Charge spectrometry with a strongly coupled superconducting single-electron transistor

We have used a superconducting single-electron transistor as a dc electrometer that is strongly coupled to the metal island of another transistor. With this setup, it is possible to directly measure the charge distribution on this island. The strong capacitive coupling was achieved by a multilayer fabrication technique that allowed us to make the coupling capacitance bigger than the junction capacitances. Simulations of this system were done using the orthodox theory of single-electron tunneling and showed excellent agreement with the measurements.