基于冷原子系综的非经典关联光子源研究
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摘要
量子信息技术是实现未来安全通信和高速计算的主要途径,其物理实现将导致现有信息科学模式的重大变革,并且深化我们对客观世界规律的理解。建立广域量子通信网络的关键问题是实现对纠缠资源进行交换传递和纯化的量子中继器,其第一步是要能制备纠缠光子对,并利用光子与更长相干保持时间媒介之间的相互作用,实现纠缠信息的远距离传输。由于冷原子体系具有的优越性,现有的中继器实验方案大都寻求在冷原子系综上实现。
本文的主体分为以下部分:
第一章绪论部分介绍了激光冷却原子的基本原理,冷原子的发展与应用,量子中继器方案和利用原子系综制备纠缠光子对等背景知识,阐述了本文工作的目的。
然后是我们开展的一系列实验工作。
第二章,为了得到制备纠缠光子对所必需的冷原子团,我们构建了磁光阱系统,包含了超高真空腔体,磁光阱光路,背景磁场和监视装置等,另外为了保证后续实验的完成,我们实现了可调谐外腔式半导体激光器的稳频移频技术和周期性关断磁场和光路的时序控制电路,在具备了以上条件的前提下,先后制得了稳定的~(85)Rb和~(87)Rb原子团。在获得冷原子团之后,利用荧光法和吸收法对冷原子团的基本参数进行了测量,通过增大冷却光强、囚禁光尺寸,改进铷单质释放源,选择合适的磁场梯度、冷却光频率失谐量等方法提高了冷原子团的光学厚度,为进一步的实验提供了基础。
第三章,冷原子的四波混频。模式匹配的好坏是制各非经典关联光子对方案的关键。在弱光探测条件下难以实现一对散射光空间方向的直接选择,因此我们利用双Λ能级结构原子中的四波混频过程搭建了可用于拉曼散射光子对探测的光路,实验证实混频效率最大时也即最优的拉曼散射光空间匹配情况。我们测量了冷Rb原子的吸收光谱,利用Λ型结构原子在不同基态上的相干布居效应,观察到电磁诱导透明现象,再联合与不同激发态能级共振的控制光场,在共线和非共线情形下,分别制备了较强的四波混频光场,测定了冷原子中三阶非线性极化的能量转换效率,分析了影响混频效率的因素。
第四章,非经典关联光子对的制备和检验。在撤去耦合光场,探测光频率锁定于一定失谐位置后,在原来的混频光位置上可以收集到向相反方向发射的单光子量级的拉曼散射光。通过对这样一对方向上的Stokes和反Stokes光子进行符合探测后我们发现其归一化交叉关联函数与自关联函数可以违背Cauchy-Schwartz不等式,证明它们之间存在非经典关联性。我们分别改变了探测光频率失谐量,探测光强,控制光强以及前后向散射光收集光路上的偏振片方向,测量了关联度与以上实验参数的关系,并与理论计算作了比较。
第五章,单光子与原子系综激发态的关联。同时作用的探测光和控制光还不能表明原子中一定产生相应的原子激发。我们错开控制光与探测光的作用次序,时序控制的写光脉冲在原子中激发出反Stokes光子被收集,经过一段可控延迟后的读光脉冲使冷原子团中的集体自旋激发态还原为沿反向传播的Stokes光子,我们对这两种光子进行符合测量,发现在一定延迟时间下仍然可以满足对Cauchy-Schwartz不等式的违背,这表明这种非经典关联性在原子系综中被保持,或者可以说写光激发的反Stokes光子与原子中的集体自旋激发态具备了非经典关联性。
另外本文还包含了一些理论工作,第六章提出了两种利用原子系综和线性光学方法几率性制备GHZ态的方案,该方案具有对位相涨落不敏感的优点,更易于在实验上被实现。
本文的创新点在于研究了利用自发四波混频制备非经典关联光子对过程中写光失谐等参数对获得的二阶关联函数的影响,实验结果和理论预言作了比较,为选择合适的写光参数最大程度优化关联光子对提供了基础。在原子团具有一定光学厚度和收集光子具有固定的耦合传输损失率条件下,增大探测光失谐量和减小探测光强以降低写光对基态原子的激发几率,是提高二阶互关联度的方法,但两者都会伴随单路光子计数率的下降,从而增加背景噪声的影响,如何选择合适的激发光参数,是保证关联或纠缠资源得到最大利用的关键。
Building the quantum repeater for entanglement swapping,transmission and purification is a key point to realize long distance communication.First step is achieving effective transformation of the entanglement from the photon carrier to other media in which coherent information could be preserved much longer,that is, quantum memory.For some advantages,most of the schemes are based on atomic ensembles.
In this article,the design of the whole experimental setup for cooling and trapping the atoms is described,which consists of the ultra-high vacuum system,the light path and background magnetic field for the MOT,the monitor system for real time observation,especially,the frequency stabilization and frequency shift of the diode lasers,which is very important to obtain a stable cold atomic cloud.
The fluorescence intensity and absorption rate is measured to know how many atoms are trapped in the cloud.The optical depth,which is of great importance of the subsequent experiments,is greatly improve by replacing the Rb source with a better one,increasing the intensity and diameter of the trapping beams and choosing proper frequency detuning of the light.
Spatial mode coupling is important for the generation and collection of non-classical correlated photon pairs.But it is difficult to achieve mode coupling in actual experiments as the scattering light is very weak.So we establish a stimulated Raman scattering set-up via four-wave mixing process in a double A-type atomic ensemble.Subsequent experiments verify the relation between spatial mode coupling and the efficiency of the wave mixing.As the probing and coupling beams are applied on the atoms simultaneously,the electromagnetically induced transparency signal is observed due to the coherent population trapping effects.The mixing light is collected and measured while the third pumping field exists,both in the collinear and non-collinear situation.
While the coupling field is removed and the frequency of the probing light is locked to off resonance.The weak anti-Stokes signal forward and the Stokes signal backward are collected by single photon diode detectors.The coincidence detecting of them indicates non-classical correlation and violation of the Cauchy-Schwartz inequality.The experimental parameters,such as the intensity and frequency of the input field,the polarization angles,are varied to see their influence on the correlation, which is compared with the theoretic predictions.
The writing and reading light are modulated to pulses by temporal sequence signal.The read pulse is delayed for several hundred nanoseconds.The normalized cross-correlation function of Stokes and anti-Stokes signal is measured and we find non-classical correlation still exists between them,that is,we establish the non-classical correlation between the anti-Stokes photon and the stimulated collective spin excitation mode in the cold atoms.
Some other theoretic work is involved is this article.A scheme of probabilistic and robust preparation of a GHZ-type state via atomic ensembles and linear optics is proposed.
There are some innovations in this article.Non-classical correlated Raman scattering photon pairs are generated in a noncollinear configuration and the influence of the experimental parameters such as the frequency detuning of the writing light on the cross-correlation function is investigated,both in experimental analysis and theoretic calculations.If the optical depth of the atomic cloud and the transmission efficiency of the generated photons cannot be improved a lot,a suitable choice of the parameters is of great importance to the non-classical property of the correlation source.
本文的主体分为以下部分:
第一章绪论部分介绍了激光冷却原子的基本原理,冷原子的发展与应用,量子中继器方案和利用原子系综制备纠缠光子对等背景知识,阐述了本文工作的目的。
然后是我们开展的一系列实验工作。
第二章,为了得到制备纠缠光子对所必需的冷原子团,我们构建了磁光阱系统,包含了超高真空腔体,磁光阱光路,背景磁场和监视装置等,另外为了保证后续实验的完成,我们实现了可调谐外腔式半导体激光器的稳频移频技术和周期性关断磁场和光路的时序控制电路,在具备了以上条件的前提下,先后制得了稳定的~(85)Rb和~(87)Rb原子团。在获得冷原子团之后,利用荧光法和吸收法对冷原子团的基本参数进行了测量,通过增大冷却光强、囚禁光尺寸,改进铷单质释放源,选择合适的磁场梯度、冷却光频率失谐量等方法提高了冷原子团的光学厚度,为进一步的实验提供了基础。
第三章,冷原子的四波混频。模式匹配的好坏是制各非经典关联光子对方案的关键。在弱光探测条件下难以实现一对散射光空间方向的直接选择,因此我们利用双Λ能级结构原子中的四波混频过程搭建了可用于拉曼散射光子对探测的光路,实验证实混频效率最大时也即最优的拉曼散射光空间匹配情况。我们测量了冷Rb原子的吸收光谱,利用Λ型结构原子在不同基态上的相干布居效应,观察到电磁诱导透明现象,再联合与不同激发态能级共振的控制光场,在共线和非共线情形下,分别制备了较强的四波混频光场,测定了冷原子中三阶非线性极化的能量转换效率,分析了影响混频效率的因素。
第四章,非经典关联光子对的制备和检验。在撤去耦合光场,探测光频率锁定于一定失谐位置后,在原来的混频光位置上可以收集到向相反方向发射的单光子量级的拉曼散射光。通过对这样一对方向上的Stokes和反Stokes光子进行符合探测后我们发现其归一化交叉关联函数与自关联函数可以违背Cauchy-Schwartz不等式,证明它们之间存在非经典关联性。我们分别改变了探测光频率失谐量,探测光强,控制光强以及前后向散射光收集光路上的偏振片方向,测量了关联度与以上实验参数的关系,并与理论计算作了比较。
第五章,单光子与原子系综激发态的关联。同时作用的探测光和控制光还不能表明原子中一定产生相应的原子激发。我们错开控制光与探测光的作用次序,时序控制的写光脉冲在原子中激发出反Stokes光子被收集,经过一段可控延迟后的读光脉冲使冷原子团中的集体自旋激发态还原为沿反向传播的Stokes光子,我们对这两种光子进行符合测量,发现在一定延迟时间下仍然可以满足对Cauchy-Schwartz不等式的违背,这表明这种非经典关联性在原子系综中被保持,或者可以说写光激发的反Stokes光子与原子中的集体自旋激发态具备了非经典关联性。
另外本文还包含了一些理论工作,第六章提出了两种利用原子系综和线性光学方法几率性制备GHZ态的方案,该方案具有对位相涨落不敏感的优点,更易于在实验上被实现。
本文的创新点在于研究了利用自发四波混频制备非经典关联光子对过程中写光失谐等参数对获得的二阶关联函数的影响,实验结果和理论预言作了比较,为选择合适的写光参数最大程度优化关联光子对提供了基础。在原子团具有一定光学厚度和收集光子具有固定的耦合传输损失率条件下,增大探测光失谐量和减小探测光强以降低写光对基态原子的激发几率,是提高二阶互关联度的方法,但两者都会伴随单路光子计数率的下降,从而增加背景噪声的影响,如何选择合适的激发光参数,是保证关联或纠缠资源得到最大利用的关键。
Building the quantum repeater for entanglement swapping,transmission and purification is a key point to realize long distance communication.First step is achieving effective transformation of the entanglement from the photon carrier to other media in which coherent information could be preserved much longer,that is, quantum memory.For some advantages,most of the schemes are based on atomic ensembles.
In this article,the design of the whole experimental setup for cooling and trapping the atoms is described,which consists of the ultra-high vacuum system,the light path and background magnetic field for the MOT,the monitor system for real time observation,especially,the frequency stabilization and frequency shift of the diode lasers,which is very important to obtain a stable cold atomic cloud.
The fluorescence intensity and absorption rate is measured to know how many atoms are trapped in the cloud.The optical depth,which is of great importance of the subsequent experiments,is greatly improve by replacing the Rb source with a better one,increasing the intensity and diameter of the trapping beams and choosing proper frequency detuning of the light.
Spatial mode coupling is important for the generation and collection of non-classical correlated photon pairs.But it is difficult to achieve mode coupling in actual experiments as the scattering light is very weak.So we establish a stimulated Raman scattering set-up via four-wave mixing process in a double A-type atomic ensemble.Subsequent experiments verify the relation between spatial mode coupling and the efficiency of the wave mixing.As the probing and coupling beams are applied on the atoms simultaneously,the electromagnetically induced transparency signal is observed due to the coherent population trapping effects.The mixing light is collected and measured while the third pumping field exists,both in the collinear and non-collinear situation.
While the coupling field is removed and the frequency of the probing light is locked to off resonance.The weak anti-Stokes signal forward and the Stokes signal backward are collected by single photon diode detectors.The coincidence detecting of them indicates non-classical correlation and violation of the Cauchy-Schwartz inequality.The experimental parameters,such as the intensity and frequency of the input field,the polarization angles,are varied to see their influence on the correlation, which is compared with the theoretic predictions.
The writing and reading light are modulated to pulses by temporal sequence signal.The read pulse is delayed for several hundred nanoseconds.The normalized cross-correlation function of Stokes and anti-Stokes signal is measured and we find non-classical correlation still exists between them,that is,we establish the non-classical correlation between the anti-Stokes photon and the stimulated collective spin excitation mode in the cold atoms.
Some other theoretic work is involved is this article.A scheme of probabilistic and robust preparation of a GHZ-type state via atomic ensembles and linear optics is proposed.
There are some innovations in this article.Non-classical correlated Raman scattering photon pairs are generated in a noncollinear configuration and the influence of the experimental parameters such as the frequency detuning of the writing light on the cross-correlation function is investigated,both in experimental analysis and theoretic calculations.If the optical depth of the atomic cloud and the transmission efficiency of the generated photons cannot be improved a lot,a suitable choice of the parameters is of great importance to the non-classical property of the correlation source.
引文
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