Shot noise in the current of a surface acoustic-wave-driven single-electron pump

We have measured the noise at approximately 1.6 MHz in the current produced by a single-electron pump that uses an approximately 2.7 GHz surface acoustic wave (SAW). The current can be varied by altering the voltage applied to surface gates. Over the range of gate voltage where the current is close to the quantized value corresponding to one electron being transported per cycle of the SAW, the noise in the current is dominated by shot noise, whereas away from this range the noise mostly arises from switching the charge states of electron traps in the material. By combining measurements of the shot noise and the current, we determined how the error rates--the probabilities of transporting zero or two electrons in a cycle--vary with gate voltage when the current is close to the quantized value. The results obtained suggest that these two probabilities are controlled by closely linked mechanisms.