量子記憶體是一個專門為讓量子態或量子糾纏存取的必要元件，也是量子計算和長距離量子傳輸中的不可或缺的重要元件之一。量子記憶體在近幾年也快速成長，目前量子記憶體已成功實現量子態在量子記憶體存取具有多模態長時間及高保真度和高效率的存取。在本論文中將主要說明與介紹如何利用電磁波引發透明效應實現量子記憶體高存取率。為了達成這個目標，高光學密度及基態之低非同調率為重要的關鍵。在本文中將仔細介紹在電磁波引發透明效應的實驗系統中，如何實現量子光學記憶體達92%。然而在高光學密度的系統下非線性光學的效應也引入在此系統中，例如光子開關效應和四波混頻效應。這兩種效應將會顯著將降低量子記憶體的存取效率和保真度。針對電磁波引發透明效應的原子系統下我們提出解決此兩個效應的方法。首先我們利用塞曼光學幫浦將原子分佈集中在近似單一的塞曼能階中，此動作可以大幅減少光子開關效應的影響。此外我們也藉由改變探測光和控制光的相對相位角度來破壞四波混頻系統中的相位匹配條件，這將可以避免四波混頻效應對於量子光學記憶體保真度的影響。為了真實實現量子態的存取，我們利用週期性之非線性晶體建立了自發性參量下轉換之共振腔的單光子源。我們也使用條件性的量測方法來診斷單光子的相對巧合函數值g(2)。透過此量測方法的量測，我們可以透過計算量子態在量子光學記憶體中的存取效率。在本實驗中已經實現量子態在量子記憶體之存取效率為35% 且相對巧合函數值g(2)為 7.6。;Quantum memory is a device that can store quantum state or quantum entanglement and replay on demand. It is a crucial component in linear-optics-based quantum computation and long-distance quan- tum communication. There have been tremendous progress in the development of quantum memories, for example, quantum memories with long storage time, high efficiency, and high fidelity or of highly multimode have been developed. The fundamental requirements to achieve high storage efficiency are atomic media with high optical depths and low ground-state decoherence rates. However, at high optical depths, nonlinear optical effects such as photon switching and four-wave mixing may become signifi- cant and lead to the degradation of storage efficiency or fidelity of the optical memory. In this thesis we attempt to solve this problem by focusing on developing a high storage efficiency optical quantum memory base on the electromagnetically induced transparency (EIT) protocol. We will describe our development on the EIT protocol experimental apparatus and the method to achieve a storage efficiency of 92% in EIT-based coherent optical memory. By implementing the EIT memory in cesium D1 tran- sition and performing Zeeman optical pumping to prepare population in nearly single Zeeman state, the photon switching effect can be minimized.Furthermore, the introduction of a small angle between the control and probe beams to break the phase matching condition, the four-wave mixing effect can be significantly reduced. To enter the quantum storage regime, we developed a photon-pair source based on cavity-enhanced spontaneous parametric down conversion using periodical polled KTP crys- tal. Through coincidental measuring of the photon pairs and determining the cross-correlation function g (z) before and after the storage process in memory, we were able to demonstrate that the quantum nature of the photon pairs can be preserved in EIT-memory. With such method, quantum storage of heralded single photons with an efficiency of 35% and a g (z) of 7.6 can be achieved.
