  We demonstrate single-electron pumping in a gate-defined carbon nanotube
double quantum dot. By periodic modulation of the potentials of the two quantum
dots we move the system around charge triple points and transport exactly one
electron or hole per cycle. We investigate the pumping as a function of the
modulation frequency and amplitude and observe good current quantization up to
frequencies of 18 MHz where rectification effects cause the mechanism to break
down.
