The spectrum of the hydrogen atom has played a central part in fundamental physics in the past 200 years. Historical examples of its significance include the wavelength measurements of absorption lines in the solar spectrum by Fraunhofer, the identification of transition lines by Balmer, Lyman et al., the empirical description of allowed wavelengths by Rydberg, the quantum model of Bohr, the capability of quantum electrodynamics to precisely predict transition frequencies, and modern measurements of the 1S–2S transition by Hänsch¹ to a precision of a few parts in 10¹⁵. Recently, we have achieved the technological advances to allow us to focus on antihydrogen—the antimatter equivalent of hydrogen^2,3,4. The Standard Model predicts that there should have been equal amounts of matter and antimatter in the primordial Universe after the Big Bang, but today’s Universe is observed to consist almost entirely of ordinary matter. This motivates physicists to carefully study antimatter, to see if there is a small asymmetry in the laws of physics that govern the two types of matter. In particular, the CPT (charge conjugation, parity reversal, time reversal) Theorem, a cornerstone of the Standard Model, requires that hydrogen and antihydrogen have the same spectrum. Here we report the observation of the 1S–2S transition in magnetically trapped atoms of antihydrogen in the ALPHA-2 apparatus at CERN. We determine that the frequency of the transition, driven by two photons from a laser at 243 nm, is consistent with that expected for hydrogen in the same environment. This laser excitation of a quantum state of an atom of antimatter represents a highly precise measurement performed on an anti-atom. Our result is consistent with CPT invariance at a relative precision of ~2 × 10⁻¹⁰.For the first time, researchers have probed the energy difference between two states of the antimatter atom.

The best known research at CERN centers on collisions of particles accelerated to higher and higher energies. But for the past 30 years, the lab has also hosted several research teams working to decelerate antiprotons, combine them with positrons, and cool and trap the resulting atoms of antihydrogen. A main goal of that research is to perform precision spectroscopic measurements that might reveal differences between matter and antimatter—and help to explain why the universe contains so much more of the former than the latter. (See the Quick Study by Gerald Gabrielse, Physics Today, March 2010, page 68.) Now CERN’s ALPHA collaboration has achieved the first spectroscopic success: observing the transition between antihydrogen’s 1S and 2S states.

The standard technique for atomic spectroscopy—exciting atoms with a laser and detecting the photons they emit—is unsuitable for antihydrogen. First, the coils and electrodes required to magnetically trap the antihydrogen, as shown here, leave little room for optical detectors. Second, the researchers trap only 14 antihydrogen atoms at a time, on average, so the optical signals would be undetectably weak.

Happily, antimatter offers an alternative spectroscopic method that works well for small numbers of atoms. When an antihydrogen atom is excited out of its 1S (or ground) state, it can be ionized by absorbing just one more photon. The bare antiproton, no longer confined by the magnetic field, quickly collides with the wall of the trap and annihilates, producing an easily detectable signal.

When the researchers tuned their excitation laser to the exact frequency that would excite atoms of hydrogen, about half of the antihydrogen atoms were lost from the trap during each 10-minute trial. When they detuned the laser by just 200 kHz—about 200 parts per trillion—all the antihydrogen remained in the trap. By repeating the experiment for many more laser frequencies, the ALPHA team hopes to get a detailed measurement of the transition line shape. But that will have to wait until the experiment resumes in May 2017.