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.     

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.     

Experimental study of vapor-cell magneto-optical traps for efficient trapping of radioactive atoms

We have studied magneto-optical traps (MOTs) for efficient on-line trapping of radioactive atoms. After discussing a model of the trapping process in a vapor cell and its efficiency, we present the results of detailed experimental studies on Rb MOTs. Three spherical cells of different sizes were used. These cells can be easily replaced, while keeping the rest of the apparatus unchanged: atomic sources, vacuum conditions, magnetic field gradients, sizes and power of the laser beams, detection system. By direct comparison, we find that the trapping efficiency only weakly depends on the MOT cell size. It is also found that the trapping efficiency of the MOT with the smallest cell, whose diameter is equal to the diameter of the trapping beams, is about 40% smaller than the efficiency of larger cells. Furthermore, we also demonstrate the importance of two factors: a long coated tube at the entrance of the MOT cell, used instead of a diaphragm; and the passivation with an alkali vapor of the coating on the cell walls, in order to minimize the losses of trappable atoms. These results guided us in the construction of an efficient large-diameter cell, which has been successfully employed for on-line trapping of Fr isotopes at INFN’s national laboratories in Legnaro, Italy.     

高精细度光学微腔中原子的偶极俘获

利用梯度光场产生的光学偶极力对原子的作用是实现原子俘获的重要途径.分析高精细 度光学微腔中的偶极阱，讨论了由腔内驻波场、侧向和横向约束光构成光学势阱的特性，说 明在高精细度光学微腔中可以产生尺度为亚微米，阱深为mK量级的纯光学阱，并获得单原子 与光场的强耦合作用.还讨论了激光线宽对微腔中偶极阱阱深的影响. 关键词： 光学微腔 偶极俘获 单原子     

Magnetic trapping of long-lived cold Rydberg atoms

We report on the trapping of long-lived strongly magnetized Rydberg atoms. 85Rb atoms are laser cooled and collected in a superconducting magnetic trap with a strong bias field (2.9 T) and laser excited to Rydberg states. Collisions scatter a small fraction of the Rydberg atoms into long-lived high-angular momentum "guiding-center" Rydberg states, which are magnetically trapped. The Rydberg atomic cloud is examined using a time-delayed, position-sensitive probe. We observe magnetic trapping of these Rydberg atoms for times up to 200 ms. Oscillations of the Rydberg-atom cloud in the trap reveal an average magnetic moment of the trapped Rydberg atoms of approximately -8microB. These results provide guidance for other Rydberg-atom trapping schemes and illuminate a possible route for trapping antihydrogen.     

利用原子的塞曼光谱对半导体激光器进行稳频

中性原子的超精细能级在磁场中产生塞曼分裂 ,另外 ,左旋和右旋圆偏振光激发下原子的跃迁选择定则不同 ,因此 ,原子在超精细塞曼能级间的吸收谱峰相对无磁场条件下的吸收谱峰有一定的移动。利用这一点 ,验证了一种简单、灵活的方法对半导体激光器进行稳频 ,使激光器的线宽稳到小于 1MHz。通过对实验结果的分析 ,发现由左旋和右旋圆偏振光激发引起的原子吸收谱峰移动之和与饱和吸收峰半高宽相等时 ,稳频效果最好     

Quantum statistics of field after double-beam two-photon absorption

Two-photon absorption for a collection of unexcited two-level atoms from two beams of light is considered and the statistical properties of the beams after the interaction are investigated. The dynamical behaviour of the beams is described by assuming the atoms maintained with a nearly fixed population distribution. By applying iteration methods the time development for the paired generating functional of the field is obtained and by means of this generating functional paired modal densities are written. The modal densities are then used for discussing the statistical properties of light after the interaction.     

Laser frequency stabilization for atom cooling and magnetic-field compression of the trap

We report a frequency stabilization technique of a diode laser in a Doppler-free atomic transition used for obtaining the magneto-optical trapping of Rb atoms. This technique, based on side locking to an atomic transition using a servo controller, is very simple and can be implemented straightforwardly to lock the laser at the red detuned frequency position required for laser cooling experiments. The number of trapped atoms and the temperature of the cold cloud have been determined. The effect of trapping the magnetic field on the cloud radius has also been analyzed.     

Production and guidance of pulsed atomic beams on chip

We demonstrated two experimental methods of producing and guiding pulsed atomic beams on chip. One is to trap atoms first in a U-type magneto-optical trap on the chip, then transfer them to the magnetic guide field and push them simultaneously by a continuous force from the power imbalance of the magneto-optical trap laser beams hence the pulsed cold atom beams are produced and move along the magnetic guide to the destination. The other is to trap atoms directly by a H-type magneto-optical trap, then push them to make them move along the magnetic guide field, thus high rate cold atom beams can be produced and guided on the chip.     

Directly Trapping Atoms in a U-Shaped Magneto-Optical Trap Using a Mini Atom Chip

We experimentally demonstrate the trapping of ^85Rb atoms directly on a chip-size U-shaped magneto-optical trap （U-MOT）. The trap includes a U-shaped wire on the chip, two bias magnetic field coils and laser beams. The capture volume of the U-MOT is theoretically calculated, and the trap is experimentally realized. With 2 A current applied to the U-shaped wire and 2-Gauss horizontal bias field, more than 2 × 10^6 atoms are trapped. In contrast with an ordinary mirror-MOT, this U-MOT captures atoms directly from the background, thus the trap size is greatly reduced. Based on this mini trap scheme, it is possible to realize a chip-size atom trap array for quantum information processing.     

A cold 87Rb atomic beam

We demonstrate an experimental setup for the production of a beam source of cold ⁸⁷Rb atoms. The atoms are extracted from a trapped cold atomic cloud in an unbalanced three-dimensional magneto-optical trap. Via a radiation pressure difference generated by a specially designed leak tunnel along one trapping laser beam, the atoms are pushed out continuously with low velocities and a high flux. The most-probable velocity in the beam is varied from 9 m/s to 19 m/s by varying the detuning of the trapping laser beams in the magneto-optical trap and the flux can be tuned up to 4×10⁹ s^-1 by increasing the intensity of the trapping beams. We also present a simple model for describing the dependence of the beam performance on the magneto-optical trap trapping laser intensity and the detuning.     

Trapping of a supersonic beam in a traveling magnetic wave

Here we report on a new approach to the magnetic deceleration of supersonic beams, based on the generation of a propagating wave of magnetic field. Atoms and molecules possessing a magnetic dipole moment, in so-called low field seeking quantum states, are trapped around a node of the propagating wave. The wave travels at a desired velocity in the direction of the supersonic beam, which can be chosen to match a velocity class populated in the beam. An additional quadrupole guide provides transverse confinement, independently of the decelerator itself. Our technique has been conceived to generate a smooth motion of the magnetic wave, which should optimize the efficiency of the trapping during a future Zeeman deceleration of the beam. We demonstrate the trapping of metastable argon atoms in a magnetic wave traveling at selected, constant velocities.     

High contrast atomic magnetometer based on coherent population trapping

We present an experimental and theoretical investigation of the coherent population trapping （CPT） resonance excited on the D1 line of 87Rb atoms by bichromatic linearly polarized laser light. The experimental results show that a lin｜｜lin tran- sition scheme is a promising alternative to the conventional circular-circular transition scheme for an atomic magnetometer. Compared with the circular light transition scheme, linear light accounts for high-contrast transmission resonances, which makes this excitation scheme promising for high-sensitivity magnetometers. We also use linear light and circular light to detect changes of a standard magnetic field, separately.     

Magnetic trapping of strongly-magnetized Rydberg atoms

Effective magnetic moments of drift Rydberg atoms in strong magnetic fields are obtained for different energy and angular-momentum states. Classical two-body trajectory calculations and quantum-mechanical one-body calculations are employed. For heavy atoms such as rubidium, the trapping dynamics can largely be explained by the net magnetic moment due to the cyclotron and the magnetron motion of the Rydberg electron. In light Rydberg atoms such as hydrogen, the intrinsic two-body nature of the dynamics becomes manifest in that the ionic motion significantly contributes to the effective magnetic moment. Also, light drift Rydberg atoms exhibit an anisotropic response to field-inhomogeneities parallel and transverse to the magnetic-field lines. The results are relevant to magnetic trapping of Rydberg atoms in strong-magnetic-field atom traps.     

Coherence of a squeezed sodium atom laser generated from Raman output coupling

The coherence of a squeezed sodium atom laser generated from a Raman output coupler, in which the sodium atoms in Bose-Einstein condensate （BEC） interact with two light beams consisting of a weaker squeezed coherent probe light and a stronger classical coupling light, is investigated. The results show that in the case of a large mean number of BEC atoms and a weaker probe light field, the atom laser is antibunching, and this atom laser is second-order coherent if the number of BEC atoms in traps is large enough.     

Laser-induced fluorescent measurements of magnetic field contoursin a low-pressure discharge

Faraday rotation of a laser beam and emission spectroscopy to resolve Zeeman splitting provide information about the plasma magnetic field, integrated along the line of sight. Information about the local magnetic field strength can be obtained using a dye laser tuned off the center of an atomic or ionic transition by an amount δλ_z. If the absorption linewidth of the transition probed is less than the Zeeman splitting, only those atoms/ions residing in a magnetic field where the Zeeman splitting is δλ_z will resonantly absorb energy from the laser and fluoresce. The feasibility of this magnetic field contour technique was studied in a low-pressure neon discharge. A conductor insulated from the discharge generated a large magnetic field in the discharge free of the Stark broadening effects associated with large plasma currents. The laser-induced fluorescence (LIF) intensity profile measured along the laser beams had peaks at those spatial locations where local magnetic fields, inferred from the conductor current, agreed with the Zeeman shifted wavelength of the laser     

Optical pumping of the Zeeman components in the rubidium vapor

We investigated the optical pumping of the Zeeman components of rubidium atoms, in the presence of the external magnetic field ranging from the geomagnetic up to 130 Gauss. Using the saturated absorption spectroscopy with linearly polarized pump and probe laser beams, the rubidium Doppler-free spectra at different magnetic field strengths were measured. The dips (negative intensity signals) in the saturated absorption spectra of the ⁸⁷Rb hyperfine transition lines were observed. They come as a result of the alignment process induced by the incoherent population transfer due to the hyperfine optical pumping. By inspection of the dips for different magnetic field strengths we were able to conclude about the dynamics of the alignment process in the external magnetic field. Present work is a part of the investigations concerning the influence of the magnetic field on the velocity selective optical pumping of the rubidium atoms induced by femtosecond frequency comb [D. Aumiler, T. Ban, H. Skenderovi?, G. Pichler, Phys. Rev. Lett. 95 (2005) 233001; T. Ban, D. Aumiler, H. Skenderovi?, G. Pichler, Phys. Rev. A 73 (2006) 043407].     

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.     

Umbesetzung der 2S 1/2-Hyperfeinstrukturniveaus im metastabilen Deuteriumatomstrahl einer Lambshift-Quelle durch Anwendung von statischen Magnetfeldern

Polarized deuteron beams may be obtained from metastable atomic beams by quenching three of the six hyperfinestructure levels. Using a field distribution which crosses zero on the beam axis the occupation numbers of the hyperfinestructure levels may be changed by a nonadiabatic process. The special conditions for this magnetic field are discussed by comparing the corresponding Larmor frequency of the metastable atoms with the angular frequency of the magnetic field vector. Experimental investigations were carried out for field gradients between 1.9 and 5.4 Oe/cm and for superposed homogeneous transverse fields of up to 6 Oe. With deuterons a tensorpolarization ofP ₃₃=?0.70±0.01 was obtained at a negative ion beam current of 42 nA.     

Near-resonant Rayleigh scattering and atomic flame flourescence spectroscopy

Near-resonant Rayleigh scattering and collision-induced resonance fluorescence of sodium have been observed in a combustion environment at atmospheric pressure. Sodium atoms in the equilibrium region of a flat-flame burner were excited by a pulsed-dye laser. The behavior of the two signals as the laser was detuned from the resonance line was studied and was found to agree with theoretical predictions. The results have important implications for atomic flame-fluorescence spectroscopy in that trapping is avoided at large atom number densities.