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arXiv:2005.02675 · Applied Physics B

Mass-selective removal of ions from Paul traps using parametric excitation

Julian Schmidt, Daniel Hönig, Pascal Weckesser, Fabian Thielemann, Tobias Schaetz, Leon Karpa

Published Oct 9, 2020• 21 citations
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Abstract

Abstract We study a method for mass-selective removal of ions from a Paul trap by parametric excitation. This can be achieved by applying an oscillating electric quadrupole field at twice the secular frequency $$\omega _{\text {sec}}$$ ω sec using pairs of opposing electrodes. While excitation near the resonance with the secular frequency $$\omega _{\text {sec}}$$ ω sec only leads to a linear increase of the amplitude with excitation duration, parametric excitation near $$2\, \omega _{\text {sec}}$$ 2 ω sec results in an exponential increase of the amplitude. This enables efficient removal of ions from the trap with modest excitation voltages and narrow bandwidth, therefore, substantially reducing the disturbance of ions with other charge-to-mass ratios. We numerically study and compare the mass selectivity of the two methods. In addition, we experimentally show that the barium isotopes with 136 and 137 nucleons can be removed from small ion crystals and ejected out of the trap while keeping $$^{138}\text {Ba}^{+}$$ 138 Ba + ions Doppler cooled, corresponding to a mass selectivity of better than $$\Delta m / m = 1/138$$ Δ m / m = 1 / 138 . This method can be widely applied to ion trapping experiments without major modifications since it only requires modulating the potential of the ion trap.

DOI
10.1007/s00340-020-07491-8

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Abstract We study a method for mass-selective removal of ions from a Paul trap by parametric excitation. This can be achieved by applying an oscillating electric quadrupole field at twice the secular frequency ωsec\omega _{\text {sec}}ωsec​ ω sec using pairs of opposing electrodes. While excitation near the resonance with the secular frequency ωsec\omega _{\text {sec}}ωsec​ ω sec only leads to a linear increase of the amplitude with excitation duration, parametric excitation near 2 ωsec2\, \omega _{\text {sec}}2ωsec​ 2 ω sec results in an exponential increase of the amplitude. This enables efficient removal of ions from the trap with modest excitation voltages and narrow bandwidth, therefore, substantially reducing the disturbance of ions with other charge-to-mass ratios. We numerically study and compare the mass selectivity of the two methods. In addition, we experimentally show that the barium isotopes with 136 and 137 nucleons can be removed from small ion crystals and ejected out of the trap while keeping 138Ba+^{138}\text {Ba}^{+}138Ba+ 138 Ba + ions Doppler cooled, corresponding to a mass selectivity of better than Δm/m=1/138\Delta m / m = 1/138Δm/m=1/138 Δ m / m = 1 / 138 . This method can be widely applied to ion trapping experiments without major modifications since it only requires modulating the potential of the ion trap.
Excitation of the motional frequencies of clouds of H2+ and N2+ ions, confined in a Paul trap, by an additionally applied radiofrequency quadrupole field leads to the observation of parametric resonances that are predicted to occur at frequencies 2ωz/n, n=1,2,3…, where ωz is the axial secular frequency. In the case of clouds of N2+ ions resonances up to n=10 are detected. They can be explained by parametric instabilities of the center-of-mass motion in the axial direction. The fractional resonances are observable only if the excitation field surpasses a critical strength. We find an odd-even staggering of the thresholds.
We report the observation of the first ten collective parametric resonances of an uncooled N2+{N}_{2}^{+}N2+​ ion cloud in a Paul trap driven at simple fractions of twice the secular frequency of the trap by an additionally applied quadrupole field. The fractional resonances are observable only if the excitation field surpasses a critical strength. Odd-even staggering of the thresholds is observed.
Ions in radio-frequency traps are widely used in various fields of applied and fundamental physics, such as metrology and quantum computing. One of the important tasks required for modern experiments is deep cooling of ion crystals. The results of simulations of an increase in the efficiency of deep cooling of linear ion crystals by the method of electromagnetically induced transparency (EIT cooling) by imposing an optical lattice on the radio-frequency trap have been reported. It has been shown that this method makes it possible to narrow the frequency range occupied by various vibrational modes of ions and to increase their axial frequencies of motion without violating the linear configuration of the crystal. Thus, for a crystal of eight ions in a Paul trap with secular frequencies ω z = 2 π × 100 kHz and ω r = 2 π × 650 kHz, the application of an optical lattice allows the reduction of the frequency range occupied by vibrational modes by a factor of 2. The dependence of the optimal power of the optical lattice for narrowing the vibrational spectrum on the number of particles in the trap and its parameters has been investigated.
Simple and effective techniques for loading barium ions into linear Paul traps are demonstrated. Two-step photoionization of neutral barium is achieved using a weak intercombination line ($6{s}^{2}\phantom{\rule{0.2em}{0ex}}^{1}S_{0}\ensuremath{\leftrightarrow}6s6p\phantom{\rule{0.2em}{0ex}}^{3}P_{1}$, $\ensuremath{\lambda}=791\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) followed by excitation above the ionization threshold using a nitrogen gas laser $(\ensuremath{\lambda}=337\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. Isotopic selectivity is achieved by using a near Doppler-free geometry for excitation of the triplet 6s6p3P16s6p\phantom{\rule{0.2em}{0ex}}^{3}P_{1}6s6p3P1​ state. Additionally, we report a particularly simple and efficient trap loading technique that employs an inexpensive uv epoxy curing lamp to generate photoelectrons.
In an experiment using a Paul trap, we create a H2+{\mathrm{H}}_{2}^{+}H2+​ ion cloud by electron ionization of the background gas at ${10}^{\ensuremath{-}9}$-mbar residual pressure. Exciting the ions parametrically at twice the frequency of the secular motion of ions in the rrr or zzz direction, we observe a narrow resonance at some distance from the motional resonance center if the amplitude of the exciting field exceeds a threshold value. The threshold value decreases with increasing ion number. Since the narrow resonance does not shift with ion number, we interpret it as a collective resonance of the center of mass of the ion cloud. The resonance shape exhibits the typical form of a driven anharmonic oscillator. The conclusions drawn from the experiments are supported by detailed analytical and numerical computations.