In a major technological advance, physicists shone a laser on trapped anti-atoms to detect whether they behaved any differently to atoms.
The work could shed light on one of the enduring mysteries about antimatter.
Although the Big Bang produced matter and antimatter in equal amounts, today, the Universe overwhelmingly consists of matter - and current theories cannot explain why.
Antimatter is incredibly difficult to produce and then capture and hold on to - not least because it gets annihilated on contact with ordinary matter.
But by using a specially-designed magnetic trap, researchers working on Cern's Alpha experiment near Geneva, Switzerland, were able to study properties of anti-hydrogen - the antimatter form of hydrogen.
CERN reported the observation of the 1S-2S transition in magnetically trapped atoms of antihydrogen in the ALPHA-2 apparatus at CERN. They determine that the frequency of the transition, driven by two photons from a frequency stabilised laser at 243 nm, is consistent with that expected for hydrogen in the same environment. This represents the first laser excitation of an internal quantum state of an atom of antimatter, and the most precise measurement performed on an anti-atom. Our result is consistent with CPT invariance at a relative precision of ~ 2x10^-10.
The first observation of the 1S-2S transition in trapped antihydrogen has been published in Nature and is the first time a spectral line has been observed in antihydrogen. This builds on years of work, developing techniques to manipulate super-cold antiprotons and positrons, create trapped antihydrogen and detect the very few atoms that are available to the experiment. It is another crucial step towards precision comparisons of antihydrogen and hydrogen.
Directly measuring if there are any differences between the antimatter partners may help us understand why our Universe is made almost entirely of matter, even though matter and antimatter should have been produced in equal quanitites in the Big Bang.
In our experiment, we trapped antihydrogen atoms in our magnetic trap and illuminated them with laser light with a wavelength close to 243nm. In one series of runs, we tuned the light so that it is in resonance with the 1S-2S transition in hydrogen, and in a second series, so that it was detuned by 200 kHz. Interactions between the laser and the trapped atoms should cause atoms to be lost from the trap.
In each run, after 600s of illumination, we counted the number of atoms left in the trap using our annihilation imaging detector. When the laser was tuned to resonance, we observed 67 atoms in 11 runs; when the laser was detuned, we counted 159 atoms in the same number of runs. We also searched for signs of the atoms annihilating as they left the trap while the laser illuminated. When the laser was on-resonance, we observed 79 events that pass our criteria for inclusion, and 27 when off-resonance. Both of these comparisons help us conclude that the on-resonance laser light is interacting with the antihydrogen atoms via their 1S-2S transition.
This first result implies that the 1S-2S transition in hydrogen and antihydrogen are not too different, and the next steps are to measure the transition's lineshape and increase the precision of the measurement.