Transferring Electrons One By One

Abstract

This chapter describes experiments in which the transfer of charge through small capacitance tunnel junction devices is controlled at the single electron level. From a fundamental point of view, these devices provide an insight into the basic concepts behind single charge tunneling such as global rules discussed in Chap. 2. More particularly, the detailed understanding of the transfer accuracy involves recently discovered effects, such as co-tunneling through several junctions discussed in Chap. 6. From a practical point of view, these devices could lead to metrological applications such as the accurate measurement of the fine structure constant a and the realisation of a current standard. We describe below three devices: 1) The single electron box [1], which provides the simplest example of the macroscopic quantization of the charge on an “island” of a small junction circuit on an integer number of electron charges. (We call an island a metallic electrode that electrons can enter or leave only by tunneling through a junction.) The charge on this island can thus be controlled at the single electron level with a gate voltage. This circuit is the basic building block for more elaborate designs. 2) The single electron turnstile [2], which was the first device to produce a current clocked electron by electron with an external periodic signal. This device requires a bias voltage and a gate voltage upon which the rf clock signal is superimposed. The direction of the current is determined by the sign of the bias voltage, and one electron is transferred through the circuit for each cycle of the rf signal. The transfer accuracy is limited mainly by electron heating and by co-tunneling. Electron heating results from the conversion of electrostatic energy into electron kinetic energy in the device, thus causing unwanted thermally activated tunneling events which degrade the regularity of the charge transfer. Co-tunneling across two or more junctions is also the source of some unwanted transitions, further degrading the control of electron transfer. 3) The single electron pump [3], which also produces a current clocked by an external periodic source. In contrast with the turnstile, this device requires two gate voltages upon which properly phase-shifted rf signals of the same frequency are superimposed. Moreover, the direction of the current is not determined by the sign of the bias voltage but by the phase-shift between the rf gate voltages. At low frequencies, energy is exchanged between the gate sources and the bias source, without any energy transfer to the electrons: the pump is therefore a reversible device. As a consequence, there is no heating at low frequencies, so the charge transfer accuracy of the pump is mainly limited by co-tunneling.