基于人工神经网络在线学习方法优化磁屏蔽特性参数

磁屏蔽在磁场敏感的装置如原子钟、原子干涉仪等精密设备中发挥重要的作用,在变化外磁场下的某个磁屏蔽内部剩余磁场,可以通过Jiles-Atherton磁滞模型和磁屏蔽系数计算得出,根据计算结果可以进行主动补偿来抵消内部磁场的改变.然而实际应用中磁滞模型中五个与磁屏蔽相关的参数以及磁场衰减的两个参数的准确值的获得是比较困难的,通常根据实测磁滞回线人工匹配调节参数会花费大量时间且很难确保最终参数是全局最优值.基于人工神经网络的机器学习方法已经成为一种对复杂模型进行参数优化的有效手段,得益于现代计算机强大的运算能力,该过程通常远远快于人工参数调节,并有更大概率找到全局最优值,获得优于手工调节的参数值.本文利用人工神经网络在线机器学习的方法,对磁滞模型的五个参数与磁屏蔽的另外两个屏蔽相关参数进行优化测定,并对模拟卫星磁场环境下磁屏蔽内剩余磁场进行预测.通过实际测量屏蔽筒内剩余磁场与预测值比对,发现通过机器学习方法得到的磁屏蔽特性参数优于手动找到的参数,且所用时间大大缩短.该结果不仅有助于更好地进行磁场补偿,用于冷原子系统参数优化调整,更重要的是验证了神经网络在多参数物理系统中的应用,可以使其他多参数共同作用的物理实验进行最优参数的快速确定.     

Miniaturized optical system for atomic fountain clock

Using modularized components, we have built a miniaturized optical system for 87Rb atomic fountain clock that is fitted on an 80 cm × 60 cm optical breadboard. Compared with the conventional optical setup on the table, our system is more compact, more robust and miniaturized. Taking advantage of this system, laser beams are transmitted through eight optical fibre patch cords from the optical breadboard to an ultra high vacuum system. This optical setup has operated for five months in our fountain system and required no alignment.     

拉曼喷泉原子钟

利用托曼光场代替喷泉原子钟的微波腔实现拉曼喷泉原子钟.将分离托曼光场技术与冷原子喷泉技术相结合.避免了存真空腔内放置微波腔,简化了真空系统.同时还保持了很高的准确度.采用半经典理论研究了冷原子喷泉与托曼光场的相互作用过程.得到了冉赛(Ramsey)条纹.比较了托曼喷泉原子钟与热铯束拉曼原子钟,前者有更小的体积和功耗,其精度可能达到或超过商用小铯钟.还比较了拉曼喷泉原子钟与微波喷泉原子钟的差别,分析了光子反冲的影响,提出利用同向传播和相向传播的两台拉曼原子钟测最精细结构常数.     

Coherent Scattering of Light by Bose-Einstein Condensation in a Tight Confinement

We have observed strong scattering of a probe light by dilute Bose-Einstein condensate （BEC） ^87Rb gas in a tight magnetic trap. The scattering light forms fringes at the image plane. It is found that we can infer the real size of the condensation and the number of the atoms by modelling the imaging system. We present a quantitative calculation of light scattering by the condensed atoms. The calculation shows that the experimental results agree well with the prediction of the generalized diffraction theory, and thus we can directly observe the phase transition of BEC in a tight trap.     

高密度冷原子云辐射陷获效应造成的荧光减弱现象

实验测量了磁光阱的磁场关断之后的荧光变化,观察到几十ms量级的荧光上升过程. 根据实验条件进行分析,在高密度冷原子云中辐射陷获效应造成额外的非相干抽运光场是造成磁光陷阱紧束缚状态下荧光减弱的原因. 同时还进行了简单的数值模拟,结果表明只有考虑到散射光场的频率重分布效应,计算结果方能与实验符合. 关键词： 冷原子云 辐射陷获 频率重分布     

Optimization of temperature characteristics of a transportable ~(87)Rb atomic fountain clock

A high-performance transportable fountain clock is attractive for use in laboratories with high-precision time-frequency measurement requirements. This Letter reports the improvement of the stability of a transportable rubidium-87 fountain clock because of an optimization of temperature characteristics. This clock integrates its physical packaging, optical benches, microwave frequency synthesizers, and electronic controls onto an easily movable wheeled plate. Two optical benches with a high-vibration resistance are realized in this work. No additional adjustment is required after moving them several times. The Allan deviation of the fountain clock frequency was measured by comparing it with that of the hydrogen maser. The fountain clock got a short-term stability of 2.3 × 10~(-13) at 1 s and long-term stability on the order of 10~(-16) at 100,000 s.     

Loss of cold atoms due to collisions with residual gases in free flight in a magneto-optical trap

The loss rate of cold atoms in a trap due to residual gas collisions differs from that in a free state after the cold atoms are released from the trap. In this paper, the loss rate in a cold rubidium-87 atom cloud was measured in a magneto-optical trap(MOT) and during its free flight. The residual gas pressure was analyzed by a residual gas analyzer, and the pressure distribution in a vacuum chamber was numerically calculated by the angular coefficient method. The decay factor, which describes the decay behavior of cold atoms due to residual gas collisions during a free flight, was calculated. It was found that the decay factor agrees well with theoretical predictions under various vacuum conditions.     

QUIC阱中紧束缚状态下87Rb原子气体的玻色-爱因斯坦凝聚体相变的直接观测

在QUIC阱中经蒸发冷却获得了2×10⁵个|F=2,m_F=2〉态的⁸⁷Rb原子气体的玻色-爱因斯坦凝聚.验证了紧束缚状态下原子云的轴向尺寸的变化作为BEC相变的判据，观察了从热原子气体到玻色-爱因斯坦凝聚的相变过程，测量了自由膨胀过程中BEC的纵横比变化，并和理论预言进行了对比. 关键词： 玻色-爱因斯坦凝聚 激光冷却与囚禁     

Surface Planar Ion Chip for Linear Radio-Frequency Paul Traps

We propose a surface planar ion chip which forms a linear radio frequency Paul ion trap. The electrodes reside in the two planes of a chip, and the trap axis is located above the chip surface. Its electric field and potential distribution are similar to the standard linear radio frequency Paul ion trap. This ion trap geometry may be greatly meaningful for quantum information processing.