Top: KRISS single electron pump, defined by surface gates on GaAs. Botttom: Schematic of electron pump profile for different values of plunger gate voltage.

Tunable barrier pumps in semiconductor devices can drive a clock-controlled pump current and thus represent a prototype quantum current standard. A problem facing any practical electron pumping scheme is that accurate pumping cannot take place at arbitrarily high frequency. In practice this limits accurate pumping to a frequency of order 1 GHz. Scientists at KRISS, South Korea have studied this effect in devices with additional control over the pump potential shape and have developed a model which captures some of these operation frequency limitations.

Single-parameter pumping relies on the creation of a confined region in which single electrons can be trapped by modulation of the potential on one of the confining gates. Pumping generally only occurs for certain range of pump geometry, control voltages and operation frequency. Understanding what determines this operating envelope is a challenging problem.

In a recently published paper by e-SI-Amp partner KRISS, the effective pumping frequency limit is extracted from pumped current measured in different pumping configurations. This takes advantage of their highly tunable multi-gate electron pump, where a plunger gate can be used to control the depth of the quantum dot, something which influences the error rate of the pump and the maximum frequency of operations.

Interestingly, the maximum frequency at which both successful loading and ejecting survive can be enhanced by changes to the dot shape, but at the cost of pumping accuracy. Understanding the details of these frequency-accuracy trade-offs is useful when developing devices for higher pumping frequencies f>> 1 GHz.

Upper frequency limit depending on potential shape in a QD-based single electron pump
Ye-Hwan Ahn, Changki Hong, Young-Seok Ghee, Yunchul Chung, Young-Pyo Hong, Myung-Ho Bae, and Nam Kim Journal of Applied Physics  122, 194502 (2017); doi: 10.1063/1.5000319