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Physical Review A

Laser-spectroscopy measurement of the fine-structure splitting 23P1–23P2 of He4

Gaoping Feng, Xin Zheng, Yu Sun, Shui-Ming Hu

Published Mar 23, 2015• 24 citations
Publisher

Abstract

Laser spectroscopy has been performed on a beam of neutral $^{4}\mathrm{He}$ atoms. By using transverse laser cooling and focusing, we are able to prepare a bright beam of atoms in the metastable state $2\phantom{\rule{0.16em}{0ex}}{}^{3}{S}_{1}$ 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.

DOI
10.1103/physreva.91.030502

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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 ($\ensuremath{\delta}{v}_{z}<\ifmmode\pm\else\textpm\fi{}4.5$ 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.
The hyperfine transitions in the 399 nm ${^{1}S}_{0}\ensuremath{\leftrightarrow}{^{1}P}_{1}$ line in 173Yb^{173}\text{Y}\text{b}173Yb overlap with the transition in 172Yb^{172}\text{Y}\text{b}172Yb. We use transverse laser cooling on this line to selectively deflect 173Yb^{173}\text{Y}\text{b}173Yb atoms from a thermal beam containing both isotopes. We then use an acousto-optic modulator to span the frequency difference between the hyperfine transitions, and hence measure hyperfine structure in the 1P1{^{1}P}_{1}1P1​ state. Our precise values for the hyperfine constants, ${A}_{173}=57.693\ifmmode\pm\else\textpm\fi{}0.006\text{ }\text{MHz}$ and ${B}_{173}=609.028\ifmmode\pm\else\textpm\fi{}0.056\text{ }\text{MHz}$, improve previous values significantly.
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The First Measurement of the 23S1→33P−23P2^{3}S_{1} \rightarrow 3^{3}P - 2^{3}P23S1​→33P−23P Tune-Out Wavelength in He*
The workhorse of atomic physics, quantum electrodynamics, is one of the best-tested theories in physics. However recent discrepancies have shed doubt on its accuracy for complex atomic systems. To facilitate the development of the theory further we aim to measure transition dipole matrix elements of metastable helium (He*) (the ideal 3 body test-bed) to the highest accuracy thus far. We have undertaken a measurement of the `tune-out wavelength' which occurs when the contributions to the dynamic polarizability from all atomic transitions sum to zero; thus illuminating an atom with this wavelength of light then produces no net energy shift. This provides a strict constraint on the transition dipole matrix elements without the complication and inaccuracy of other methods. Using a novel atom-laser based technique we have made the first measurement of the tune-out wavelength in metastable helium between the 33P1,2,33^{3}P_{1,2,3}33P1,2,3​ and 23P1,2,32^{3}P_{1,2,3}23P1,2,3​ states at 413.07(2) nm which compares well with the predicted value\cite{Mitroy2013} of 413.02(9) nm. We have additionally developed many of the methods necessary to improve this measurement to the 100 fm level of accuracy where it will form the most accurate determination of transition rate information ever made in He* and provide a stringent test for atomic QED simulations. We believe this measurement to be one of the most sensitive ever made of an optical dipole potential, able to detect changes in potentials of $\sim$200 pK and is widely applicable to other species and areas of atom optics.