Papers - Hasegawa, Taro

Quantum mechanics in rotating-radio-frequency traps and Penning traps with a quadrupole rotating field

Kazuya Abe, Taro Hasegawa

Physical Review A (American Physical Society)  81 ( 3 ) 033402-1 - 033402-7 2010.03

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted

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Quantum-mechanical analysis of ion motion in a rotating-radio-frequency (rrf) trap or in a Penning trap with a quadrupole rotating field is carried out. Rrf traps were introduced by Hasegawa and Bollinger [Phys. Rev. A 72, 043404 (2005)]. The classical motion of a single ion in this trap is described by only trigonometric functions, whereas in the conventional linear radio-frequency (rf) traps it is by the Mathieu functions. Because of the simple classical motion in the rrf trap, it is expected that the quantum-mechanical analysis of the rrf traps is also simple compared to that of the linear rf traps. The analysis of Penning traps with a quadrupole rotating field is also possible in a way similar to the rrf traps. As a result, the Hamiltonian in these traps is the same as the two-dimensional harmonic oscillator, and energy levels and wave functions are derived as exact results. In these traps, it is found that one of the vibrational modes in the rotating frame can have negative energy levels, which means that the zero-quantum-number state (“ground” state) is the highest energy state.

Laser frequency locking by dispersive response of atoms in transversal magnetic field

Taro Hasegawa, Mitsuyasu Deguchi

Journal of Optical Society of America B (The Optical Society)  26 ( 6 ) 1216 - 1220 2009.06

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted

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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.

Rotating-radio-frequency ion traps

Taro Hasegawa, John J. Bollinger

Physical Review A (American Physical Society)  72 ( 4 ) 043403-1 - 043403-8 2005.10

Research paper (scientific journal), Joint Work, Lead author, Accepted

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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.

Stability of a Penning trap with a quadrupole rotating electric field

Taro Hasegawa, Marie J. Jensen, John J. Bollinger

Physical Review A (American Physical Society)  71 ( 2 ) 023406-1 - 023406-6 2005.02

Research paper (scientific journal), Joint Work, Accepted

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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.

Rapid Heating of a Strongly Coupled Plasma near the Solid-Liquid Phase Transition

Marie J. Jensen, Taro Hasegawa, John J. Bollinger, Daniel H. E. Dubin

Physical Review Letters (American Physical Society)  95 ( 2 ) 025001-1 - 025001-4 2005.01

Research paper (scientific journal), Joint Work, Accepted

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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).

Temperature and heating rate of ion crystals in Penning traps

Marie J. Jensen, Taro Hasegawa, John J. Bollinger

Physical Review A (American Physical Society)  70 ( 3 ) 033401-1 - 033401-13 2004.09

Research paper (scientific journal), Joint Work, Accepted

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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.

Limiting temperature of sympathetically cooled ions in a radio-frequency trap

Taro Hasegawa, Tadao Shimizu

Physical Review A (American Physical Society)  67 ( 1 ) 013408-1 - 013408-4 2003.01

Research paper (scientific journal), Joint Work, Lead author, Accepted

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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.

Resonant oscillation modes of sympathetically cooled ions in a radio-frequency trap

Taro Hasegawa, Tadao Shimizu

Physical Review A (American Physical Society)  66 ( 6 ) 063404-1 - 063404-5 2002.12

Research paper (scientific journal), Joint Work, Lead author, Accepted

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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.

Spin dynamics of dense alkali-metal atoms

Kiyoshi Ishikawa, Togo Kojima, Taro Hasegawa, Yoshihiro Takagi

Physical Review A (American Physical Society)  65 ( 3 ) 032511-1 - 032511-9 2002.02

Research paper (scientific journal), Joint Work, Accepted

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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.

Observation and Temperature Measurement of Sympathetically Cooled Ions in an RF Trap

Taro Hasegawa, Tadao Shimizu

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT (IEEE)  50 ( 2 ) 556 - 558 2001.04

Research paper (scientific journal), Joint Work, Lead author, Accepted

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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.

Removal of irrelevant isotope ions in the presence of laser cooling in a rf trap

Taro Hasegawa, Tadao Shimizu

Applied Physics B (Springer)  70 ( 6 ) 867 - 871 2000.06

Research paper (scientific journal), Joint Work, Lead author, Accepted

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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.

Charge transfer between a laser-cooled ion and a thermal atom in a radio-frequency trap

Taro Hasegawa, Tadao Shimizu

AIP Conference Proceedings (American Physical Society)  500   704 - 708 2000.02

Research paper (international conference proceedings), Joint Work, Lead author, Accepted

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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.

レーザー冷却・共同冷却されたrfトラップ中のイオンの運動

長谷川太郎

博士学位論文(東京大学)    1 - 119 2000.01

Doctoral thesis, Single Work, Lead author, Last author, Corresponding author, Accepted

Frequency-doubled Hermite-Gaussian beam and the mode conversion to the Laguerre-Gaussian beam

Taro Hasegawa, Tadao Shimizu

Optics Communications (Elsevier)  160 ( 1-3 ) 103 - 108 1999.02

Research paper (scientific journal), Joint Work, Lead author, Accepted

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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.

Distortion of spectral line profiles of laser-cooled ions in a radio-frequency ion trap

Taro Hasegawa, Daisuke Tanooka, Tadao Shimizu

Physical Review A (American Physical Society)  58 ( 3 ) 2327 - 2331 1998.09

Research paper (scientific journal), Joint Work, Lead author, Accepted

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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.

Nonlinear dynamics of two particles in the rf trap

Taro Hasegawa, Kiyoji Uehara

Progress in Crystal Growth and Characterization of Materials (Elsevier)  33 ( 1-3 ) 401 - 403 1996.01

Research paper (scientific journal), Joint Work, Lead author, Accepted

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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.

Dynamics of a single particle in a Paul trap in the presence of the damping force

Taro Hasegawa, Kiyoji Uehara

Applied Physics B (Springer)  61   159 - 163 1995.08

Research paper (scientific journal), Joint Work, Lead author, Accepted

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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.

Dynamics of two particles in an rf trap

Taro Hasegawa

修士学位論文(慶應義塾大学)    1 - 46 1995.03

Master’s thesis, Single Work, Lead author, Last author, Corresponding author, Accepted