Surface acoustic waves are sound waves which propagate in a thin layer along the surface of an elastically deformable material, much like seismic waves along the earth’s surface. The layer thickness roughly equals the sound wavelength which is less than a micrometer at sound frequencies in the GHz range and for typical sound velocities of solid media.
In piezo-electric crystals like Gallium Arsenide the sound wave is accompanied by a wave of electric potential, which can be used to drag the electrons of a two dimensional system beneath the surface of the crystal. This generates an electric current without the application of an electrical voltage.
If on limits the current path by an artificial constriction to a width of less than a micrometer electrons are moved through the constriction individually and sequentially. In the ideal case, realised at low temperature, there is one electron in each minimum of the acousto-electric wave. This results in a current I = e·f, where f is the sound frequency and e is the charge of an electron.
Central to our investigations is the question how close one can approach the ideal case, i.e. how precise is the relation I = e·f fulfilled. If one wants to trace back the unit of electric current Ampere to the elementary charge e and to the very well known unit of frequency the error should be less than 0.1 µA/A in order to be at least as precise as when reproducing the Ampere by classical means. Although this is currently not the case it can be assumed that the improved understanding of the microscopic details of the transport will lead to optimised devices and an improved precision.
In any case, there are obvious advantages of an acousto-electric current source in its higher operating frequency compared to other single-electron sources as well as in the possibility to further increase the current by operating more than one channel in parallel which we have already demonstrated. We have also achieved the operation of an acousto-electric current source at a world-record frequency of 4.78 GHz.