自发参量下转换过程的单光子测量及量子态重构

【作者】 李永明；

【导师】 郜江瑞； 张俊香；

【作者基本信息】 山西大学 ， 光学， 2004， 硕士

【摘要】 单光子态作为一种非经典光场，在基础量子光学实验研究、量子通讯、量子计算等领域有着重要的应用。由于单光子的量子性，避免了在通讯传输过程中的信号窃取，在确保信息安全等方面具有经典信息无法比拟的优越性。目前有多种方案可以制备单光子态，如直接将激光衰减至单光子水平、量子点、单荧光分子以及金刚石纳米颗粒中的N-V色心、腔QED及自发参量下转换等。其中，自发参量下转换因其成对的产生下转换光子，可以通过探测其中之一来确认另一个是否为单光子态，这样就可以在不破坏单光子的情况下确认单光子的存在。另外，参量下转换产生的光子对具有时间、空间、偏振、能量等多种纠缠，因而在理论和实验上都得到了广泛的研究。 本文是单光子态制备及其Wigner函数重构课题中的部分内容，主要包括两个方面内容，即脉冲抽运的Ⅰ类非共线相位匹配BBO晶体的自发参量下转换过程和利用量子层析摄影术重构光量子态的Wigner函数。其主要内容包括 1．计算了BBO晶体中下转换光的分离角与晶体切割角的关系曲线。在中心波长425nm飞秒脉冲抽运、下转换光频率简并的情况下，选择晶体的切割角θ为28.7度，对应下转换光与抽运光在晶体外夹角为5度。 2．使用CCD测量了自发参量下转换光分布。 3．分析了抽运光的走离效应和群速失配对下转换过程的影响。非共线匹配对群速失配有一定的补偿作用。当下转换光与抽运光夹角达到13.4度时，非共线相位匹配基本可以消除晶体中群速失配对下转换过程的影响。 4．在抽运光脉冲重复频率为76MHz的情况下，得到了每秒50500次的下转换光子记数，其中单光子概率大干99％。 5．作为重构单光子Wigner函数的实验准备，重构出连续光真空态和相干态的Wigner函数，并考虑了kc值和实验采集的数据点数量对实验结果的影响。更多还原

【Abstract】 As a nonciassical state, the single-photon state has applications in many areas, such as in basic quantum optics research, quantum communication, quantum computation etc. Because of its quantum property, single-photon state can avoid eavesdropping and has more advantages in information security transferring than the classical state. There are many ways to generate single-photon state, e.g. attenuated laser, quantum dot, pulsed excitation of a single dipole, single nitrogen-vacancy color center in a diamond nanocrystal, cavity-QED and spontaneous parametric down-conversion (SPDC). Because SPDC creates photons in pairs, if one photon is found in one arm of the twin-photon, we know there must exists a single-photon state in another arm. So, we can assure when the single-photon state generate without detecting it. And the SPDC has also been widely studied for its entanglement of time, space, polarization, and energy.In order to get single-photon state and reconstruct its Wigner function, The two aspects, SPDC of pulse pumped nonlinear phase match type I BBO crystal and reconstruction of Wigner function of quantum state, are studied in this thesis.1. The relationship of cut-angle and the degree between pump beam and signal beam in BBO crystal pumped by 425nm pulse is theoretically calculated. 28.7° cut-angle BBO crystal is chose for our use, which correspond to a 5° angle between pump beam and signal beam.2.We measure the down-conversion ring with CCD and analysis the thickness of the down-conversion ring.3. The effect of pump beam’s walk-off and the group-velocity-mismatch in BBO crystal is discussed, the group-velocity-mismatch can be compensated by non-collinear phase-match when the angle between signal and idler beams equals to 14°.4. The maximum 50500 photo counts per second with 76MHz pump pulse frequency is detected. The probability to be single-photon state of every count is better than 99%5. As a primary work for reconstructing Wigner function of single photon state, we reconstruct Wigner function of vacuum and coherent state. The effect of kc and the number of the data is discussed in the process of reconstruction.更多还原