We present measurements of an electron pump based on the Coulomb block
ade in two multiple-tunnel junctions (MTJ's) connected in series, util
izing a single source of radio-frequency signal. The electron pump rea
lizes both forward and reverse electron flow. In the reverse flow, the
electrons move from the drain to the source in an opposite direction
to the bias-voltage polarity. Moreover, the current polarity due to th
e electron pump can be switched by the two dc side-gate voltages. The
bidirectional electron pump is attributed to asymmetric Coulomb blocka
de gaps at positive and negative sides in the current-voltage characte
ristics of the MTJ's.