Observation of the 1S-2P Lyman-α transition in antihydrogen

Abstract

In 1906, Theodore Lyman discovered his eponymous series of transitions in the extreme-ultraviolet region of the atomic hydrogen spectrum^1,2. 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 antihydrogen atom, the antimatter counterpart 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 antihydrogen with those of its well-studied matter equivalent allow precision tests of fundamental symmetries between matter and antimatter. Alongside the ground-state hyperfine^4,5 and 1S-2S transitions^6,7 recently observed in antihydrogen, the Lyman-α transition will permit laser cooling of antihydrogen^8,9, 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 cooling of antihydrogen, and the extension of antimatter spectroscopy to quantum states possessing orbital angular momentum.

Fig. 1. Experimental set-up.

a, The three-layer silicon vertex annihilation detector is shown schematically in green; the external solenoid magnet for the Penning traps is not shown in this diagram. Laser light enters from the positron (e⁺) side (right) and is transmitted to the antiproton ($\overline{p}$) side (left) through vacuum-ultraviolet-grade MgF₂ ultrahigh-vacuum windows. The laser beam crosses the trap axis at an angle of 2.3°. The transmitted 121.6-nm pulses are detected by a photomultiplier at the antiproton side. b, Axial magnetic well formed by the five mirror coils and responsible for the axial confinement of cold (less than 0.5 K) anti-atoms. c, Radial magnetic octupole field profile. PMT, photomultiplier tube; THG, third-harmonic generation.

Fig. 2. Trappable and untrappable energy levels in hydrogen.

This plot reports the calculated energies for the sublevels of the 1S, 2S and 2P states for hydrogen as functions of the magnetic field strength. Note that the centroid energy difference E_1S–2S = 2.4661 × 10¹⁵ Hz is suppressed on the vertical axis. The vertical red arrow indicates the one-photon laser transition addressed here; the dashed red arrow illustrates the decay to the same trappable level, whereas the dashed black arrow corresponds to the decay to the untrappable level. Antihydrogen that decays to the untrapped 1S level escapes from the trap and can be detected by the annihilation detector.

Fig. 3. Temporal and spatial distribution of detected events.

a, Temporal distribution of detected signals between −2 ms and 10 ms with respect to the laser pulse. The laser frequency was at the resonance of the 1S–2P transition (detuning 0 GHz). The black trace shows the raw detector triggers. The red trace shows events after background suppression due to an MVA-based machine-learning algorithm. The y axis is the total number of counts (bin size, 50 μs) for 24,000 pulses at the resonant frequency. b, Same as a, but the laser frequency was off-resonance; the detuning was −2.47 GHz. c, The axial position of the annihilation events is plotted versus the TOF. The detuning was 0 GHz. d, Same as c, but the detuning was −2.47 GHz. Events within the dashed box for each laser frequency were selected for the spectral shape determination (Fig. 4a).

Fig. 4. 1S–2P line shape, time and axial distributions.

a, The 1S–2P spectral line shape. Black, detected events, normalized to the total number of trapped antihydrogen atoms and scaled to the result obtained for a 600-s irradiation at 0.55 nJ. The error bars represent the statistical counting uncertainties. Red and blue, simulated line shapes for the initial conditions of n = 1 (simulation 1, red) and n = 30 (simulation 2, blue) (Methods). The statistical uncertainties of the simulation are within the size of each marker symbol. b, Black, the TOF distribution of all the detected annihilation events, measured relative to the laser pulse. Red, the simulated TOF distribution reflecting the experimental condition. Green and blue, simulated distributions for trapped antihydrogen atoms in thermal equilibrium at 100 mK and 10 mK, respectively. The simulated curves are normalized by area to the data (black). c, Comparison of the measured and simulated axial position distributions of the annihilations. The simulated distributions include the detector response function. Black, measured distribution. Red, simulated distribution. Green and blue, simulations with thermal distributions of 100 mK and 10 mK, respectively.

Extended Data Fig. 1. Laser system.

The figure shows a schematic of the 121.6-nm laser system for driving the 1S–2P transition. See Methods for details.