Coherent Manipulation Of Quantum State In Cold Atomic Ensemble

Quantum memories are of great importance in a number of contexts, including long-distance quantum communication, optical quantum computation, and quantum metrology. Recent years, a lot of great progresses of optical quantum memory have been implemented in different systems for its potential applications. Among them, cold atomic ensembles are currently one of the most important systems for its over-all high quality, and have witnessed remarkable progresses these years. This thesis will present several experimental demonstrations of coherent manipulation of spinwaves in cold atomic ensemble quantum memories, which is an essential part to improve the quality of memory and realize high-fidelity quantum operations.By employing stimulated Raman transitions, we have successfully managed to op-erate the spin echo technique for an ensemble-based quantum memory in the single-quanta regime for the first time. Through manipulating the spin-wave’s wavevector with Raman beams, we have successfully eliminated the motion-induced dephasing and ex-tended the lifetime to the order of millisecond. What’s more, by carefully investigating the noise induced by π-pulse error, we have demonstrated that both previous theoretical studies are solid. For the former spin echo technique needs extremely high-fidelity e-cho pulses, we demonstrate another convenient technique to freeze the motion-induced dephasing by manipulating the spin-wave using Raman transitions without population inversion, which can be used in large storage angles. The lifetime has been extend-ed by one order of magnitude to the limit of the thermal expansion. Notably, high non-classical correlation(g(2)) above 20 has been achieved to guarantee the memory in quantum regime. Besides, by making use of stimulated Raman transition and controlled Larmor procession jointly, we have realized high-fidelity single-qubit operations, such as arbitrary rotation and single-qubit gate, for single spin-waves in an atomic-ensemble quantum memory for the first time. Finally, by employing the Rabi oscillations driven by a standing wave light field, we have experimentally demonstrated a novel quantum lithography scheme proposed by Liao et al for the first time. Combined with high-resolution imaging system, we have observed 9 peaks in a single λeff/2 range success-fully, thus the resolution down to one-ninth of the Rayleigh limit has been realized. What’s more, we have also spacially localized the atoms in one half-wavelength range with multiple Raman pulses, and the smallest observed width of the atomic pattern is about one-seventh of the diffraction limit.In summary, by coherently manipulating the spinwaves, the overall quality of the quantum memory has been improved. Our work enriches the toolbox of coherently ma- nipulating the spin-waves in the atomic-ensemble quantum memory, thus could make such high-performance quantum memory more general and scalable, which will be use-ful for future development of quantum information process, such as quantum repeator, long-distance quantum communication, linear optical quantum computaion, and so on.