Resonant Quantum Transitions in Trapped Antihydrogen Atoms

Since 2002 it has been possible, using specialised equipment available at CERN, the European Laboratory for Particle Physics, to create antihydrogen. This atom is the antimatter equivalent of hydrogen, the most abundant atom in the Universe, and is comprised of an antiproton as a nucleus, with a positive electron, the positron, orbiting. The antihydrogen atoms were created in vacuum, but until very recently they quickly migrated to the walls of the apparatus where they annihilated on contact with matter.

However in the last couple of years, teams of physicists, again based at CERN, have learnt how to capture some of the antihydrogen atoms in a magnetic minimum neutral atom trap. This type of trap exploits the fact that the anti-atoms, just like hydrogen, have a tiny magnetic moment, and this can result in some of them being attracted to a minimum in a magnetic field that was engineered within the apparatus using a complex coil design.

A few of these anti-atoms have been held for periods longer than 15 minutes. In a very recent advance, the ALPHA antihydrogen collaboration [1] has observed the first resonant quantum transition in an anti-atom [2]. Microwaves shone onto the trapped antihydrogen reversed the direction of the magnetic moment with respect to the magnetic field, such that the anti-atoms left the trap and annihilated when the microwaves were of the correct frequency: i.e., on resonance.

These experiments will be described, as well as the motivation for undertaking them. The latter involves one of nature’s great conundrums: the absence of bulk antimatter in the current epoch of the Universe. Some non-technical accounts of aspects of our work can be found in references [3]-[5].

References

1. ALPHA website - http://alpha-new.web.cern.ch/

2. C. Amole et al. (ALPHA collaboration), Resonant quantum transitions in trapped antihydrogen atoms, Nature 483 (2012) 439

3. M. Charlton, Antihydrogen on tap, Physics Education 40 (2005) 229

4. Mike Charlton and Jeffrey Hangst, Probing the antiworld, Physics World 18 {issue 10} (2005) 22

5. M. Charlton, S. Jonsell, L.V. Jørgensen, N. Madsen and D.P. van der Werf, Antihydrogen for precision tests in physics, Contemporary Physics 49 (2008) 29