Examination of the Charge Quantum in a Single-Electron Pump

In single-electron tunneling (SET) circuits, charge moves in discrete quanta that are generally assumed to carry a charge of exactly $e$, the free electron charge. To the extent that this is true, SET devices have an important role to play in fundamental metrology by providing a solid-state current source that is directly linked to a fundamental constant of nature. But is the SET charge quantum in fact exactly $e$? We discuss why this is not a trivial question and present an experimental answer to the question: by placing a known number of SET charge quanta onto a known capacitor, and by measuring the resulting voltage across the capacitor using a Josephson voltage standard, we compare the SET charge quantum to $e$. We find that the SET charge quantum is equal to $e$ within a relative standard uncertainty of 1 part in $10^6$, a constraint that is $\sim 100$ times smaller than the best previous result. This measurement is expected to reach an uncertainty $\sim 3$ parts in $10^7$ in the near future, at which point it will also give useful information on possible corrections to the Josephson constant, $K_J = 2e/h$.