Published papers

A mechanically robust Sagnac loop employing a polarized beam displacer and a hollow retroreflector is developed for laser frequency stabilization to a sub-Doppler spectral line of Rb. In the experiment, a 1560 nm laser is frequency-doubled and stabilized to the D2 transition of 85Rb at 780 nm. The optical frequency of the stabilized laser is evaluated by heterodyne detection using an optical frequency comb. As a result, a frequency uncertainty of ~1 kHz is achieved with an averaging time of 8 s.

Twisted radio frequency (rf) ion traps, which have electrode configurations that are twisted variants of conventional linear rf ion traps, have been proposed and successfully demonstrated using charged alumina dust particles. When constructed from thin metal wire electrodes, the twisted traps offer excellent optical accessibility to the trapped ions, making them suitable for applications in quantum information technology. In the experimental demonstration, 14 μm-diameter alumina dust particles were charged via discharge and successfully confined in the twisted trap. A linear chain of trapped particles was observed within the trap.

The charge transfer process between a laser-cooled magnesium ion and a thermal barium atom in a radio-frequency ion trap in extremely low energy regime is observed. The merit of using the trap is that the generated ions are also confined in the trap. The occurrence of the charge transfer process is confirmed by the detection of the laser-induced fluorescence from the generated ions. The cross section of the process is estimated to be 10^-3 A^2 ~ 10^-1 A^2.

Laser frequency stability locked to D2 transition of 85Rb at 780 nm by the Sagnac-loop stabilization scheme was evaluated using the heterodyne measurement with an optical frequency comb. This scheme does not require modulation and magnetic field to generate the feedback error signal. Therefore, the setup for the laser frequency stabilization is simple. Because of the mechanical stability of the Sagnac interferometer, the dispersion of atom transition, which is used as an error signal for feedback loop, can be observed with negligibly small effects of mechanical fluctuations. In the experiment, the Allan variance for relative frequency uncertainty was 2.6×10-12 over 64 s averaging.

The motion of a single charged particle in a Paul trap in the presence of the damping force is investigated theoretically and the modified stability diagrams in the parameter space are calculated. The results show that the stable regions in the a-q parameter plane are not only enlarged but also shifted. Consequently, the damping force causes instability in some cases, contrary to intuition. As a by-product of the calculation, we derive new theoretical approximate expressions for the secular-oscillation frequency. In the limiting case of no damping, these formulas are in good agreement with early measurements done by Wuerker et al.

The dynamics of two particles in the rf trap is investigated theoretically. To solve tile nonlinear equations of motion nmnerieally, we introduce a new method based upon the Fourier expansion. Several new kinds of solutions including those that have a period two or four times the rf period are found and their stability diagrams in the parameter space are obtained. The results exhibit the bistability which may be related to the hysteresis observed in the phase transitions between the "crystal" state and the "cloud" state.

The laser-cooled ions in a radio-frequency ion show a variety of peculiar line profiles as functions of laser intensity and trap potential. All these line profiles are well reproduced by a calculation based on the present theoretical model, in which the ion motion in a trap in the presence of laser radiation pressure is analyzed. The results give information about collisions of ions, and are utilized for the accurate spectroscopic measurement of the ion responses.

Frequency-doubled beam of the higher-order Laguerre–Gaussian mode is obtained with a high power (2 mW) aiming at an investigation of the interaction between the Laguerre–Gaussian beam and ions in a radio-frequency ion trap. For a sufficient beam power, the Hermite–Gaussian beam is obtained at first, frequency-doubled, and then mode-converted to the Laguerre–Gaussian mode. The beam profile is analyzed theoretically.

Barium ions are trapped simultaneously with laser-cooled magnesium ions in a radio-frequency (RF) trap and cooled sympathetically. The barium ions are produced by charge transfer with the magnesium ions and are caught in a shallow trapping potential. The temperature of the trapped barium ions is measured, and it is confirmed that sympathetic cooling is effective. It is demonstrated that charge-transfer ionization is applicable to produce other ion species.

An additionally applied rf perturbation excites the secular motion of isotope ions selectively in a rf trap. Simultaneous application of laser cooling has achieved complete removal of irrelevant isotope ions and kept a large number of relevant ones.

The free induction signals of dense Rb atoms are observed with an injection-seeded power laser diode. The decay rate of signals is found to depend on the number density of Rb atoms, the static magnetic field, the electron spin polarization, and the pulse area of the oscillating magnetic field. We note that the observed broadening of the magnetic resonance line, appearing at high polarization and large pulse area, is the key to understanding the spin dynamics of the precessing atoms which interact with each other. The feature of broadening is quantitatively interpreted by numerically calculating, in a wide range of parameters, the nonlinear Liouville equation that includes the spin-exchange interaction between Rb atoms.

Sympathetic cooling of Ca+, Zn+, Sr+, Ba+, and Yb+ as guest ions with laser-cooled 24Mg+ as host ions in a rf ion trap is carried out, and resonant frequencies of their motion in the trap potential are measured. Various oscillation modes of the sympathetically cooled ions are observed. The resonant frequency of the oscillation mode is different from the frequency of either the collective oscillation frequency of the trapped ions or the oscillation frequency of each ion without host ions. This difference is well explained by a theoretical model in which coupled equations of motion of the host ion cloud with a single guest ion are considered.

The limiting temperature achieved by sympathetic cooling in an rf trap is calculated with a theoretical model in which no fitting parameters are used. The calculated result agrees well with observation. The dependence of the temperature on trapping parameters and ion mass is also analyzed. The results can be used for designing an rf trap system.

We have determined the temperature and heating rate of laser-cooled ions in a Penning trap using Doppler laser spectroscopy. Between 104 and 106 9Be+ ions are trapped in a Penning trap and Doppler laser cooled to temperatures of a few millikelvin, where they form ion crystals. This system is an example of a strongly coupled one-component plasma. The ion temperature was measured as a function of time after turning off the laser-cooling. In the solid phase, we measured a heating rate of ~65 mK/ s. Information about possible heating mechanisms was obtained directly from temperature measurements, and also from measurements of the rate of radial expansion of the ion plasma. We determined that the observed heating is due to collisions with the ~4x 10−9 Pa residual gas of our vacuum system.

Between 104 and 106 9Be+ ions were trapped in a Penning trap and laser cooled to ~1 mK, where they formed a crystalline plasma. We measured the ion temperature as a function of time after turning off the laser cooling and observed a rapid temperature increase as the plasma underwent the solid-liquid phase transition at T ’~ 10 mK ( Γ ~ 170). We present evidence that this rapid heating is due to a sudden release of energy from weakly cooled degrees of freedom involving the cyclotron motion of trapped impurity ions. This equilibration of cyclotron motion with motion parallel to the magnetic field is more than 10 orders of magnitude faster than that predicted by currently available theory, which is valid only in the absence of correlations (Γ<< 1).

We present theoretical and experimental studies of the center-of-mass (c.m.) stability of ions in a Penning trap with a quadrupole rotating electric field. The rotation frequency of an ion cloud in a Penning trap determines the cloud density and shape, and it can be precisely controlled by a rotating electric field. The quadrupole rotating-field scheme can control pure single-species plasmas in contrast to the dipole field, which is effective only for plasmas composed of two or more species of ions. However, the quadrupole field can modify the trap stability because of the spatial dependence of the electric field. In this study, we theoretically and experimentally determine the c.m. stability condition for ions in a Penning trap with a rotating quadrupole field. The experimental results agree well with the theoretical prediction. In the limit of zero magnetic field we obtain a type of rf trap which uses a rotating quadrupole field and in which the c.m. motion is analytically solvable.

We discuss a radio-frequency (rf) ion trap, the rotating-radio-frequency (rotating-rf) trap, in which the motion of a charged particle is described by trigonometric functions rather than the usual Mathieu functions of a normal rf trap. In the rotating-rf trap, a rotating quadrupole electric field confines charged particles, whereas in a normal rf trap, an oscillating quadrupole electric field does. Ion motion in a rotating-rf trap is a superposition of two nondegenerate circular secular motions and two corresponding circular micromotions. The cases of applying a uniform dc magnetic field or a quadrupole dc electric field in addition to the rotating-rf field are also discussed. Confinement in a rotating-rf trap can be tighter than in a normal linear rf trap with the same experimental parameter values.

A robust laser-frequency-locking scheme to atomic transitions is proposed and demonstrated. In this scheme, dispersion of the Zeeman-shifted atoms instead of absorption in the dichroic atomic vapor laser lock (DAVLL) [Appl. Opt. 37, 3295 (1998)] is measured and used as an error signal for stabilization feedback. The error signal of the proposed scheme can provide a wide locking range. Experimental demonstration with the transition between 5d 2D3/2 and 6p 2P1/2 of Ba+ is carried out for a grating feedback external-cavity laser diode at 650 nm.