冷原子系统及窄线宽光子对的产生与调控研究

发布时间：2019-01-13 08:05

【摘要】：非经典关联光子对是一种非常重要的量子光源,能探索未知的量子世界,其在量子密钥分布、量子信息技术、量子计算、量子存储和通讯方面有着重要的应用。非线性晶体通过自发参量效转换,在相位匹配条件下产生一对非经典关联光子对,其线宽通常达到THz量级。为适用于长程量子通信和基于光作为飞行比特和原子作为网点的量子网络方案,关联光子对的线宽需要与原子能级带宽量级匹配。原子系综能级易于调控,从其中通过非线性过程制备的光子对必定匹配原子能级的带宽,因此从原子系综制备窄线宽关联光子对在近十多年中成为量子光学的一个重要课题。另一方面,随着激光冷却与囚禁技术的发展和广泛应用,冷原子系综成为量子光学、量子信息技术等领域的重要介质源。结合冷原子实验技术和量子光学前沿问题,本博士论文主要完成了以下三项工作。1.我们搭建了一个85Rb二维磁光阱,光学深度为100。磁光阱被广泛应用在量子光学和原子分子物理领域,为基础科学研究提供冷原子源。我们搭建的二维磁光阱的最大特点是沿长轴方向具有高光学深度和具有较低的原子基态退相干率,适合用于制备窄线宽非经典关联光子对。2.在冷原子系综中利用DLCZ方案产生关联光子对。写光先作用到原子系综中,通过拉曼过程制备原子集体激发态,即原子自旋波,并同时产生一个斯托克斯光子。双光子波函数的时间宽度可以通过写光的脉冲宽度进行调控。反斯托克斯光子的读取时间由读光的作用时间决定,因此在原子自旋波的寿命内能任意控制光子对的延迟时间。这种方式产生的光子对在波函数上能写为两个光子波函数的乘积,它们在时间和频率域上是相互独立的。3.在高光学厚度的冷原子团中在连续光作用下通过自发四波混频过程制备窄线宽光子对,并且这种光子对在时间频率维度上能用一个波函数表示,因此具有时间频率纠缠。光子对中的反斯托克斯光子经过一窄带的电磁诱导透射窗口得以延迟。影响双光子相干时间的因素主要有原子系综的光学深度OD,耦合光的拉比频率Ωc,基态退相干率γ12。在一定的范围内,OD越大、耦合光的拉比频率越低,相干时间越长。在耦合光的拉比频率降低到一定程度,致使电磁诱导透射的透射窗口的吸收不能被忽略时,基态退相干率γ12是限制相干时间的唯一因素,决定了光子对相干时间的极限。我们通过光学厚度达到100的冷原子系综,在γ12为283kHz的低退相干率条件下,光子对的相干时间首次突破2微秒(长达2.34μs)。
[Abstract]:Non-classical correlated photon pair is a very important quantum light source, which can explore the unknown quantum world. It has important applications in quantum key distribution, quantum information technology, quantum computing, quantum storage and communication. A pair of nonclassical correlated photon pairs is produced by spontaneous parametric effect conversion in nonlinear crystals under the condition of phase matching. The linewidth of the pair is usually in the order of THz. In order to be suitable for long range quantum communication and quantum network schemes based on light as flying bits and atoms as dot the linewidth of correlated photon pairs needs to match the order of atomic bandwidth. The atomic ensemble energy level is easy to be controlled, and the photon pair produced by nonlinear process must match the bandwidth of atomic energy level. Therefore, the generation of narrow-linewidth correlated photon pairs from atomic ensemble has become an important subject in quantum optics in the past ten years or so. On the other hand, with the development and wide application of laser cooling and trapping technology, cold atom ensemble has become an important dielectric source in the field of quantum optics, quantum information technology and so on. Combining the cold atom experiment technique and the quantum optics frontier problem, this doctoral thesis has accomplished the following three works. 1. We have constructed a 85Rb two-dimensional magneto-optic trap with an optical depth of 100. Magneto-optic traps are widely used in quantum optics and atomic and molecular physics to provide cold atomic sources for basic scientific research. Our two-dimensional magneto-optical trap is characterized by its high optical depth and low atomic ground state decoherence along the long axis, which is suitable for the preparation of narrow linewidth nonclassical correlated photon pairs. In cold atomic ensemble, the DLCZ scheme is used to generate correlated photon pairs. The writing light is first applied to the atomic ensemble, and the collective excited state of the atom, that is, the spin wave of the atom, is prepared by the Raman process, and a Stokes photon is produced at the same time. The time width of the two-photon wave function can be adjusted by the pulse width of writing light. The reading time of the anti-Stokes photon is determined by the action time of the read light, so the delay time of the photon pair can be controlled arbitrarily within the lifetime of the atomic spin wave. The photon pair produced in this way can be written as the product of two photon wave functions on the wave function, and they are independent of each other in the time and frequency domain. In a cold atomic cluster with high optical thickness, a narrow linewidth photon pair is prepared by spontaneous four-wave mixing under the action of continuous light, and the pair can be represented by a wave function in the dimension of time and frequency, so it has time-frequency entanglement. The anti-Stokes photon in the photon pair is delayed through a narrow-band electromagnetic induced transmission window. The main factors affecting the two-photon coherence time are the optical depth of atomic ensemble, the Rabi frequency 惟 _ c of the OD, coupling light, and the ground state decoherence rate 纬 _ (12). In a certain range, the larger the OD, the lower the Rabi frequency of the coupling light and the longer the coherent time. When the Rabi frequency of the coupled light is reduced to a certain extent and the absorption of the transmission window induced by electromagnetic transmission cannot be neglected, the ground state decoherence rate 纬 12 is the only factor limiting the coherence time, which determines the limit of the photon pair coherence time. By using the cold atomic ensemble with optical thickness up to 100, under the condition that 纬 12 is the low decoherence rate of 283kHz, the coherence time of photon pair for the first time exceeds 2 microseconds (up to 2.34 渭 s).
【学位授予单位】：华东师范大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O431.2

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