The 1S-2P Lyman-α transition in antihydrogen has been observed.

[NNadir]

Very, very, very cool...

The Lyman series played a huge role in the foundation of modern quantum mechanics when it was explained by the young Neils Bohr, bringing him to the forefront of physics, in the history of which, he played such a huge role.

Now a transition from this series has been observed in antimatter in an elegant experiment.

From the abstract:

In 1906, Theodore Lyman discovered his eponymous series of transitions in the extreme-ultraviolet region 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-? 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-? forest’3 of absorption lines at different redshifts. Here we report the observation of the Lyman-? transition in the antihydrogen atom, the antimatter counterpart of hydrogen...

From the introduction:

Challenges to antimatter Lyman-? spectroscopy include the difficulty of fabricating optical components and continuous-wave11 or pulsed laser sources at these extremely short wavelengths, as well as the scarcity of anti-atoms. The current observation was made possible by a number of technical advances, including the development of a solid-state-based, pulsed Lyman-? source12, implementation of innovative plasma control techniques13, and the ALPHA Collaboration’s recent, marked improvement in antihydrogen trapping and accumulation or ‘stacking’ rate. Stacking provides a sample of several hundred anti-atoms14, accumulated over several hours. Taking advantage of a nanosecond-scale laser pulse and our low-background annihilation detection, we have also inferred information on the antihydrogen velocity distribution.

Because matter and antimatter annihilate each other when they meet, antihydrogen must be created and then trapped in strong, inhomogeneous magnetic fields in an ultrahigh-vacuum chamber. The ALPHA-2 apparatus (Fig. 1a) is designed to combine antiprotons from CERN’s Antiproton Decelerator15 with positrons from a positron accumulator16,17 to produce and to trap atoms of antihydrogen.

A picture of the apparatus:

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 (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.

The paper is apparently open sourced, and should you be interested, and inspired to read it yourself, it's here: Observation of the 1S–2P Lyman-? transition in antihydrogen.

Cool, I think, very cool...