Observation of the 1S–2P Lyman-α transition in antihydrogen
2018
In 1906, Theodore Lyman discovered his
eponymousseries of transitions in the
extreme-ultravioletregion of the atomic hydrogen spectrum1,2. The patterns in the hydrogen spectrum helped to establish the emerging theory of quantum mechanics, which we now know governs the world at the atomic scale. Since then, studies involving the Lyman-α line—the 1S–2P transition at a wavelength of 121.6 nanometres—have played an important part in physics and astronomy, as one of the most fundamental atomic transitions in the Universe. For example, this transition has long been used by astronomers studying the intergalactic medium and testing cosmological models via the so-called ‘Lyman-α forest’3 of absorption lines at different redshifts. Here we report the observation of the Lyman-α transition in the
antihydrogenatom, the
antimattercounterpart of hydrogen. Using narrow-line-width, nanosecond-pulsed laser radiation, the 1S–2P transition was excited in
magnetically trapped
antihydrogen. The transition frequency at a field of 1.033 tesla was determined to be 2,466,051.7 ± 0.12 gigahertz (1σ uncertainty) and agrees with the prediction for hydrogen to a precision of 5 × 10−8. Comparisons of the properties of
antihydrogenwith those of its well-studied matter equivalent allow precision tests of fundamental symmetries between matter and
antimatter. Alongside the ground-state hyperfine4,5 and 1S–2S transitions6,7 recently observed in
antihydrogen, the Lyman-α transition will permit
laser coolingof antihydrogen8,9, thus providing a cold and dense sample of anti-atoms for precision spectroscopy and gravity measurements10. In addition to the observation of this fundamental transition, this work represents both a decisive technological step towards
laser coolingof
antihydrogen, and the extension of
antimatterspectroscopy to quantum states possessing orbital angular momentum.
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