Cold atom trap with zero residual magnetic field: the ac magneto-optical trap

A novel atom trap is described using alternating current to generate the magnetic B field, together with high speed polarization switching of the damping laser field. This combination produces a trap as effective as a standard magneto-optical trap (MOT), with the advantage that the average B field is zero. No net current is hence induced in surrounding conductive elements, and the B field produced by the ac MOT is found to switch off >300 times faster than a conventional MOT. New experiments can hence be performed, including charged particle probing or detection of the cold target ensemble.     

磁光阱中单原子荧光信号的优化及单原子的高效装载

实验中首先通过增大四极磁场梯度、提高背景真空度、缩小冷却俘获激光光束直径的方法获得了磁光阱中单原子的装载.其次,通过减小冷却光失谐量、适当增加其光强、同时使用偏振光谱锁频技术抑制冷却光噪声的方法得到了磁光阱中高信号背景比的单原子荧光信号.此外,通过实时反馈控制磁光阱四极磁场梯度的方法,在实验中实现了单原子98%的装载概率.使用Hamburg Brown-Twiss方案测量了磁光阱中的单原子在连续光激发下所辐射荧光的光子统计特性,得到二阶关联度g(2)(τ=0)=0.09.     

中性原子的表面双磁光阱及其应用

提出了一种新的采用载流导线的表面双磁光阱(MOT)方案(即双U型导线磁光阱方案)。通过改变中间U型导线中的电流大小,即可将一个双磁阱连续地合并为一个单磁阱,反之亦然。详细计算和分析了上述双U型载流导线磁光阱方案的磁场及其梯度的空间分布,研究发现当导线中的电流为600 A,z方向均匀偏置磁感应强度为-4.0×10-3 T时,双U型导线方案产生的两个磁阱中心的磁场梯度约为1.5×10-3~2.5×10-3 T/cm,结合通常制备磁光阱时所用的三维粘胶(Molasses)光束即可在基底表面附近形成一双磁光阱。理论分析表明在弱光近似下,每个磁光阱中所能俘获的85Rb原子数约为106 量级,相应的磁光阱温度约为270μK。由于双磁光阱可以独立制备,所以双U型导线方案特别适用于制备双样品磁光阱,并用于研究双原子样品的冷碰撞性质。     

A compact high-efficiency cold atom beam source

We report on a compact high-efficiency Cs slow atom beam source based on a retro-reflected two-dimensional magneto-optical trap (2D MOT). Employing two laser beams in an angled retro-reflected setup, we achieve 3D MOT loading rates greater than 8?×?10⁹?atoms/s using only 20?mW of total laser power for the source.     

Laser cooling without repumping: a magneto-optical trap for erbium atoms

We report on a novel mechanism that allows for strong laser cooling of atoms that do not have a closed cycling transition. This mechanism is observed in a magneto-optical trap (MOT) for erbium, an atom with a very complex energy level structure with multiple pathways for optical-pumping losses. We observe surprisingly high trap populations of over 10(6) atoms and densities of over 10(11) atoms cm(-3), despite the many potential loss channels. A model based on recycling of metastable and ground state atoms held in the quadrupole magnetic field of the trap explains the high trap population, and agrees well with time-dependent measurements of MOT fluorescence. The demonstration of trapping of a rare-earth atom such as erbium opens a wide range of new possibilities for practical applications and fundamental studies with cold atoms.     

Semiclasical model of magneto-optical trap depth

We report the results of trap depth calculations based on the semiclassical theory of interaction of light field with atom in magneto-optical trap (MOT). In our model of magneto-optical trap we take into account Zeeman shift of ground and excited states in local magnetic field and only Doppler cooling force are considered. The calculated trap depth has been compared with value of trap depth estimated from experimental results for ⁷Li.     

A continuous cold atomic beam from a magneto-optical trap

We have developed and characterized a new method to produce a continuous beam of cold atoms from a standard vapour-cell magneto-optical trap (MOT). The experimental apparatus is very simple. Using a single laser beam it is possible to hollow out in the source MOT a direction of unbalanced radiation pressure along which cold atoms can be accelerated out of the trap. The transverse cooling process that takes place during the extraction reduces the beam divergence. The atomic beam is used to load a magneto-optical trap operating in an ultra-high vacuum environment. At a vapour pressure of 10^-8mbar in the loading cell, we have produced a continuous flux of 7×10⁷atoms/s at the recapture cell with a mean velocity of 14 m/s. A comparison of this method with a pulsed transfer scheme is presented. Received 19 February 2001     

Narrow-line magneto-optical cooling and trapping of strongly magnetic atoms

Laser cooling on weak transitions is a useful technique for reaching ultracold temperatures in atoms with multiple valence electrons. However, for strongly magnetic atoms a conventional narrow-line magneto-optical trap (MOT) is destabilized by competition between optical and magnetic forces. We overcome this difficulty in Er by developing an unusual narrow-line MOT that balances optical and magnetic forces using laser light tuned to the blue side of a narrow (8 kHz) transition. The trap population is spin polarized with temperatures reaching below 2 muK. Our results constitute an alternative method for laser cooling on weak transitions, applicable to rare-earth-metal and metastable alkaline earth elements.     

Characterizing optical dipole trap via fluorescence of trapped cesium atoms

Optical dipole trap (ODT) is becoming an important tool of manipulating neutral atoms. In this paper ODT is realized with a far-off resonant laser beam strongly focused in the magneto-optical trap (MOT) of cesium atoms. The light shift is measured by simply monitoring the fluorescence of the atoms in the magneto-optical trap and the optical dipole trap simultaneously. The advantages of our experimental scheme are discussed, and the effect of the beam waist and power on the potential of dipole trap as well as heating rate is analyzed.     

Loading of a krypton magneto-optical trap with two hollow laser beams in a Zeeman slower

A significant enhancement in the number of cold atoms in an atomic-beam-loaded magneto-optical trap (MOT) for metastable krypton atoms is observed when hollow laser beams are used in a Zeeman slower instead of a Gaussian laser beam. In the Zeeman slower setup, a combination of two hollow laser beams, i.e., a variable-diameter hollow beam generated using a pair of axicon lenses superimposed on a fixed-diameter hollow beam, has been used to reduce the longitudinal velocity of the atoms in the atomic beam below the capture speed of the MOT. The observed enhancement in the number of atoms in the MOT is attributed to reduced destruction of the atom cloud in the MOT and increased cooling of the off-axis atoms in the atomic beam, resulting from the use of hollow beams in the Zeeman slower.     

An atom faucet

We present a simple and efficient source of slow atoms. From a background vapour loaded magneto-optical trap (MOT), a thin laser beam extracts a continuous jet of cold rubidium atoms. The jet that is typical to leaking MOT systems is created without any optical parts placed inside the vacuum chamber. We also present a simple three dimensional numerical simulation of the atomic motion in the presence of these multiple saturating laser fields combined with the inhomogeneous magnetic field of the MOT. At a pressure of P _Rb87 = 10^-8 mbar and with a moderate laser power of 10 mW per beam, we generate a flux Φ = 1.3×10⁸ atoms/s with a mean velocity of 14 m/s and a divergence of 10 mrad. Received 13 January 2001     

Improved atom number with a dual color magneto-optical trap

We demonstrate a novel dual color magneto–optical trap (MOT), which uses two sets of overlapping laser beams to cool and trap 87 Rb atoms. The volume of cold cloud in the dual color MOT is strongly dependent on the frequency difference of the laser beams and can be significantly larger than that in the normal MOT with single frequency MOT beams. Our experiment shows that the dual color MOT has the same loading rate as the normal MOT, but much longer loading time, leading to threefold increase in the number of trapped atoms. This indicates that the larger number is caused by reduced light induced loss. The dual color MOT is very useful in experiments where both high vacuum level and large atom number are required, such as single chamber quantum memory and Bose–Einstein condensation (BEC) experiments. Compared to the popular dark spontaneous-force optical trap (dark SPOT) technique, our approach is technically simpler and more suitable to low power laser systems.     

~(87)Rb玻色-爱因斯坦凝聚体的快速实验制备

采用二维磁光阱产生了-个快速~(87)Rb原子流,并在高真空的三维磁光阱中实现了~(87)Rb原子的快速俘获,进一步采用射频蒸发冷却技术实现了原子云的预冷却,然后将原子转移到远失谐的光学偶极阱中蒸发得到了玻色-爱因斯坦凝聚体.实验上可以在25 s内完成三维磁光阱的装载(约1.0×10~(10)个~(87)Rb原子),然后经过16 s的冷却过程最终在光学偶极阱中获得5.0×10~5个原子的玻色-爱因斯坦凝聚体.实验重点研究了二维磁光阱的优化设计和采用蓝失谐大功率光束对四极磁阱零点的堵塞,抑制四极磁阱中原子的马约拉纳损耗,更加有效地对原子云进行预冷却.     

Laser cooling of thulium atoms

We demonstrated laser cooling and trapping of thulium atoms at sub-Doppler temperatures in a magneto-optical trap (MOT). Up to 3 × 10⁶ thulium atoms were trapped in the MOT at temperatures down to 25(5) μK which is approximately 10 times lower than the Doppler limit. The lifetime of atoms in the MOT varied between 0.3–1.5 s and was restricted mostly by optical leaks from the upper cooling level. The lower limit for the leaking rate was estimated to be 22(6) s⁻¹. Due to a big magnetic moment of Tm atoms, a part of them were trapped in a magnetic trap from the quadrupole field of the MOT. We observed about 3 × 10⁴ purely magnetically trapped atoms at temperature of 25 μK with a lifetime in the trap of 0.5 s. Also we set up a “dark” MOT consisting of six crossed hollow beams which increased the number of trapped atoms by a factor of 5 leading to 1.5 × 10⁷ atoms at the expense of higher temperature.     

Miniaturized magnetic guide for neutral atoms

We describe the principle and realization of a miniaturized magnetic guide for neutral atoms. The magnetic guide in our experiment is formed by a micrometer-sized current-carrying wire which is attached to a second, thick wire. The conductors are electrically insulated from each other. The combined magnetic field of both conductors provides an approximately linear trapping potential which establishes a magnetic guide along the surface of the thin wire. The miniaturized waveguide is filled with rubidium atoms from a magneto-optical trap (MOT) by first loading the atoms into a spherical magnetic quadrupole trap which is subsequently transformed into the linear potential of the waveguide. As thermal source for Rb atoms we use an alkali metal dispenser which is located close to the center of the MOT. This novel method is compatible with ultrahigh vacuum conditions and we achieved lifetimes of the magnetically trapped atoms up to 100 s. Received: 18 October 1999 / Published online: 24 March 2000     

中性汞原子磁光阱装载率的优化

量子投影噪声是影响光晶格钟的一个重要参数,提高磁光阱中装载率有利于降低量子投影噪声,可提升光晶格钟的性能.针对实验所用的汞原子单腔磁光阱,本文分析并计算了磁光阱中汞原子受力情况和一维运动规律,在此基础上用随机数方法对磁光阱中汞原子三维装载进行了数值计算,获得了磁光阱中的稳态原子数,研究了磁光阱的冷却激光的光强、失谐量以及磁场梯度等参数对稳态原子数的影响,得出了获得最优装载率的实验参数.涉及的计算方法和结论对汞原子光晶格钟的实验设计具有参考价值.     

用于原子干涉仪实验的锂原子的塞曼减速与磁光囚禁

为了制备适于原子干涉仪实验的低温锂原子样品,开展了锂原子的塞曼减速及与磁光阱囚禁相关的实验研究.设计并实现了一种结构紧凑的腔体内冷式多级线圈叠加的塞曼减速器,将速度小于600 m/s的7Li原子减速到60 m/s,磁光阱装载速率为5×108/s,囚禁原子数目1×109个,原子团的最低温度约为220±30μK.研究了光学黏胶中7Li原子的寿命与囚禁光频率失谐量的关系.这些结果为进一步开展7Li原子亚多普勒冷却、光势阱蒸发冷却以及原子干涉仪实验奠定了基础.     

Single atoms transferring between a magneto-optical trap and a far-off-resonance optical dipole trap

Based on our work on single cesium atoms trapped in a large-magnetic-gradient vapour-cell magneto-optical trap (MOT), the signal-to-noise ratio (SNR) is remarkably improved. Also a far-off-resonance optical dipole trap (FORT) formed by a strongly-focused 1064~nm single frequency Nd:YVO₄ laser beam is introduced. One cesium atom is prepared in the MOT, and then it can transfer successfully between the MOT and the FORT which is overlapped with the MOT. Utilizing the effective transfer, the lifetime of single atoms trapped in the FORT is measured to be 6.9± 0.3~s. Thus we provide a system where the atomic qubit can be coherently manipulated.     

用吸收法对铯原子磁光阱中冷原子数目的测量

介绍了吸收法测量冷原子数的原理以及对铯原子气室磁光阱中俘获的冷原子数目的测量过程及结果. 与通常的荧光收集法相比，在原理上与静止二能级原子同共振单模光场作用的模型更加接近，同时大大减小了测量中的误差累积，提高了测量精度. 测得的冷原子数为(8±0.3)×10⁶，同时还利用测得的阱中俘获的稳态冷原子数和磁光阱中冷原子的寿命间接获得了磁光阱的俘获率. 关键词： 激光冷却与俘获 磁光阱 冷原子数目 俘获率     

应用于铯原子喷泉钟的二维磁光阱研制

理论模拟研究了二维磁光阱原子束流量与饱和蒸汽压、冷却光强、激光失谐量的关系, 构建了二维磁光阱(2D-MOT)装置, 实验上实现了大流量的慢速原子束, 其测量值为2.1× 10⁹/s.利用荧光法测量了各实验参数与流量的关系, 测量结果与数值模拟结果符合较好. 关键词： 2D-MOT 流量 慢速原子束 铯原子喷泉钟