Confirming antihydrogen neutrality with voltage bias

Neutral atoms appear to have literally zero net electric charge. The consistency of quantum field theory requires that the proton and electron have the same charge magnitude e, and observations of several atoms and molecules have confirmed neutrality to 10⁻²¹e. Theory demands that antiatoms are likewise strictly neutral. But direct experimental confirmation of the theoretical expectation has been problematic. Now the ALPHA collaboration at CERN has measured the charge of antihydrogen to be −1.3 × 10⁻⁸ e, consistent with zero to within about one standard deviation and six orders of magnitude more precise than the previous best antihydrogen measurement. To obtain its result, the collaboration returned to data it had taken a few years ago on antihydrogen in its cryogenic trap, a detail of which is shown in the figure. As part of those experiments, group members released antihydrogen atoms and then detected...