Kinetic Energy of Trapped Ions Cooled by Buffer Gas

The average kinetic energy of 40 Ca＋ ions is measured by the method of evaporating ions in an rf ion trap. The kinetic energy of the ion 40Ca＋ varies from 0.5eV to 0.2eV with changing buffer gas pressure from 10^-7 mbar to 10^-5 mbar. The Brownian motion model is also introduced to calculate the average kinetic energy of the trapped ions.     

On the sensitivity of ion traps for spectroscopic applications

Ba⁺ ions, created by surface ionization near one endcap of an rf quadrupole trap were slowed down by collisions with the background gas. At He pressures of 10^?6 mbar or more 2% of the primary ions could be trapped. The sensitivity of ion detection by fluorescence radiation allows spectroscopic experiments, starting from less than 10⁷ particles. The observation of the ground-state hyperfine splitting of¹³⁷Ba⁺ is given as an example.     

Fluorescence Detection and Buffer Gas Cooling of Trapped Mercury Ions in Paul Trap

Pig ions are confined in a hyperboloid ion trap. With the rf discharge ^202 Hg isotope lamp, the fluorescence signal of trapped Hg ions is observed. By means of buffer gas cooling, the ionic temperature is reduced. As a result, the trapping time is increased and the signal-to-noise ratio （SNR） of the fluorescent signal is improved. The temperature of ion cloud is estimated by measuring the space charge shift.     

Trapping of ions from high energy sources into a radiofrequency ion trap

An attempt is described to confine ions, created externally and accelerated to some energy, in an rf quadrupole trap. 4 keV Ba⁺ ions were stopped on a Ni foil, placed in an aperture of one trap electrode. The Ba then was evaporated from the heated foil and ionized by electron impact. At background pressure of about 10^–5 mbar of various light buffer gases (He, H₂, N₂), the trap was filled once with 10⁵ ions, at a minimum primary ion number of 10¹⁰. The storage time was 10 min. From the data obtained the possibility of spectroseopic experiments on rare isotopes, created with accelerators or nuclear reactors, is discussed.     

Measurement of storage time, estimation of ion number and study of absorption line profile in a Paul trap

Europium (Eu⁺) ions were confined in a Paul trap and detected by non-destructive method. Storage time of Eu⁺ ions achieved in vacuum was improved by orders of magnitude employing buffer gas cooling. The experimentally detected signal was fitted to the ion response signal and the total number of ions trapped was estimated. It is found that the peak signal amplitude as well as the product of FWHM and the peak signal amplitude is proportional to the total number of trapped ions. The trapped ion secular frequency was swept at different rates and its effect on the absorption line profile was studied both experimentally and theoretically.     

Laser spectroscopy of trapped 7Be and 10Be at a prototype slow RI-beam facility of RIKEN

A next-generation slow radioactive nuclear ion beam facility (SLOWRI) which provides slow, high-purity and small emittance ion beams of all elements is being build as one of the principal facilities at the RIKEN RI-beam factory (RIBF). High energy radioactive ion beams from the projectile fragment separator BigRIPS are thermalized in a large gas catcher cell. The thermalized ions in the gas cell are guided and extracted to a vacuum environment by a combination of dc electric fields and inhomogeneous rf fields (rf carpet ion guide). From there the slow ion beam is delivered via a mass separator and a switchyard to various devices: such as an ion trap, a collinear fast beam apparatus, and a multi-reflection time of flight mass spectrometer. In the R&D works at the present RIKEN facility, an overall efficiency of 5% for a 100A MeV ⁸Li ion beam from the present projectile fragment separator RIPS was achieved and the dependence of the efficiency on the ion beam intensity was investigated. Recently our first spectroscopy experiment at the prototype SLOWI was performed on Be isotopes. Energetic ions of ¹⁰Be and ⁷Be from the RIPS were trapped and laser cooled in a linear rf trap and the specific mass shifts of these isotopes were measured for the first time.     

Optical detection of Yb+ trapped in an rf trap

The fluorescence from Yb⁺ ions trapped in an rf trap was detected by driving the²S_1/2−²P_1/2 transition at 369.52 nm with the radiation generated by sum-frequency mixing of diode-laser and argon-ion-laser radiation. The rf resonance absorption signal as well as the fluorescence signal, when the Yb⁺ ions were continuously irradiated by the resonant uv radiation, faded out with a decay time shorter than the storage time. This observation suggests that the Yb⁺ ions disappeared from the trap with the irradiation of the resonant uv radiation.     

Detection of parametric resonance of trapped ions for micromotion compensation

We have detected excess micromotion of trapped ions by modulating the trapping voltage. This radio-frequency (rf) modulation induces parametric resonance and excites secular motion of the trapped ions when they possess excess motion. This technique has been applied to laser-cooled ions in a linear rf trap and it provides optimum values for compensating the trapping field. We found that the technique has sensitivity equal to or greater than the conventional method for detecting excess micromotion. Because any laser propagation direction can be used, this method is expected to be applied to surface-electrode traps.     

Observation of a saturation dip in the fluorescence light from electrodynamically trapped Ba+ ions

Ba⁺ ions are trapped in a Paul rf quadrupole trap and irradiated by D1 resonance light at 493.4 nm from a tunable laser. At ion densities of about 10⁵ cm^-3 a saturation lamp dip was easily observed when two laser beams of opposite directions were used. This should allow Doppler-free optical spectroscopy on a small number of ions in the future.     

The energy distribution of an ion in a radiofrequency trap interacting with a nonuniform neutral buffer gas

An ion in a radiofrequency (rf) trap sympathetically cooled by a simultaneously trapped neutral buffer gas exhibits deviations from thermal statistics caused by collision-induced coupling of the rf field to the ion motion. For a uniform density distribution of the buffer gas, the energy distribution of the ion can be described by Tsallis statistics. Moreover, runaway heating of the ion occurs if the buffer gas particles are sufficiently heavy relative to the ion. In typical experiments, however, ultracold buffer gases are confined in traps resulting in localised, non-uniform density distributions. Using a superstatistical approach, we develop an analytical model for an ion interacting with a localised buffer gas. We demonstrate theoretically that limiting collisions to the centre of the ion trap enables cooling at far greater mass ratios than achievable using a uniform buffer gas, but that an upper limit to the usable mass ratio exists even in this case. Furthermore, we analytically derive the functional form of the energy distribution for an ion interacting with a buffer gas held in a harmonic potential. The analytical distribution obtained is found to be in excellent agreement with the results of numerical simulations.     

A compact Penning trap for light ions

We describe the construction of a novel compact Penning trap from strong permanent magnets for trapping light ions. Our cylindrically symmetric, iron-free magnetic configuration allows fully analytical treatment, is easy to handle and to optimize. The magnetic field inhomogeneity is less than 1% in a volume of 1 cm³ at 0.7 T. The stored H⁺ and H ₂ ⁺ ions in this trap are detected electrically by the rf absorption method. The charge density, total number and storage time of the trapped ions are measured.Dedicated to H. Walther on the occasion of his 60th birthday     

Optically-driven red blood cell rotor in linearly polarized laser tweezers

We have constructed a dual trap optical tweezers set-up around an inverted microscope where both the traps can be independently controlled and manipulated in all the three dimensions. Here we report our observations on rotation of red blood cells (RBCs) in a linearly polarized optical trap. Red blood cells deform and become twisted in hypertonic phosphate buffer saline and when trapped, experience an unbalanced radiation pressure force. The torque generated from the unbalanced force causes the trapped RBC to rotate. Addition of Ca⁺⁺ ions in the solution, keeping the osmolarity same, makes the cell membranes stiffer and the cells deform less. Thus the speed of rotation of the red blood cells can be controlled, as less deformation and in turn less asymmetry in shape produces less torque under the radiation pressure resulting in slower rotation at the same laser power     

联合阱中稳定囚禁离子的实验研究

实验观察到了小型联合阱稳定囚禁离子的射频信号增强现象，并以此对联合阱囚禁离子的特性进行了讨论；同时，通过实验检测到的稳定囚禁离子的工作点范围与磁场强度的关系，在一定程度上验证了联合阱囚禁离子稳定区随磁场强度的变化趋势。     

Temperature measurement of 6He?+? ions confined in a transparent Paul trap

The LPCTrap setup is a transparent Paul trap dedicated to the measurement of the ???C?? correlation coefficient a _?|? in the ?? decay of trapped radioactive nuclides. In a first experiment, the system has been used to record ??10⁵ coincidences between the ?? particles and recoiling ions emitted from the decay of ⁶He^?+? ions. The analysis of the collected data has already shown that the size of the ⁶He^?+? ion cloud confined in the Paul trap is a critical parameter, potentially limiting the accuracy on the a _?|? measurement. We report here the precise determination of the trapped ion cloud temperature and size. This was performed by extracting the trapped ions toward a position sensitive micro channel plate detector at different phases of the RF driving field. We find a temperature T _exp ?= 0.107(7) eV, consistent with the temperature values inferred using two other observables but 20% higher than the temperature T _sim ?= 0.09 eV predicted by realistic simulations of the ions interacting with the H₂ buffer gas.     

Observation of laser-cooled Be+ -ion clouds in a Penning trap

Be⁺ ions trapped in a Penning trap are laser-cooled to about 10 mK. The excitation spectra of ion clouds containing about 500 ions are obtained by scanning the frequency of the cooling laser and discontinuities in these spectra are observed because of phase transitions. When the cooled ions are heated electrically by applying an rf voltage, no phase transition occurs and the spectra become continuous. Two-dimensional measurement of the ion clouds is carried out and the abrupt change in the shape of the ion cloud due to the phase transition is observed. When many ions are trapped and cooled, the phase transition occurs partially and a transient state where two states are mixed can be observed. The static properties of the ions are also measured by using an additional probe laser and the results of experimental measurements are compared with theoretical predictions.     

Micromotion compensation in a surface electrode trap by parametric excitation of trapped ions

We report a surface electrode trap with a relatively large trap depth (0.6–1.0?eV). The trap electrodes are formed by gold plating an alumina substrate. Calcium ions are trapped approximately 400?μm above the trap surface. We demonstrate micromotion compensation based on parametric resonance for surface electrode traps. Unlike the conventional method based on radio-frequency (rf)–photon correlation in which the wave vector of the laser beam must have a component parallel to the micromotion to be detected, the proposed method is independent of the laser propagation direction. This enables the micromotion component normal to the electrode surface to be detected without increasing the scattered light.     

A high-precision segmented Paul trap with minimized micromotion for an optical multiple-ion clock

We present a new setup to sympathetically cool ¹¹⁵In⁺ ions with ¹⁷²Yb⁺ for optical clock spectroscopy. A first prototype ion trap made of glass-reinforced thermoset laminates was built, based on a design that minimizes axial micromotion and offers full control of the ion dynamics in all three dimensions. We detail the trap manufacturing process and the characterization of micromotion in this trap. A calibration of the photon-correlation spectroscopy technique demonstrates a resolution of 1.1 nm in motional amplitude of our measurements. With this method, we demonstrate a sensitivity to systematic clock shifts due to excess micromotion of $|(\Updelta\nu/\nu)_{\rm mm}|=7.7\times10^{-20}$ along the direction of the spectroscopy laser beam. Owing to our on-board filter electronics on the ion trap chips, no rf phase shifts could be resolved at this level. We measured rf fields over a range of 400 μm along the ion trap axis and demonstrated a region of 70 μm where an optical frequency standard with a fractional inaccuracy of ≤1 × 10^?18 due to micromotion can be operated.     

All-diode-laser cooling of single Ca+ ions

+ ions. The Ca⁺ ions are trapped in a miniature rf Paul trap and irradiated by light from a frequency-doubled diode laser at 397 nm and by light from a diode laser at 866 nm. We are able to cool a single ion and observe its fluorescence continuously with the laser diode locked to the external frequency-doubling cavity. Quantum jumps in the fluorescence light of a single ion and of a small cloud of five ions have been induced by driving the “clock” transition at 729 nm. We were able to resolve the influence of the micromotion on the excitation spectrum of the small ion cloud. Received: 10 July 1997/Revised version: 17 November 1997     

Subpicosecond pulse generation in synchronously pumped energy transfer dye lasers

We describe a laser-cooling experiment on Mg⁺ ions confined in an electromagnetic trap (Penning trap or rf trap) and give the preliminary experimental results. In particular, we have observed a laser cooling in the Penning trap configuration in which a measured temperature of about 1 K has been obtained.