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Coherent conversion between a Raman pump field and its Stokes field is observed in a Raman process with a strong atomic spin wave initially prepared by another Raman process operated in the stimulated emission regime. The oscillatory behavior resembles the Rabi oscillation in atomic population in a two-level atomic system driven by a strong light field. The Rabi-like oscillation frequency is found to be related to the strength of the prebuilt atomic spin wave. High conversion efficiency of 40% from the Raman pump field to the Stokes field is recorded and it is independent of the input Raman pump field. This process can act as a photon frequency multiplexer and may find wide applications in quantum information science.

In spontaneous Raman process in atomic cell at high gain, both the Stokes field and the accompanying collective atomic excitation (atomic spin wave) are coherent. We find that, due to the spontaneous nature of the process, the phases of the Stokes field and the atomic spin wave change randomly from one realization to another but are anti-correlated. The phases of the atomic ensembles are read out via another Raman process at a later time, thus realizing phase memory in atoms. The observation of phase correlation between the Stokes field and the collective atomic excitations is an important step towards macroscopic EPR-type entanglement of continuous variables between light and atoms. PACS numbers: 42.25Kb,42.25Hz,42.65.Dr Correlations in a quantum system played an important role in the test of foundation of quantum mechanics [1,2], and in the applications of quantum metrology [3] and quantum information [4]. Recently, atomic memory for correlated photons is demonstrated [5,6] based on the DLCZ scheme [7] in a collective Raman process in spontaneous emission regime for the application in quantum repeaters for long distance quantum communication.But what is demonstrated so far is the intensity correlation between the Stokes field and the atomic excitations in the spontaneous Raman process. The phase correlation between the related optical fields and atomic medium has not been explored. With photon correlation demonstrated [5,6,8], we believe that there is a strong correlation in the phases of the atomic ensemble and the Stokes field as well.In this paper, we report on an experiment in which we measure the phase difference of two spatially separated atomic spin waves created via spontaneous Raman processes in high gain regime and directly confirm a phase anti-correlation between the Stokes field and the corresponding atomic spin wave in the collective Raman process. The phase of the atomic spin wave is retrieved after a time delay of the phase measurement of the Stokes field, thus realizing the memory of phase information of optical fields.The conceptual sketch of the experiment is shown in Fig.1. The basic process is a collective Raman scattering process in atoms with a lambda structure: an excited state |e and two meta-stable lower level states |g , |m (insets of Fig.1). N atoms are initially prepared in the ground state |g by optical pumping. There are two stages of operation, depicted in Figs.1a and 1b, respectively. In the first stage shown in Fig.1a, two Raman pump pulses (W 1 , W 2 ) start the Raman scattering to produce random Stokes fields (S W 1 , S W 2 ) and the corresponding atomic spin wavesŜ a1,2 ≡ (1/ √ N ) i |g 1,2 i m 1,2 | between two lower states in two separate atomic ensembles. We superimpose the generated Stokes fields to measure their relative phase. The second stage reads out the atomic spin waves by another Raman process as shown in Fig.1b and the relative phase of the readout fields are measured by interference methods. We look for the correlation between the two measured phases.The int...

We experimentally demonstrate efficient Raman conversion to respective Stokes and anti-Stokes fields in both pulsed and continuous modes with a Rb-87 atomic vapor cell. The conversion efficiency is about 40-50% for the Stokes field and 20-30% for the anti-Stokes field, respectively. This efficient conversion process is realized with coherent feedback of both the Raman pump and the frequency-converted fields (Stokes or anti-Stokes). The experimental setup is simple and can be applied easily to produce light sources with larger frequency shifts using other Raman media with long coherence time. They may have potential applications in nonlinear optics, Raman spectroscopy and precision measurement.

We investigate experimentally and theoretically the onset of parametric oscillation in a single-pass atomic Raman scattering process without the optical feedback. We find that this Raman oscillation effect is due to the coherent buildup of the atomic spin waves, and therefore the threshold of the Raman pump power for the onset of the oscillation depends on the decoherence time of the atomic spin waves and the Raman coupling constant but is not sensitive to the loss of the Stokes field, unlike the effect of amplified spontaneous emission. A simple theory of atomic Raman process fits the experimental results very well. The long decoherence time of the atomic spin waves in the atomic Raman process leads to a threshold power as low as a few hundred microwatts.