Comparative short-term stability for a Cs beam and an expanding cold atomic cloud of Cs used as atomic frequency standards

We have compared the stability of two distinct atomic frequency standards. One is based on an atomic cold cloud of ¹³³Cs atoms in free expansion, interrogated by a Rabi resonance. The other standard is based on a thermal Cs beam interrogated by the separated oscillatory field method (Ramsey’s method). The atomic expanding cloud standard shows better stability and technical advances based on its simplicity.     

A slow gravity compensated atom laser

We report on a slow guided atom laser beam outcoupled from a Bose–Einstein condensate of ⁸⁷Rb atoms in a hybrid trap. The acceleration of the atom laser beam can be controlled by compensating the gravitational acceleration and we reach residual accelerations as low as 0.0027 g. The outcoupling mechanism allows for the production of a constant flux of 4.5×10⁶ atoms per second and due to transverse guiding we obtain an upper limit for the mean beam width of 4.6 μm. The transverse velocity spread is only 0.2 mm/s and thus an upper limit for the beam quality parameter is M ²=2.5. We demonstrate the potential of the long interrogation times available with this atom laser beam by measuring the trap frequency in a single measurement. The small beam width together with the long evolution and interrogation time makes this atom laser beam a promising tool for continuous interferometric measurements.     

Polarized velocity selective spectroscopy of atomic rubidium using counterpropagating beams

A scheme to obtain dispersion-like profiles using polarized velocity selective spectroscopy is presented. A circularly polarized pump laser beam whose frequency is scanned, and a linearly polarized, probe beam locked to a resonant frequency in the atom cross at a rubidium absorption cell. The transmitted intensities of the probe beam, with mutually perpendicular polarization directions are detected as the frequency of the pump beam is scanned. The sum of these two signals gives absorption profiles, while the difference results in dispersion profiles. This scheme is tested in the D₂ manifold of atomic rubidium. Weaker cross-over lines are found to be present and the slopes of their dispersion profiles are found to be opposite to those of the atomic transitions. This allowed an unambiguous determination of the atomic lines in both ⁸⁵Rb and ⁸⁷Rb, something that is particularly useful for the identification of the repumping transition in neutral atom trapping experiments. The dispersion profiles obtained are also suitable for frequency locking to atomic transitions or cross-over lines in both isotopes.     

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.     

The Effect of an Antirelaxation Coating on Absorption in the <Emphasis Type="Italic">D</Emphasis><Subscript>2</Subscript> Lines of Alkali Metals

The specific features of absorption in a cell with an antirelaxation coating related to optical pumping and a finite rate of laser-frequency scanning are studied. The internal state of an atom in such cells is likely to remain unaltered after a collision with a wall. This results in optical pumping of an atomic ensemble over all velocities and the entire cell volume. Both the frequency corresponding to the maximum absorption in the D₂ line of ¹³³Cs and the absorption maximum itself depend on the sign of laser-frequency scanning. A theoretical model attributing the power dependence of asymmetry of the absorption contour of the ¹³³Cs D₂ line to the presence of cyclic transition levels in the system is presented.     

Improved uncertainty budget for optical frequency measurements with microkelvin neutral atoms: Results for a high-stability 40Ca optical frequency standard

Using a Ca optical frequency standard at 657 nm, we demonstrate a method that reduces uncertainties in absolute frequency measurements of optical transitions using freely expanding neutral atoms. Working with atoms that have been laser cooled to 10 μK, we have developed and employed a new technique that combines launching of cold atom clouds with atom interferometry to measure and optimise spectroscopy beam parameters. When applied to a frequency standard with laser beams of high spatial quality, this approach can potentially reduce residual Doppler effect uncertainties to well below one part in 10¹⁶. With Doppler uncertainties greatly suppressed, we investigate other potential shifts at the 1-Hz level with a multiplexed measurement system that takes advantage of the low instability of the calcium frequency standard (4×10^-15 at 1 s). The resultant fractional frequency uncertainty for the standard is 6.6×10^-15, the lowest uncertainty reported to date for a neutral atom optical standard. PACS 06.30.Ft; 32.30.-r; 39.20.+q     

铯原子束频标中的Majorana跃迁频移

本文对铯原子频标中的Majorana跃迁频移进行理论上的探讨。通过经受低频和微波多重共振的多能级系统的铯原子量子态变化的分析,认为产生这种频移的物理机制在于C_s¹³³原子穿过空间变化磁场时感应了塞曼子能级间的跃迁,扰动了频标赖以工作的两个能级相应态的相位,这个过程又相干于Ramsey共振过程中,最后导致Ramsey共振频移。基于此机制,解释一些有关实验现象并提出消除和减少Majorana跃迁的判断方法。     

Nuclear quadrupole coupling interactions in the rotational spectrum of tryptamine

Four conformers of tryptamine have been detected in a supersonic expansion and characterized by laser ablation molecular beam Fourier transform microwave spectroscopy LA-MB-FTMW in the 5–10 GHz frequency range. The quadrupole hyperfine structure originated by two ¹⁴N nuclei has been completely resolved for all conformers and used for their unambiguous identification. Nuclear quadrupole coupling constants of the nitrogen atom of the side chain have been used to determine the orientation of the amino group involved in N–H?π interactions: to the π electronic system of the pyrrole unit in the Gauche-Pyrrole conformers (GPy) or to the phenyl unit in the Gauche-Phenyl ones.     

A high-resolution time-of-flight mass spectrometer for the detection of ultracold molecules

We have realized a high-resolution time-of-flight mass spectrometer combined with a magneto-optical trap. The spectrometer enables excellent optical access to the trapped atomic cloud using specifically devised acceleration and deflection electrodes. The ions are extracted along a laser beam axis and deflected onto an off-axis detector. The setup is applied to detect atoms and molecules photoassociated from ultracold atoms. The detection is based on resonance-enhanced multi-photon ionization. Mass resolution up to m/Δm_rms=1000 at the mass of ¹³³Cs is achieved. The performance of this spectrometer is demonstrated in the detection of photoassociated ultracold ⁷Li¹³³Cs molecules near a large signal of ¹³³Cs ions. PACS 07.75.+h; 32.80.Rm; 37.10.Gh     

Frequency stability of a mercury ion frequency standard

Observation with an improved signal-to-noise ratio of the hyperfine resonance pattern of¹⁹⁹Hg⁺ stored in a radiofrequency trap is reported. A frequency control loop which locks the frequency of a 5MHz quartz crystal oscillator to the hyperfine transition of stored mercury ions, close to 40.5 GHz, is described. In this system, pulses delivered by the photomultiplier are processed digitally. Optimal conditions for the interrogation of the hyperfine transition are specified. A fractional frequency stability σ_y(τ)=3.6×10⁻¹¹τ^−1/2 for 10s<τ<3500s has been obtained. This frequency stability is comparble to that of commercially available cesium beam frequency standards. Prospect for improvement by a factor of at least 10 are discussed. This work has been sponsored by DRET     

Longitudinal ramsey-fringe spectroscopy in a calcium beam

For ultra-high resolution spectroscopic applications such as optical frequency standards, the value of thermal sources such as atomic beams is currently limited by secondorder Doppler broadening. The use of a longitudinal interaction geometry in which an atomic beam crosses the counter-propagating laser fields at a small angle is able to reduce second-order Doppler broadening to an insignificant level as well as to provide long interaction times without the necessity of large-diameter optical beams. We have analyzed this geometry for the case of the long-lived calcium intercombination line, and conclude that when combined with pulsed (Ramsey) excitation, the longitudinal interaction geometry could be used with a thermal calcium beam to create an optical frequency standard with a reproducibility of the order of 10^?14 for a few seconds averaging time. Our initial experimental results have demonstrated the first use of the longitudinal geometry.     

Electron scattering based on a novel internal target technique: SCRIT

A novel technique for forming internal targets, named SCRIT (Self-Confining RI Ion Target), has been developed, which can make electron scattering off short-lived radioactive nuclei possible in an electron storage ring. SCRIT confines the ions of interest by utilizing the “ion trapping” phenomenon in the electron storage ring. Approximately 10⁷ stable ¹³³Cs ions were trapped in a three-dimensional configuration along the electron beam axis at an electron beam current of 75mA. The angular distribution of the electrons scattered from the trapped ¹³³Cs ions was successfully measured, and a collision luminosity of 10²⁶/(cm² s) was achieved.     

激发态铯原子间的碰撞能量转移

用激光抽运基态Cs₂分子,通过预离解或碰撞转移,由一部分的激发态Cs₂产生Cs原子6P,5D激发态。测量了Cs(5D)+Cs(6P)→Cs(7D_J)+Cs(6S)碰撞激发能量转移截面σ_(7DJ),以及Cs(7D_J)+Cs(6S)→除Cs(7D)外的态的截面σ_tr。结果是,对于J=5/2,3/2,σ_(7DJ)(以 关键词：     

Doubly charged clusters - neutral atom charge transfer: the role of the Coulombic repulsion

We have studied experimentally the collisional charge transfer between a neutral atom and a multicharged metal-atom cluster. The charge transfer cross section measured for Na ₃₁ ^{+ +} + Cs is in the range of 400 ?². The time-of-flight mass analysis of the singly charged collision products demonstrates that an energy of about 0.5 eV is deposited in the cluster fragment during the charge transfer collision. This effect can be interpreted as a charge transfer to an excited state of the metal cluster. The measured cross section for Na ₃₁ ^{+ +} + Cs is larger than the one for Na ₃₁ ⁺ + Cs collisions. This difference between these two systems is due to the existence, for the first one, of a Coulombic repulsion term in the collision output channel. Received 24 October 2000     

Resonances in the cesium atom yield in electron-stimulated desorption from tungsten coated with a germanium monolayer

The yield and energy distributions of Cs atoms emerging from cesium layers, which are adsorbed on tungsten coated with a thin germanium film (1-to 2-monolayers thick), have been measured as a function of the incident electron energy, the amount of adsorbed cesium, and the substrate temperature. The measurements were performed by the time-of-flight technique with a surface ionization detector. At low cesium coverages (Θ < 0.1), the Cs atom appearance threshold at a substrate temperature T = 160 K is ～24 eV, which correlates with the Cs 5s-level ionization energy. As the electron energy is increased, the yield passes through a broad plateau and reaches saturation. The signal intensity in the plateau region decreases gradually with increasing cesium coverage and tends to zero for Θ > 0.14. For Θ ≥ 0.15, the cesium atom appearance threshold shifts to ～30 eV, which corresponds to the Ge 3d-level ionization energy and the plateau is replaced by a resonance peak at ～38 eV, which can be identified with the ionization energy of the W 5p _3/2 level. This peak is observed only for Θ < 0.3 and T = 160 K. For Θ ≥ 0.3, there appears a resonance peak at ～50 eV, and for Θ ≥ 0.5, another resonance peak appears at ～80 eV. These peak positions correlate with the ionization energies of the W 5p _1/2 and W 5s levels, and their intensity is maximum at Θ = 1. The Cs atom energy distributions for Θ < 0.15 consist of a bell-shaped peak with a maximum at ～0.55 eV, and those for Θ ≥ 0.15 contain two nearly resolved maxima, a broad one peaking at ～0.5 eV and a narrow one at ～0.35 eV. The above results argue for the existence of three channels of Cs atom desorption. One channel involves reverse motion of the Cs²⁺ ion; another channel, neutralization of the adsorbed Cs⁺ ion following the Auger decay of a vacancy in the Ge atom; and the third channel involves desorption of a CsGe molecule as it is repelled from a W core exciton.     

The Hyperfine Transition for the Definition of the Second

The unit of time of the International System of Units (SI), the “atomic second” was defined through a constant of physics in 1967. It is derived from the frequency of the hyperfine transition of the atom of cesium 133. From the astronomical definition of the second until today, the accuracy of the realization of the second has improved by eight orders of magnitude, with a rate that has increased since the development of the cesium frequency standards, to reach parts in 10¹⁶ for the best clocks today. In 2018, when the new SI was adopted, the time metrology community proved that a new generation of frequency standards operating in optical wavelengths has uncertainties at the level of 10^–18, and challenge the implementation of high accurate frequency and time comparison techniques to decide on a revision of the definition of the second. Herein, the progress in the definition and realization of the second from astronomy until today is reviewed, an inventory of the present resources is assembled and a brief view for the future given.     

Target mass correction on the 3He polarized structure function

The diffusion-effusion model has been used to analyse the release and yields of Fr and Cs isotopes from uranium carbide targets of very different thicknesses (6.3 and 148 g/cm²) bombarded by a 1 GeV proton beam. Release curves of several isotopes of the same element and production efficiency versus decay half-life are well fitted with the same set of parameters. Comparison of efficiencies for neutron-rich and neutron-deficient Cs isotopes enables separation of the contributions from the primary (p + ²³⁸U) and secondary (n + ²³⁸U) reactions to the production of neutron-rich Cs isotopes. A rather simple calculation of the neutron contribution describes these data fairly well. The FLUKA code describes the primary and secondary-reaction contributions to the Cs isotopes production efficiencies for different targets quite well.     

Polarization spectroscopy of Rb and Cs

Theoretical calculation of ⁸⁵Rb and ¹³³Cs D₁ signals in polarization spectroscopy is presented by using the method of velocity-selective optical pumping in a four-level system. Since good agreement between theory and experiment has been found in Na D₁ polarized signals, the theoretical calculation can be also applied to Rb and Cs D₁ lines. Rb and Cs atoms have higher total angular momentum F in the hyperfine structures than Na atom and then the calculation is more complicated. The relative signal intensities in polarization spectroscopy are compared with those in saturation spectroscopy.     

Core polarization of the133Cs atom by the 7p electron

The hfs of the 7² P _1/2 state of Cs has been measured by optical double resonance yieldingA(7² P _1/2,¹³³Cs)=94.35 (4) MHz. The core polarization contribution to the hfs and the value 〈r ^?3〉 _j =2.54 · 10⁺²⁴ cm^?3 of the 7p electron of Cs has been calculated from the experimental data and was compared with current theories indicating still an appreciable uncertainty in the atomic wave functions of this one-electron atom.     

Rydberg hydrogen atom in the presence of uniform magnetic and quadrupolar electric fields

Laser cooling and trapping offers the possibility of confining a sample of radioactive atoms in free space. Here, we address the question of how best to take advantage of cold atom properties to perform the observation of as highly forbidden a line as the 6S-7S Cs transition for achieving, in the longer term, atomic parity violation (APV) measurements in radioactive alkali isotopes. Another point at issue is whether one might do better with stable, cold atoms than with thermal atoms. To compensate for the large drawback of the small number of atoms available in a trap, one must take advantage of their low velocity. To lengthen the time of interaction with the excitation laser, we suggest choosing a geometry where the laser beam exciting the transition is colinear to a slow, cold atomic beam, either extracted from a trap or prepared by Zeeman slowing. We also suggest a new observable physical quantity manifesting APV, which presents several advantages: specificity, efficiency of detection, possibility of direct calibration by a parity conserving quantity of a similar nature. It is well adapted to a configuration where the cold atomic beam passes through two regions of transverse, crossed electric fields, leading both to differential measurements and to strong reduction of the contributions from the M₁-Stark interference signals, potential sources of systematics in APV measurements. Our evaluation of signal-to-noise ratios shows that with available techniques, measurements of transition amplitudes, important as required tests of atomic theory, should be possible in ¹³³Cs with a statistical precision of 10^-3 and probably also in Fr isotopes for production rates of Fr atoms s^-1. For APV measurements to become realistic, some practical realization of the collimation of the atomic beam as well as multiple passages of the excitation beam matching the atomic beam looks essential.Received: 5 March 2003, Published online: 17 July 2003PACS: 32.80.Ys Weak-interaction effects in atoms - 32.70.Cs Oscillator strengths, lifetimes, transition moments - 32.80.Pj Optical cooling of atoms; trapping - 39.90.+d Other instrumentation and techniques for atomic and molecular physicsS. Sanguinetti: Also at E. Fermi Physics Dept., Pisa Univ., Pisa, Italy.