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.
Researchers at Chalmers University of Technology in Gothenburg, Sweden, first developed a way to observe individual electrons within a current in March 2005.
However, their set-up could not tell the direction in which it an electron was moving. This is important for extremely precise measurements as, even when electrons are flowing mostly one way, a few can go backwards.
The Japanese scientists behind the invention believe it could be used to make an extremely sensitive ammeter, a device for measuring electric current. It could then be used, for example, to count the number of electrons that pass through the nanoscopic features of a microchip. By contrast, a standard ammeter needs millions of electrons to flow before it can provide a measurement of current.
Independent experts say the device could be used to study the fundamental behaviour of electrons and as critical components inside quantum computers, which must exploit quantum physics to perform calculations.
"These are fundamental subatomic particles," says Alexander Rimberg, a physicist at Dartmouth College in New Hampshire, US, who was not involved with the work. "To the extent that we want to be able to harness the power of quantum mechanics for something practical, we need to be able to manipulate and study these fundamental particles."
Journal reference: Science (vol 312, p 1634)
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