The Quantum Noise Of Probe Light In The N-type Atomic System

Recently, the interaction between light and atom as one primary component of quantum information system and quantum engineering draws more and more attention of researchers. The initial study of this subject focused on high-efficiency coherence effect between the optical dense atomic ensembles and quantized light field in the free space. At present, it has been well demonstrated that by using atom ensembles we could easily realize high-efficiency interaction between light and atoms instead of placing a single atom inside a high finesse cavity which may enhance the interaction by many orders of magnitude.There exist different physical models to describe the interaction between light and atom, including electromagnetically induced transparency (EIT), quantum non-demolition (QND), Faraday interaction, quantum measurement and feedback, Raman interaction, and photon echo. EIT is a coherent effect which leads to an initial high opaque medium being rendered transparency over a narrow spectral range for the probe light beam, and intense dispersion also occurs within this transparency "window" which greatly slows the group velocity of light and extends the coherence time simultaneously.In this paper, we review the different physical descriptions of interaction between light and atom, and analyze the quantum noise property of quantized light field after passing through a N type four-level atomic ensembles by utilizing the semi-classical theoretical model with Heisenberg-Langevin dynamical equation. To explain our results clearly, firstly the dynamic evolution of atomic spin in the two-level and three-level atomic ensembles which interacts with light beams is given, and the quantum noise spectra of the probe light is calculated. Then the output noise spectra of initial squeezed light which has been propagated in the N type four-level atomic ensemble is discussed. It is shown that the noise of output light consists of amplitude noise, phase noise and the noise from atomic ensembles. Furthermore the condition to get the low noise transmission is analyzed. The discussion is helpful for quantum information memory.