the theoretical prediction. This work offers exciting prospects to the realization of EIT-based XPM scheme at the single-photon level and benefits experimental development in few-photon applications of EIT-based techniques for quantum optics and quantum information science.

We report on an experimental demonstration of applying the beat-note interferometer to simultaneously measure the phase and amplitude variations of light pulses after propagating through an electromagnetically induced transparency medium at femtowatt-light levels. Furthermore, we observe that the measured phase noise approaches the shot-noise level arising from the fluctuations of detected photons.