光致原子解吸附对冷原子磁光阱装载的动力学研究

在获得光致原子解吸附(light-induced atom desorption,LIAD)效应的基础上,从理论和实验方面分析了LIAD对铯原子磁光阱装载的动力学过程的影响,特别是背景原子对磁光阱的影响.通过实验获得了不同光强和照射时间下关闭解吸附光后磁光阱中铯原子的衰减过程,理论模型定量地描述了背景铯原子造成压强的变化及其对最终平衡态下真空度的影响.该研究对中性原子的长时间俘获,有效控制磁光阱中原子的装载过程具有重要意义. 关键词： 光致原子解吸附 磁光阱 激光冷却与俘获     

强耦合腔量子电动力学中单原子转移的实验及模拟

对于强耦合腔量子电动力学系统中以自由下落方式转移原子与腔模强耦合作用过程进行了实验研究,并在理论上利用蒙特卡罗方法对整个实验过程进行了模拟.根据模拟的高精度光学微腔实时记录的原子穿腔信号,获得了原子与腔模相互作用以及冷原子的参数等基本信息,包括不同初始条件下原子与腔模相互作用时腔的透射谱、单个原子在腔内的驻留时间、原子到达腔模时刻的概率分布以及原子到达腔模的动能分布等,并作为对比给出了相应的实验结果.基于模拟结果,实验上建立了腔内光学偶极阱来俘获单个原子,测量的单原子的腔内俘获寿命达到5 ms,比自由穿越时延长了约30倍.该研究对于原子-腔受限空间内,以自由下落方式转移原子以及原子与腔的耦合过程给出详细的分析,有助于对类似实验结果的分析和系统参数的优化.     

Revision of single atom local density and capture number varying with coverage in uniform depletion approximation and its effect on coalescence and number of stable clusters

In vapour deposition,single atoms (adatoms) on the substrate surface are the main source of growth.The change in its density plays a decisive role in the growth of thin films and quantum size islands.In the nucleation and cluster coalescence stages of vapour deposition,the growth of stable clusters occurs on the substrate surface covered by stable clusters.Nucleation occurs in the non-covered part,while the total area covered by stable clusters on the substrate surface will gradually increase.Carefully taking into account the coverage effect,a revised single atom density rate equation is given for the famous and widely used thin-film rate equation theory,but the work of solving the revised equation has not been done.In this paper,we solve the equation and obtain the single-atom density and capture number by using a uniform depletion approximation.We determine that the single atom density is much lower than that evaluated from the single atom density rate equation in the traditional rate equation theory when the stable cluster coverage fraction is large,and it goes down very fast with an increase in the coverage fraction.The revised equation gives a higher value for the ’average’ capture number than the present equation.It also increases with increasing coverage.That makes the preparation of single crystalline thin film materials difficult and the size control of quantum size islands complicated.We also discuss the effect of the revision on coalescence and the number of stable clusters in vapour deposition.     

Detecting a single atom in a cavity using the χ(2) nonlinear medium

We propose a protocol for detecting a single atom in a cavity with the help of the χ⁽²⁾ nonlinear medium. When the χ⁽²⁾ nonlinear medium is driven by an external laser field, the cavity mode will be squeezed, and thus one can obtain an exponentially enhanced light-matter coupling. Such a strong coupling between the atom and the cavity field can significantly change the output photon flux, the quantum fluctuations, the quantum statistical property, and the photon number distributions of the cavity field. This provides practical strategies to determine the presence or absence of an atom in a cavity. The proposed protocol exhibits some advantages, such as controllable squeezing strength and exponential increase of atom-cavity coupling strength, which make the experimental phenomenon more obvious. We hope that this protocol can supplement the existing intracavity single-atom detection protocols and provide a promise for quantum sensing in different quantum systems.     

Joint Remote State Preparation of a Single-Atom Qubit State via a GHZ Entangled State

We proposed a physical protocol for the joint remote preparation of a single-atom qubit state via a three-atom entangled GHZ-type state previously shared by the two senders and one receiver. Only rotation operations of single-atom, which can be achieved though the resonant interaction between the two-level atom and the classical field, are required in the scheme. It shows that the splitting way of the classical information of the secret qubit not only determines the success of reconstruction of the secret qubit, but also influences the operations of the senders.     

Study of two types of Ir or Rh covered single atom pyramidal W tips

The growth of a single-atom sharp pyramid tips on a tungsten substrate by depositing noble metals of the Pt family was investigated by field ion microscopy earlier. The Pd or Pt covered single-atom W pyramidal tips have been successfully prepared and established by our group. They are thermally and chemically stable and can easily be regenerated. In this study, we report the establishing and structural analysis of Rh and Ir covered single-atom W pyramidal tips. Two types of stable structures with bcc {2 1 1} facets are found for both metals. One is stacked by 1, 3, 10 atoms, and the other is stacked by 1, 6, 15 atoms for the top three layers and so on in series from the top to the deeper layers. The single atom tip is destroyed layer by layer after field evaporation to observe the structures of the different layers. However, the tip can be regenerated after it is annealed again and the two types of structures appear systematically depending on the annealing temperature. The regeneration process is investigated and the growth parameters of the two different types of Rh or Ir covered W tip are determined. The differences in the activation barrier and binding energy of these two types are calculated to be 0.08 eV and 0.064 eV for Rh covered single atom tips, and 0.03 eV and 0.14 eV for Ir covered single atom tips, respectively. Possible mechanisms and the relevance for application are discussed.     

Enhanced cooling of a V-type three-level atom in an optical cavity

We have studied theoretically and numerically the enhanced cooling of a V-type three-level atom in a high-finesse optical cavity and shown that the cooling rate can be increased by one order of magnitude over that of a two-level atom, and the momentum amplitude tends to a stationary state much smaller than that of a two-level atom. We have further shown that the cooling rate can be significantly improved by using feedback and a time-dependent pump.     

Efficient generation of cold atoms towards a source for atom lithography

We report on the efficient generation of cold rubidium atoms as a potential coherent atom source for atom lithography. We successfully trapped and cooled 2.6 × 10⁸ atoms in 5 s with a conventional magneto-optical trap simply by enlarging the diameter of the laser beam to 20 mm. The size of the laser-cooled atom cloud was measured to be 10 × 7 × 7 mm³. The number of trapped atoms was approximately 10 times as large as that of previous typical results, while the loading time of atoms remained the same.     

基于低Q腔光子Faraday旋转的远程态制备

基于低Q腔中单光子的输入与输出关系,提出了利用偏振光Faraday旋转分别遥远制备单原子态和两原子纠缠态的可行方案.研究结果表明,当初始原子态的系数为实数时,通过选择合适的偏振光、腔场与原子相互作用系统的参数,单原子态与两原子纠缠态的远程制备均可确定性地得以实现.与以前的原子态远程制备方案相比,本文方案采用光子作为飞行比特来传递量子信息,故原则上可实现原子态的真正长距离制备.由于原子态的信息编码在耗散单边腔囚禁的Λ型三能级原子的两个基态能级,且原子仅虚激发,因此本文方案对腔衰减和原子自发辐射不敏感.此外,本文所提出的两种方案不需要两体或多体正交测量,仅涉及单体直积态测量,而且两种方案都工作在低Q腔,不需要原子与光腔的强耦合,从而有效降低了实验难度.     

High efficient scheme for remote state preparation with cavity QED

In this paper, a scheme is proposed for remote state preparation （RSP） with cavity quantum electrodynamics （QED）. In our scheme, two observers share two-atom nonmaximally entangled state as quantum channels and can realize remote preparation of state of an atom. We also propose a generalization for remote preparation of N-atom entangled state by （N＋1）-atom GHZ-like state （N ≥ 2）. By this scheme, one single-atom projective measurement is enough for the RSP of a qubit or N-atom entangled state, and the probability of success for RSP is unity. Furthermore, we have considered the case where observers use W-like state as quantum channels to realize RSP of a qubit. We compare our scheme with existing ones.     

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.     

Direct loading of atoms from a macroscopic quadrupole magnetic trap into a microchip trap

We demonstrate the direct loading of cold atoms into a microchip 2-mm Z-trap, where the evaporative cooling can be performed efficiently, from a macroscopic quadrupole magnetic trap with a high loading efficiency. The macroscopic quadrupole magnetic trap potential is designed to be moveable by controlling the currents of the two pairs of anti-Helmholtz coils. The cold atoms are initially prepared in a standard six-beam magneto-optical trap and loaded into the macroscopic quadrupole magnetic trap, and then transported to the atom chip surface by moving the macroscopic trap potential. By means of a three-dimensional absorption imaging system, we are able to optimize the position alignment of the atom cloud in the macroscopic trap and the microchip Z-shaped wire. Consequently, with a proper magnetic transfer scheme, we load the cold atoms into the microchip Z-trap directly and efficiently. The loading efficiency is measured to be about 50%.This approach can be used to generate appropriate ultracold atoms sources, for example, for a magnetically guided atom interferometer based on atom chip.     

Integrated optical components on atom chips

We report on the integration of small-scale optical components into silicon wafers for use in atom chips. We present an on-chip fibre-optic atom detection scheme that can probe clouds with small atom numbers. The fibres can also be used to generate microscopic dipole traps. We describe our most recent results with optical microcavities and show that a sufficiently high finesse can be achieved to enable single-atom detection on an atom chip. The key components have been fabricated by etching directly into the atom chip silicon substrate.     

Superradiant cascade emissions in an atomic ensemble via four-wave mixing

We investigate superradiant cascade emissions from an atomic ensemble driven by two-color classical fields. The correlated pair of photons (signal and idler) is generated by adiabatically driving the system with large-detuned light fields via four-wave mixing. The signal photon from the upper transition of the diamond-type atomic levels is followed by the idler one which can be superradiant due to light-induced dipole–dipole interactions. We then calculate the cooperative Lamb shift (CLS) of the idler photon, which is a cumulative effect of interaction energy. We study its dependence on a cylindrical geometry, a conventional setup in cold atom experiments, and estimate the maximum CLS which can be significant and observable. Manipulating the CLS of cascade emissions enables frequency qubits that provide alternative robust elements in quantum network.     

Addressing individual atoms in optical lattices with standing-wave driving fields

A scheme for addressing individual atoms in one- or two-dimensional optical lattices loaded with one atom per site is proposed. The scheme is based on position-dependent atomic population transfer induced by several standing-wave driving fields. This allows various operations important in quantum-information processing, such as manipulation and measurement of any single-atom, two-qubit operations between any pair of adjacent atoms, and patterned loading of the lattice with one atom per every nth site for arbitrary n. The proposed scheme is robust against considerable imperfections and actually within reach of current technology.     

An atom and a photon

The coherent control of single-photon emitters as, e.g., single ions or atoms, is a crucial element for mapping quantum information between light and matter. The possibility of generating entanglement between a photon and the emitter system provides an interface between matter-based quantum memories and photonic quantum communication channels, which is the essential resource for quantum repeaters and other future quantum information applications. To generate entangled atom-photon states, in our experiment, we store a single ⁸⁷Rb atom in an optical dipole trap. The single-atom/single-photon character is confirmed by the observation of photon antibunching in the detected fluorescence light. The spectral properties of single photons emitted by the atom allowed us to determine the mean kinetic energy of the atom corresponding to 105 μK. We describe a single-atom state analysis method which allowed us to characterize the entanglement between the atom and a single photon emitted in the spontaneous decay. We obtain an entanglement fidelity of 89% that clearly shows the high degree of entanglement in our system and potential for further applications in quantum communication.     

Two-Channel Skyrme–Hartree–Fock Model for Bose–Einstein Condensate Near Feshbach Resonance

We apply a two-channel Skyrme–Hartree–Fock model to describe an atomic Bose–Einstein condensate near a Feshbach resonance. In this model the single-atom wave-function has two components corresponding to the two intrinsic states of the atom related to the Feshbach resonance. From the variational principle we derive the corresponding system of two coupled equations for the single-atom wave-function—a generalization of the Gross–Pitaevskii equation. We carry out an exploratory gaussian variational calculation and show that the two-component model can successfully describe the collapse of the condensate near a Feshbach resonance.     

Reliable lateral manipulation of a single Ag adatom on a Ag(1 1 1) surface with a trimer-apex tip

We study the reliability of the lateral manipulation of a single Ag adatom on a Ag(1 1 1) surface with the single-atom and trimer-apex tips based on molecular statics simulations using surface embedded-atom-method potential. The dependence of the manipulation reliability on tip height and orientation is investigated. For the single-atom tip the manipulation reliability increases monotonically with decreasing tip height, which is owing to the strengthened lateral tip-adatom interaction as the tip height lowers. For the trimer-apex tip, the manipulation reliability is sensitive to the tip orientation in the lower tip-height range, while in the higher tip-height range the manipulation reliability is independent of the tip orientation and moreover can be greatly improved due to the strong vertical attraction of the tip on the adatom as compared to the single-atom tip. We also compare these results to those for manipulating single Cu adatoms on the Cu(1 1 1) surface, reveal the underlying physics, and propose the method to improve the manipulation reliability for different systems.     

光脉冲序列对三能级原子重力仪测量精度的影响

提出了一种通过改变激光脉冲序列来提高三能级原子重力仪测量精度的方法.利用提出的描 写原子运动的3×3阶矩阵方法，分析了三能级原子与双色光场的正交相互作用，计算了 原子干涉仪中的相位差，得到了原子重力仪的相位差与重力加速度的关系.发现通过选择合 适的光脉冲序列，可以提高三能级原子重力仪的精度. 关键词： 原子光学 原子重力仪 矩阵方法     

一种新颖的全光型表面原子(分子)漏斗

提出了一种产生全光型表面原子(分子)漏斗的新方案.采用红失谐高斯激光束照明由柱面透镜组成的光学系统，可在透镜焦平面附近产生横向漏斗形光强分布，以构成一表面光波导型原子漏斗.计算了漏斗的光强分布及其光学偶极势与偶极力分布.研究结果表明：该原子漏斗可用于冷原子(分子)的表面光波导、分束器和干涉仪以及微阱囚禁的有效装载，因而在集成原子光学及其原子芯片的研究中有着重要的应用. 关键词： 原子漏斗 分子漏斗 光学偶极势 原子芯片