Measurement of single electron lifetimes in a multijunction trap

Single electron traps have been shown to hold a single charge for over 2 h at 50 mK (limited by observation time). The traps, each with an array of seven Al/${\mathrm{AlO}}_{\mathit{x}}$/Al tunnel junctions of normal state resistance R∼300 kΩ, and capacitance, C∼0.15 F, have trapped electrons with the junctions in both the superconducting and normal states. The temperature dependence of the escape time has been measured for one trap in the superconducting state near 0.35 K and observed to follow an Arrhenius law with an energy barrier ΔU/${\mathit{k}}_{\mathit{B}}$∼4 K in agreement with theoretical estimates.