Measurement of the incremental charge of a superconducting island

We have measured the total charge of a superconducting electrode or ‘island’ which is free to exchange electrons with a metallic ground through a tunnel junction and which is capacitively coupled to a voltage source at potential U. We find that the island charge varies stepwise with U, the incremental charge being equal to 2 e inside a low-temperature, low-magnetic-field domain, and to e elsewhere. In this latter case, steps corresponding to different parities of the number of electrons inside the island have unequal lengths. The odd-even free energy difference deduced from the asymmetry of the steps is well explained by the theory of dirty superconductors.