We manufacture and study positronium (Ps). This is an atom made of an electron (e-) bound to an anti-electron (e+, also known as a positron). The particle/ antiparticle components have equal but opposite electric charge and are held together in a similar way to the electron and proton in a hydrogen atom. However, because an anti-electron has the same mass as an electron (0.00054 × the mass of a proton), Ps is much lighter than any of the elements found in the periodic table.
Unlike hydrogen, Ps is inherently unstable and the electron and anti-electron usually annihilate with each other within a few hundred billionths of a second, producing gamma-ray photons. Fortunately, the short lifetime of ground-state Ps is not too short for us to probe it using pulsed lasers. These can be used to resonantly drive the atoms to highly-excited Rydberg states that are comparatively long lived, making it possible to perform experiments over much longer time scales.
We’re working on techniques to manipulate Ps atoms using electric fields (exploiting the large electric dipole moments of Rydberg states), and hope to eventually make a slow beam for a precise gravity measurement. Because “How does antimatter interact with gravity?” is still one of the most important unanswered questions of modern physics.
We are grateful to the EPSRC (Grant No. EP/K028774/1, EP/R006474/1), ERC (Marie Sklodowska Curie action (MSCA) Career Integration Grant No. 630119) as well as the Leverhulme Trust (Grant No. RPG-2013-055) and Lundbeck Foundation (Grant No. R49-A5860) for providing funding.