Physicists at Northwestern University have coaxed a single positron (the antimatter counterpart of an electron) into an exquisitely controlled quantum state for the first time. The scientists cooled the fast-moving particle to nearly absolute zero after capturing it from radioactive decay, and then suspended it in a near-perfect vacuum, resulting in a state where the positron behaved much like an artificial atom with quantum energy levels scientists can precisely control and observe. The technique could enable the most precise test yet of a fundamental symmetry between electrons and their antimatter counterparts.