Interference And Nonclassical Excitation In The Four-level Atomic System

The interaction between the light and the atomic system is one of the most important aspect in quantum optics, which provides us abundant of interesting phenomenon and application. The electromagnetically induced transparency(EIT) occurred when the lasers are arranged in the near resonant region of a coherent medium. The EIT is the basis of many optical experiments in atomic system.There are many possible usage of atomic ensemble in the quantum information. For example, the EIT effect can be used to control the delay of the wave packets; the atomic ensemble can be used to generate the nonclassical correlated photon pairs. The controllable delay between the wave packets can keep the wave packets overlapped well. Long distant quantum communication is one of the most important aspect in quantum information. Photons are decayed exponentially as the increment of the distance, therefore quantum repeaters are needed in the long distance quantum communication. Duan et al. have shown us that the atomic ensemble can be used as the quantum repeater. Narrow bandwidth photon pairs are needed in order to use the atomic ensemble as the quantum repeater. The spontaneous parametric down conversion(SPDC) in the nonlinear crystal can generate photon pairs effectively. However the photon pairs generated by SPDC have very broad bandwidth, and can not couple with atomic ensemble effectively. The photon pairs generated in the atomic system have a very narrow bandwidth, and can couple with atomic ensemble effectively. Therefore narrow bandwidth photon pairs generated in the atomic ensemble may be very useful in the quantum communication.In this dissertation we give a scheme which can be used to control the delay of the wave packet from positive delay to negative delay effectively, and experimentally demonstrate the generation of nonclassical correlated photon pairs using spontaneous four-wave-mixing process in hot atomic ensemble. The works presented in this dissertation are listed as follows,1. Control of light speed: from slow light to superluminal light. Control of light speed has attracted much attention in the past years. We give a scheme which can be used to change the group velocity of probe field from slower-than-c to faster- than-c continuously by controlling the strength of an optical pump. We used a far detuning double-coupling structure, which will lead to two Raman absorptive peaks, and when an optical pump is added to the system the absorptive peaks will change into enhanced peaks, and the normal dispersion between the two peaks change into anomalous dispersion. Therefore the probe field can change from slow light to superluminal propagation. This scheme can control the delay of the pulse from positive to negative delay effectively.2. Computation of topological charges of optical vortex via non-degenerate four-wave-mixing. Orbital angular momentum (OAM) of light, which has many interesting property, was quite unfamiliar to the researchers until very recently. In this experiment we show that the OAM will be preserved in the four-wave-mixing process. Therefore the four-wave-mixing process can be used for computation of topological charges of optical vortex.3. Generation of nonclassical photon pairs in a hot atomic ensemble. Correlated photon pairs are very important in quantum information field. Usually, spontaneous parametric down-conversion in nonlinear crystal is used in the generation of the correlated photon pairs. However the photon pairs generated using this method have a very broad bandwidth. Narrow band photon pairs can be generated by atomic system. Although previous works had demonstrated the non-classical correlation between the Stokes and anti-Stokes photons generated in the hot atomic ensemble, there is not any previous work in the hot atomic ensemble which gives a clear picture of the time-resolved second order correlation function between the photon pairs. In the cold atomic system such a relation between the Stokes and anti-Stokes photons has been given out since the first experiment. This experiment gives the time-resolved second order correlation function between the photon pairs generated in a hot rubidium cell, and the violation of the Cauchy-Schwarz inequality intuitively. The scheme we used in this experiment is spontaneous four-wave-mixing.4. Entanglement of the orbital angular momentum states of the photons generated in a hot atomic ensemble. Quantum protocols will be more efficient with high-dimensional entangled states. Photons carrying orbital angular momenta can be used to create a high-dimensional entangled state. In this paper we experimentally demonstrate the entanglement of the orbital angular momentum between the Stokes and anti-Stokes photons generated in a hot atomic ensemble using spontaneous four- wave-mixing. This experiment also demonstrates the existence of the entanglement concerned with spatial degrees of freedom between the hot atomic ensemble and the Stokes photon.