1.National Time Service Center, Chinese Academy of Sciences, Xi’an 710600, China 2.Key Laboratory of Time and Frequency Primary Standards, Chinese Academy of Sciences, Xi’an 710600, China 3.University of Chinese Academy of Sciences, Beijing 100049, China
Fund Project:Project supported by the National Key R&D Program of China (Grant No.2016YFF0200202)
Received Date:22 January 2019
Accepted Date:05 July 2019
Available Online:01 October 2019
Published Online:05 October 2019
Abstract: The Gaussian radius and temperature of cold atomic cloud are important parameters in describing the state of cold atoms. The precise measuring of these two parameters is of great significance for studying the cold atoms. In this paper, we propose a new method named knife-edge to measure the Gaussian radius and temperature of the cold atomic cloud. A near-resonant and supersaturated laser beam, whose size is controlled by a knife-edge aperture, is used to push away the cold atoms in the free falling process of cold atomic cloud. By detecting the intensity of fluorescence signal, the numbers of residual atoms under different-sized near-resonant beams can be obtained. According to the characteristic of cold atoms′ distribution, we construct a theoretical model to derive the Gaussian radius of cold atomic cloud from the recorded residual atom number and near-resonant beam size. Since the Gaussian radius and temperature of cold atomic cloud are associated with each other, we can finally obtain the temperature of cold atomic cloud through the recorded residual atom number and beam size. By using this method, we successfully measure the Gaussian radii of cold atomic cloud at the heights of 10 mm and 160 mm below the center of 3D-MOT (three dimensional magneto-optical trap) to be (1.54 ± 0.05) mm and (3.29 ± 0.08) mm, respectively. The corresponding temperature of cold atomic cloud is calculated to be (7.50 ± 0.49) μK, which is well consistent with the experimental result obtained by using the time-of-flight method under the same condition. This experiment is conducted on the platform of Cesium atomic fountain clock of National Time Service Center, China. Keywords:knife-edge method/ cold atomic cloud/ Gaussian radius/ cold atomic cloud′s temperature
由(5)式可以得到不同冷原子团高斯半径下, 归一化剩余原子数N0与刀口光阑位置d的关系. 如图2所示, 随着刀口光阑向右移动, 剩余原子数目不断减小, 且冷原子团高斯半径越小, 剩余原子数目减小的速度就越快(对应图中曲线梯度更大). 刀口光阑位于最左端时, 剩余原子数为1, 没有原子被推除; 刀口光阑位于最右端时, 剩余原子数为0, 所有原子均被推除. 同时可以看出, 这5条曲线相交于一点(0, 0.5)处, 即刀口光阑位于冷原子团正中心时, 剩余原子数为总原子数的一半. 因此实验中可以通过刀口法测量冷原子团剩余原子数目与刀口位置关系, 拟合得到冷原子团高斯半径. 图 2 不同高斯半径的剩余原子数与刀口位置关系 Figure2. The residual atom number versus knife-edge position with different Gaussian radii.
国家授时中心铯原子喷泉装置在文献[25]中有过详尽描述, 在此系统上选择合适窗口搭建测温装置, 便可实现对冷原子团高斯半径和温度的测量, 如图3所示. 在由六束呈(1,1,1)结构的对射冷却光和反亥姆霍兹线圈组成的磁光阱中制备好冷原子团后, 关闭磁场使原子团自由膨胀35 ms获得冷原子黏团, 并对其进行偏振梯度冷却, 30 ms后关闭冷却光自由释放冷原子团. 在磁光阱中心正下方h1 = 10 mm和h2 = 160 mm处分别沿y方向横向打入一束扁平过饱和近共振推除光, 纵截面尺寸均为2 mm × 20 mm、功率为4 mW、频率锁定在铯原子D2线62S1/2F = 4 → 62P3/2F' = 5的跃迁线上, 推除光作用于原子后可使其偏离原来飞行轨道而不能到达预定探测区间. 将两个安装在精密位移台上的方形横向偏置刀口光阑沿x方向分别架设在两束推除光正前方, 两刀口光阑尺寸均为16 mm × 16 mm. 在磁光阱中心正下方h3 = 170 mm处, 将一束经由0°高反镜形成纵截面尺寸为0.5 mm × 40 mm的扁平驻波探测激光束沿y方向横向打入探测区真空腔, 功率为2 mW, 频率与推除光频率相同. 与探测光相垂直的x方向安装透镜组和光电探测器, 对飞行时间信号进行采集与探测. 光电探测器由Thorlabs公司生产, 型号为PDA36A2, 选用增益为60 dB的档位, 对应响应时间为18 μs, 探测到的原子数目与采集到的原子飞行时间信号的积分成正比[26]. 图 3 刀口法测量冷原子团温度实验装置简图 Figure3. The schematic diagram of experimental setup for measuring cold atomic cloud′s temperature by knife-edge method.
实验中, 首先调节两个精密位移台, 使两个刀口光阑均完全遮住对应位置处推除光, 此时飞行时间信号最强. 调节上面精密位移台, 使刀口光阑沿x方向以步进Δx = 0.5 mm逐渐移动, 来控制推除光推掉的原子比例. 此时, 随着刀口位置d的增加, 激光束推除的冷原子数目不断增加, 冷原子团的飞行时间信号不断下降. 在每一刀口位置处, 通过多次释放冷原子团自由下落, 并测量剩余原子数后取平均, 得到对应的剩余冷原子数目. 实验结果如图4(a)中圆点所示, 误差棒代表每一刀口位置处剩余原子数的起伏. 按照归一化原子数目与刀口位置关系(5)式拟合得到黑色曲线, 测量得到在磁光阱中心正下方10 mm处冷原子团高斯半径为σ1 = 1.54 mm, 误差为${{{\delta }}_{{{{\sigma }}_{1}}}}=0.05$ mm. 图 4 磁光阱中心正下方10 mm (a)和 160 mm (b)处的剩余原子数与刀口位置关系 Figure4. The residual atom number versus knife-edge position at height 10 mm (a) and 160 mm (b)under the center of magneto-optical trap.
为了验证刀口法测量冷原子团温度的可重复性, 分别应用刀口法和飞行时间法对喷泉系统同一冷却条件下的冷原子团温度进行了10次测量. 实验结果如图6所示, 其中方点表示刀口法测温结果, 圆点表示飞行时间法测温结果. 通过比较可以看出, 刀口法测量得到的冷原子团温度与飞行时间法测量结果均在7 μK左右, 两者偏差小于1 μK. 由于刀口法是针对多个周期原子喷泉信号的测量, 测量时间较长, 原子数目的起伏以及刀口多次往返的位移不均匀都会对测量结果产生影响, 所以刀口法测量得到的冷原子团温度的误差略大于飞行时间法的测量误差. 下一步实验可以通过优化喷泉系统实验参数降低原子数起伏及改善刀口位移的均匀性来提高刀口法测量冷原子团温度的精度. 图 6 刀口法与飞行时间法测量冷原子团温度对比 Figure6. The comparison between knife-edge and time of flight methods in measuring cold atomic cloud′s temperature