Investigation Of High-resolution Spectroscopy And Far-off-resonance Raman Memory With Atomic Ensemble

To implement the quantum information process such as long-distance quantum communication, quantum computer, quantum network and so on, quantum repeater protocol should be used, in which the quantum memories are important elements for it. Quantum memories also paly an import role in kinds of other areas including of the generation of the non-classical correlated photon pairs, the generation of the deterministic and controllable single photons, the realization of the loophole-free Bell test and the enhanced measurement precision up to the classical limit. Atomic ensemble is an ideal media for the realization of the quantum memories due to the collective enhancement effect in it. Another reason is that there are several kinds of the techniques for atomic ensemble to prolong the coherence time of the spin wave. A good quantum memory means that the long-lived entanglement could be stored. As a result, it can be viewed as an entanglement source to enhance the sensing and precision of the metrology. Utilization of the atomic ensemble can enhance the coupling between the light and atoms, by which the measurement precision could be increased when detecting external perturbation on the state of such a system with quantum mechanically limited resolution.Many process introduced above is based upon the interaction between atoms and lights, which are usually benefited from the development of laser technology and the methods to control the internal or external atomic state. High resolution laser spectroscopies are always used to detect the internal energy level structure, from which we can know more about the atoms. It helps us choose the suitable atomic energy level structure to increase the precision and the resolution of the spectra. Meanwhile, laser spectroscopy can also be used as a frequency reference in laser frequency locking and the distinguishing of the different atomic internal levels duration the process of the quantum memory and quantum information. In this thesis, I mainly introduced the research works on the subject of the high resolution laser spectroscopy and the far-off-resonance Raman quantum memory in atomic ensemble.There are many types quantum memories with atomic ensemble are proposed, such as electromagnetically induced transparency (EIT), spontaneous Raman scattering, far-off-resonance Raman memory. All of them are essentially based upon the atomic coherence effect. Actually, with the help of the atomic coherence in atomic ensemble, the resolution and the SNR (Signal-to-Noise Ratio) of the laser spectroscopy could be further enhanced. Meanwhile, high resolution laser spectroscopy caused by the atomic coherence, coherence population trapping (CPT) for example, can also be used for demonstrating and evaluating the phase coherence of the laser system, that is just what I have done as introduced in this paper. The characterized works in this paper are introduced as follows:1. By reforming the velocity-selection optical-pumping Spectra, the single-resonance optical pumping spectra (SROP) are obtained in V-type atomic system with cesium vapor cell. With the virtue of Doppler-free line-width, SNR, flat background and elimination of crossover lines, the SROP spectra of the D1or D2lines of cesium in room temperature atomic vapor cell can be utilized to measure dressed-state splitting of ground state, which is normally detected with cold atomic sample only.2. The double-resonance optical pumping spectra (DROP) in the ladder-type cesium atoms in room-temperature vapor cell are observed. It is applied to laser frequency stabilization and the measurements of atomic energy level structure. By comparison with the EIT in the ladder-type atomic system, we analyze the atomic coherence effect include in the DROP3. Two phase-locked home-made lasers with moderate output power (-150mW) and the fixed frequency difference correspond to the hyperfine splitting of the cesium atomic ground states are obtained by optical injection between two DFB diode lasers. Utilizing this kind of the phase-locked two-color lasers, we observed the CPT signal with12.3kHz line-width in a cesium vapor cell filled with20Torr neon as buffer gas. By this high-resolution spectrum, the good phase coherence between the two DFB lasers is verified.4. With the phase-locked two-color laser system, we achieved the far-off-resonant Raman memory in a room-temperature cesium vapor cell with about20MHz for the maximum bandwidth, about35%for the maximum total storage efficiency and about1μs for the longest storage time at present. We also studied experimentally the ingredients affect the storage time and efficiency of the memory, such as the pulse shape, pulse parameters, the temperature of the atomic ensemble. The partial results verify the theoretical researches that have been published. Meanwhile, we also analyze the factors currently exist limiting the bandwidth and time of the storage. The experimental results will help us deepen the understanding of the far-off-resonance Raman memory and improve its performance. It makes us walk nearly up to the quantum memory for single photons emitted from the single atoms as well as for the non-classical light.5. To further extend the storage time and storage efficiency, we plan to implement the far-off-resonance Raman memory with cold cesium atoms. Utilizing the method of the optical injection, two slaved DFB lasers with nearly the same output characteristics to the master laser are obtained and used as the three-dimensional cooling lights of the MOT in company with the master laser. In this way, the power of the cooling light is increased largely. At the same time, Cooling light beams’diameters are expanded by the telescope system to increase the trapping range of the cold atoms. After above the transformation of our old MOT, we get a cold atomic ensemble with the higher optical depth (O.D). By measuring the absorption at different point of the detuning, we obtain the resonant optical depth of the cold atomic ensemble.6. High-resolution absorption spectra between the excited states of cold atoms in the MOT are obtained experimentally. The experimental results show that this kind of the spectroscopy not only can be used to study the dressed-state splitting of the atomic excited states, but also can be develop to measure the effective Rabi frequency felt by the cold atoms in a MOT.