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Chinese Physics Letters

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

Lei She, Wenming Wang, Lei Bai, Huan-Yao Sun, Xiwen Zhu, Jiaomei Li, Kelin Gao

Published May 1, 2008• 9 citations
Publisher

Abstract

Hg 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.

DOI
10.1088/0256-307x/25/5/036

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Individual Ba ions are trapped in a gas-filled linear ion trap and observed with a high signal-to-noise ratio by resonance fluorescence. Single-ion storage times of $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{min}$ $(\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{min})$ are achieved using He (Ar) as a buffer gas at pressures in the range $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}--4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\text{torr}$. Trap dynamics in buffer gases are experimentally studied in the simple case of single ions. In particular, the cooling effects of light gases such as He and Ar and the destabilizing properties of heavier gases such as Xe are studied. A simple model is offered to explain the observed phenomenology.

arXiv:physics/0702122•Physical Review A•Aug 9, 2007•33 citations
Hg 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.
We present studies on monoisotopic Hg+ ions generation and trapping at room temperature. An ultraviolet laser system, including a frequency-controlled 194 nm laser and a 254 nm laser, is constructed. Mercury atoms are isotope-selectively excited by the 254 nm laser. The spectrum of the 6s2–6s6p transition lines for natural mercury isotopes is measured, and the uncertainty is less than 100 MHz. Then, the selected mercury isotopes are ionized by the 194 nm laser and trapped in a linear Paul trap. The fluorescence of the ion crystal at 194 nm is detected, and the temperature of the ions is estimated to be less than 10 mK.
Buffer gas cooling is the most effective and practical method to cool ions in ion trap.The kind and quantity of buffer gas are the key technologies in mercury ion microwave frequency standard experiments.Buffer gas made of helium,neon or argon was studied to cool trapped mercury ions(~(199)Hg~+) in a linear ion trap by introducing a resistance term in Mathieu's equation.It's found that the decay time of motion of ~(199)Hg~+ in argon gas is the shortest,and the frequency shift of the clock transition(40.5 GHz) is minimum when the pressure of helium is 10~(-5) Torr or the pressure of neon gas is 2.4×10~(-5) Torr.Neon is the most suitable buffer gas among helium,neon and argon,considering the decay time-constant of motion of ~(199)Hg~+ in buffer gases and ~(199)Hg~+ clock transition shift's sensitivity to the change of the pressure of buffer gas.
Singly ionized mercury ions have been stored in a miniature rf (Paul) quadruple ion trap and observed by means of resonance fluorescence. The trap electrode spacings (~0.5 mm) and rf voltage (~500 V at 20 MHz) were chosen1 so that the Lamb-Dicke criterion could be satisfied for optical transitions on a single-laser-cooled ion. The presence of a single ion was determined by a statistical loading method2; that is, the loading conditions were scaled down to a point where only occasionally would any fluorescence be observed after a loading cycle. With high probability, this fluorescence was due to a single ion. The fluorescence observed was due to spontaneous scattering from a 194-nm cw source (~3-μW power focused to a beam waist of ~20 μm) directed at the ion and tuned near the strongly allowed first resonance line.
The use of the mercury ion isotope (201)Hg(+) was examined for an atomic clock. Taking advantage of the faster optical pumping time in (201)Hg(+) reduces both the state preparation and the state readout times, thereby decreasing the overall cycle time of the clock and reducing the impact of medium-term LO noise on the performance of the frequency standard. The spectral overlap between the plasma discharge lamp used for (201)Hg(+) state preparation and readout is much larger than that of the lamp used for the more conventional (199)Hg(+). There has been little study of (201)Hg(+) for clock applications (in fact, all trapped ion clock work in mercury has been with (199)Hg(+); however, recently the optical pumping time in (201)Hg(+) has been measured and found to be 0.45 second, or about three times faster than in (199)Hg(+) due largely to the better spectral overlap. This can be used to reduce the overall clock cycle time by over 2 seconds, or up to a factor of 2 improvement. The use of the (201)Hg(+) for an atomic clock is totally new. Most attempts to reduce the impact of LO noise have focused on reducing the interrogation time. In the trapped ion frequency standards built so far at JPL, the optical pumping time is already at its minimum so that no enhancement can be had by shortening it. However, by using (201)Hg(+), this is no longer the case. Furthermore, integrity monitoring, the mechanism that determines whether the clock is functioning normally, cannot happen faster than the clock cycle time. Therefore, a shorter cycle time will enable quicker detection of failure modes and recovery from them.
Individual Ba ions are trapped in a gas-filled linear ion trap and observed with a high signal-to-noise ratio by resonance fluorescence. Single-ion storage times of $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{min}$ $(\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{min})$ are achieved using He (Ar) as a buffer gas at pressures in the range $8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}--4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.3em}{0ex}}\text{torr}$. Trap dynamics in buffer gases are experimentally studied in the simple case of single ions. In particular, the cooling effects of light gases such as He and Ar and the destabilizing properties of heavier gases such as Xe are studied. A simple model is offered to explain the observed phenomenology.