A device capable of counting the individual electrons in an electric current, by feeding them through a pair of quantum dots, has been developed by scientists in Japan. The device can even spot the "backscattering" that occurs when electrons travel the wrong way through a circuit.
Toshima Fujisawa and colleagues at NTT Basic Research Laboratories in Atsugi, Japan, created a circuit incorporating a two quantum dots - semiconducting crystals just a few nanometres in diameter - which only let a single electron pass through at a time. After switching the current on, they used another nanoscale device, called a quantum point contact, to measure the charge contained within each quantum dot. This revealed whether it contained an electron or not.
By taking measurements every 20 microseconds the researchers could count the flow of individual electrons as they passed through the quantum dots, and also determine the direction in which they were moving. Link