This is a difficult process. The magnetic forces on these atoms are quite weak, so the trap can only contain antihydrogen atoms of very low motional (kinetic) energy, that is at very low temperature. The current state-of-the-art ALPHA trap can contain antihydrogen atoms in their ground state if they are colder than about 0.5 Kelvin (i.e. 0.5 degrees above absolute zero). ALPHA is currently working on making such cold antihydrogen.