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Quantum dot moves to redefine ampere


A pump which picks electrons up one-by-one and moves them across a barrier to create a well-defined electrical current has been developed by National Physical Laboratory and University of Cambridge scientists.

The nano-device drives electrical current by manipulating individual electrons one at a time at very high speed in a technique that could replace the traditional definition of electrical current, the ampere.


The breakthrough came when researchers experimented with the exact shape of the voltage pulses that control the trapping and release of electrons. By changing the voltage slowly while trapping electrons and much more rapidly when ejecting them, researchers sped up the overall rate of pumping without compromising accuracy.


The team used a quantum dot to pump electrons through a circuit. The dot can be filled with electrons and raised in energy before all but one fall back to the source lead. The remaining electron is ejected by tilting the trap. When repeated rapidly this gives a current determined by the repetition rate and charge on each electron – a universal constant of nature and the same for all electrons.


“Our device is like a water pump in that it produces a flow by cyclical action. The tricky part is making sure exactly the same number of electronic charge is transported in each cycle,” said Masaya Kataoka from the Quantum Detection Group at NPL.


“The way that the electrons in our device behave is quite similar to water; if you try and scoop up a fixed volume of water, say in a cup or spoon, you have to move slowly otherwise you’ll spill some. This is exactly what used to happen to our electrons if we went too fast.”


The team pumped almost a billion electrons per second and although the resulting current was quite small – around 150 picoamperes – they measured the accuracy to 1.2ppm. The result is a milestone and an important step towards redefining the ampere.


“Using mechanical forces to define the ampere has made a lot of sense for the last 60 years or so, but now that we have the nanotechnology to control single electrons we can move on,” said Stephen Giblin. “A quantum system of measurement is more elegant, because you are basing your system on fundamental constants of nature.”