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Physical Review A

Speed-adjustable atomic beam of metastable helium for precision measurement

J.-J. Chen, Yu Sun, Jin-Lu Wen, Shui-Ming Hu

Published May 11, 2020• 9 citations
Publisher

Abstract

A bright beam of metastable helium atoms is useful in various studies. Laser spectroscopy of helium in an atomic beam can provide a determination of the fine-structure constant and the nuclear charge radius, towards a test of quantum electrodynamics (QED). In these measurements, it is necessary to control not only the internal quantum state but also the translation motion of the helium atom. Here we present a setup producing an intense and speed-adjustable continuous beam of helium atoms. By using a combination of laser cooling, focusing, and deflection, helium atoms are prepared at the single quantum state of ${2}^{\phantom{\rule{0.16em}{0ex}}3}{S}_{1}$ ($m=0$), with a very narrow distribution of the longitudinal speed ($\ensuremath{\delta}{v}_{z}<\ifmmode\pm\else\textpm\fi{}4.5$ m/s). At the same time, we obtain a flux of $1.8\ifmmode\times\else\texttimes\fi{}{10}^{13}$ (atoms/s)/sr with a fractional fluctuation below 0.02%. The atomic-beam quality was investigated by laser spectroscopy, indicating a considerable improvement over those used in previous helium precision-spectroscopy measurements.

DOI
10.1103/physreva.101.053824

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A bright beam of metastable helium atoms is useful in various studies. Laser spectroscopy of helium in an atomic beam can provide a determination of the fine-structure constant and the nuclear charge radius, towards a test of quantum electrodynamics (QED). In these measurements, it is necessary to control not only the internal quantum state but also the translation motion of the helium atom. Here we present a setup producing an intense and speed-adjustable continuous beam of helium atoms. By using a combination of laser cooling, focusing, and deflection, helium atoms are prepared at the single quantum state of 23S1{2}^{\phantom{\rule{0.16em}{0ex}}3}{S}_{1}23S1​ (m=0m=0m=0), with a very narrow distribution of the longitudinal speed (\ensuremathδvz<\ifmmode±\else\textpm\fi4.5\ensuremath{\delta}{v}_{z}<\ifmmode\pm\else\textpm\fi{}4.5\ensuremathδvz​<\ifmmode±\else\textpm\fi4.5 m/s). At the same time, we obtain a flux of 1.8\ifmmode×\else\texttimes\fi10131.8\ifmmode\times\else\texttimes\fi{}{10}^{13}1.8\ifmmode×\else\texttimes\fi1013 (atoms/s)/sr with a fractional fluctuation below 0.02%. The atomic-beam quality was investigated by laser spectroscopy, indicating a considerable improvement over those used in previous helium precision-spectroscopy measurements.
Laser spectroscopy has been performed on a beam of neutral 4He^{4}\mathrm{He}4He atoms. By using transverse laser cooling and focusing, we are able to prepare a bright beam of atoms in the metastable state 23S12\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}23S1​ deflected from the original effusive atomic beam. The initial state preparation is completed with optical pumping on the $2\phantom{\rule{0.16em}{0ex}}{}^{3}{P}_{1}\ensuremath{\leftarrow}2\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}$ transition at the wavelength of 1083 nm, followed by laser spectroscopy on the $2\phantom{\rule{0.16em}{0ex}}{}^{3}{P}_{1,2}\ensuremath{\leftarrow}2\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}$ transitions. The $2\phantom{\rule{0.16em}{0ex}}{}^{3}{P}_{1}\ensuremath{-}2\phantom{\rule{0.16em}{0ex}}{}^{3}{P}_{2}$ fine-structure splitting is determined to be $2\phantom{\rule{0.16em}{0ex}}291\phantom{\rule{0.16em}{0ex}}177.69\ifmmode\pm\else\textpm\fi{}0.36\phantom{\rule{0.16em}{0ex}}\mathrm{kHz}$. The quantum interference effect is included in data extraction. This is the most precise laser spectroscopy measurement of the interval. Our result is in agreement with both the latest QED-based calculation and the most precise measurement conducted with microwave spectroscopy.
Precision spectroscopy of simple atomic systems has refined our understanding of the fundamental laws of quantum physics. In particular, helium spectroscopy has played a crucial role in describing two-electron interactions, determining the fine-structure constant and extracting the size of the helium nucleus. Here we present a measurement of the doubly forbidden 1557-nanometer transition connecting the two metastable states of helium (the lowest energy triplet state 2 (3)S(1) and first excited singlet state 2 (1)S(0)), for which quantum electrodynamic and nuclear size effects are very strong. This transition is weaker by 14 orders of magnitude than the most predominantly measured transition in helium. Ultracold, submicrokelvin, fermionic (3)He and bosonic (4)He atoms are used to obtain a precision of 8 × 10(-12), providing a stringent test of two-electron quantum electrodynamic theory and of nuclear few-body theory.
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Intense, highly collimated sources of atoms have many potential applications. Bright beams will be important for competitive high flux and high resolution direct-write techniques in lithography, with the added advantage of parallel writing through laser manipulation. Intense sources will also be useful in other atom optic devices e.g. for loading atoms into hollow fiber waveguides. In atomic physics, many collision processes can only be measured with the sensitivity offered by such high flux sources. We report progress on the development of an intense, collimated beam of metastable helium atoms which improves the brightness generated by conventional nozzle discharge sources by several orders of magnitude. The system uses diode lasers to transversely collimate and then to longitudinally slow the atoms, using Zeeman tuning to compensate for the changing Doppler shift. The slowed, collimated beam is then compressed in a 2D magneto-optic trap before a final collimation stage, to achieve the required increase in intensity. Initial experiments using the helium source for some of the applications above are described.
Precision spectroscopic measurements in atoms and molecules play an increasingly important role in chemistry and physics, e.g., to characterize structure and dynamics at long timescales, to determine physical constants, or to search for physics beyond the standard model of particle physics. In this paper, we demonstrate the combination of Zeeman deceleration and transverse laser cooling to generate slow (mean velocity of 175 m/s) and transversely ultracold (${T}_{\ensuremath{\perp}}\ensuremath{\approx}135\phantom{\rule{0.28em}{0ex}}\textmu{}\mathrm{K}$) supersonic beams of metastable (1s)(2s)3S1(1s)(2s)\phantom{\rule{0.16em}{0ex}}^{3}S_{1}(1s)(2s)3S1​ He (He∗{\mathrm{He}}^{*}He∗) for precision spectroscopy. The curved-wavefront laser-cooling approach is used to achieve large capture velocities and high He∗{\mathrm{He}}^{*}He∗ number densities. The beam properties are characterized by imaging, time-of-flight, and high-resolution spectroscopic methods, and the factors limiting the Doppler widths in single-photon spectroscopic measurements of the $(1s)(40p)\phantom{\rule{0.16em}{0ex}}^{3}P_{J}\phantom{\rule{0.16em}{0ex}}\ensuremath{\leftarrow}(1s)(2s)\phantom{\rule{0.16em}{0ex}}^{3}S_{1}$ transition at UV frequencies around $1.15\ifmmode\times\else\texttimes\fi{}{10}^{15}$ Hz are analyzed. In particular, the use of skimmers to geometrically confine the beam in the transverse directions is examined and shown to not always lead to a reduction of the Doppler width. Linewidths as narrow as 5 MHz could be obtained, enabling the determination of line centers with a precision of $\mathrm{\ensuremath{\Delta}}\ensuremath{\nu}/\ensuremath{\nu}$ of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ limited by the signal-to-noise ratio. Numerical particle-trajectory simulations are used to interpret the experimental observations and validate the conclusions.
We have constructed an atomic beam of metastable helium atoms He(2 3S) with a mean velocity of 300 m/s (15 K) and a yield of 3×1012 atoms/s sr. The metastable atoms are produced in a dc discharge in a cryogenic environment cooled by liquid helium. Using a hexapole magnetic lens, we have increased further the beam intensity by focusing the metastable atoms. Initial studies show a factor of 2.5 increase in the beam flux but more is expected when the hexapole is constructed from permanent hexapole magnets. The He(2 3S) atoms are subsequently loaded into a magneto-optical trap.