COMPARISON OF SINGLE-ELECTRON TRAPS IN THE SUPERCONDUCTING AND NORMAL STATES

Traps, consisting of arrays of nano-scale aluminum tunnel junctions, have been shown to hold single electrons for over 10(4) s (limited by observation time) at 50 mK. The barrier Delta U to the change of the charge state of the trap has a measured value in the range of 5-10 K (consistent with calculations) for traps of seven junctions (in the normal state) of capacitance C approximate to 150 aF. In the superconducting state, the activation energy is increased by 2.6 +/- 0.4 K, equal to the superconducting energy gap of Al. The observed escape of a single electron from the trap is via thermal activation down to temperatures of 50 mK (where Delta U must be reduced to observe transitions).