We experimentally demonstrate a low-light-level cross-phase-modulation (XPM) scheme based on the light-storage technique in laser-cooled 87 Rb atoms. The proposed scheme can achieve a similar phase shift and has the same figure of merit as one using static electromagnetically induced transparency under the constant coupling field. Nevertheless, the phase shift and the energy loss of a probe pulse induced by a signal pulse are neither influenced by the coupling intensity nor by the atomic optical density in the lightstorage XPM scheme. This scheme enhances the flexibility of the experiment and makes possible conditional phase shifts on the order of with single photons. Atoms PD2 PD1 M Aperture Block Beam 1 OSC DL AOM Signal M DL PBS Block M Beam 2 AOM Probe BS DL M M M M M FIG. 2. The experimental setup. DL: diode laser, PD: photodetector, BS: beam splitter cube, PBS: polarizing beam splitter cube, =4: quarter wave plate, M: mirror, OSC: oscilloscope.

We have experimentally demonstrated that stored and retrieved light pulses have different frequencies but maintain phase coherence; a stored light pulse is released with a different polarization. However, the manipulation process causes an energy loss of the retrieved pulse. We have discovered that Clebsch-Gordan coefficients among the Zeeman sublevels play an important role in the energy loss and have demonstrated a solution for avoiding the energy loss.

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We describe a simple method of the beat-note interferometer that directly and dynamically measures the phase shift of light pulses induced by highly dispersive samples. Using the method, we show intuitively that the storage and retrieval of light pulses in a medium due to the effect of electromagnetically induced transparency is a coherent process and, quantitatively, that there is no observable phase shift caused by the process. This method is sensitive enough that it can be used to measure the phase of a light pulse with energy on the order of a dozen photons.