Electromagnetically induced transparency (EIT) techniques are important tools for the storage of the quantum states of light fields in atomic ensembles and for enhancement of the interaction between photons. In this paper, we briefly summarize the recent experimental studies conducted by our group on enhanced cross-phase modulation based on double EIT effects, the quantum interference of stored dual-channel spin-wave excitations and the coherent manipulation of the spin wave vector for the polarization of photons in a single tripod atomic system. The work presented here has potential application in the developing field of quantum information processing.
Frequency tunable continuous variable （CV） entangled optical beams are experimentally demonstrated from a non-degenerate optical parametric oscillator working above the threshold.The measured correlation variances of amplitude and phase quadratures are 3.2 and 1.5 dB,respectively,below the corresponding shot noise level （SNL） in the tuning range of 580 GHz （2.25 nm）.The frequency tuning is realized by simply controlling the temperature of the nonlinear crystal.
We report an experimental study of electromagnetically induced transparency（EIT）-based light storage in a cloud of cold atoms loaded into a magneto-optical-trap（MOT）. After the MOT is turned off, the retrieval efficiencies of rightand left-circularly polarized signal light fields each as a function of storage time are measured for different time delays between MOT off and the storage event, respectively. The results show that in the delay ranging from 0.015 ms to 3.5 ms,the retrieval efficiency for a zero-storage time（0.2 μs） and the storage lifetime can exceed 15% and 1.4 ms, respectively.The measured results will provide important help for optimizing the storage of the polarized entanglement photons in cold atomic ensembles.
We systematically investigate the polarization gradient cooling （PGC） process in an optical molasses of ultracold cesium atoms. The SR mode for changing the cooling laser, which means that the cooling laser frequency is stepped to the setting value while its intensity is ramped, is found to be the best for the PGC, compared with other modes studied. We verify that the heating effect of the cold atoms, which appears when the cooling laser intensity is lower than the saturation intensity, arises from insufficient polarization gradient cooling. Finally, an exponential decay function with a statistical explanation is introduced to explain the dependence of the cold atom temperature on the PGC interaction time.
We present a robust method of single-photon modulation by directly modulating the single photons and observe its fre- quency spectrum. Compared with conventional photon counting technique, the single-photon modulation spectrum shows that the method could not only realize high-frequency modulation but also obtain higher signal-to-noise ratio. Moreover, the theoretical calculations show good agreement with the experimental results.
