  We report on the operation of a single electron trap comprising a chain of
four Al/AlOx/Al tunnel junctions attached, at one side, to a memory island and,
at the other side, to a miniature on-chip Cr resistor R=50 kOhm which served to
suppress cotunneling. At appropriate voltage bias the bi-stable states of the
trap, with the charges differing by the elementary charge e, were realized. At
low temperature, spontaneous switching between these states was found to be
infrequent. For instance, at T=70 mK the system was capable of holding an
electron for more than 2 hours, this time being limited by the time of the
measurement.
