Electromechanical charge shuttle
The quest of counting electrons is one of the key challenges in metrology. It relates to the attempt of linking the electrical units directly with fundamental constants, as it is the case for voltage and resistance using the Josephson effect and the Quantum Hall effect, respectively. So far, a similar definition of current is yet to be achieved. The creation, measurement and control of current at the single electron level represents a natural limit of precision, and although realization of this ultimate current standard has yet to be achieved, mechanical electron shuttles provide a promising approach.
The shuttle is realized by a gold island hosted in the center of a doubly-clamped silicon nitride nanostring which is situated in a gap between the source and drain electrode. Oscillation of the beam brings the island into contact with the electrodes, and in the presence of a DC bias the repetitive charging of the island results in a current mediated by the moving island.
See: König et al., Nature Nano 3, 482 (2008), König et al., Appl. Phys. Lett. 101, 213111 (2012).