A team of scientists at the National Physical Laboratory (NPL) and University of Cambridge has made a significant advance in using nano-devices to create accurate electrical currents. Electrical current is composed of billions and billions of tiny particles called electrons. NPL scientists have developed an electron pump – a nano-device – which picks these electrons up one at a time and moves them across a barrier, creating a very well-defined electrical current.
The device drives electrical current by manipulating individual electrons, one-by-one at very high speed. This technique could replace the traditional definition of electrical current, the ampere, which relies on measurements of mechanical forces on current-carrying wires.
The key breakthrough came when scientists experimented with the exact shape of the voltage pulses that control the trapping and ejection of electrons. By changing the voltage slowly while trapping electrons, and then much more rapidly when ejecting them, it was possible to massively speed up the overall rate of pumping without compromising the accuracy.
By employing this technique, the team were able to pump almost a billion electrons per second, 300 times faster than the previous record for an accurate electron pump set at the National Institute of Standards and Technology (NIST) in the USA in 1996.
Although the resulting current of 150 picoamperes is small (ten billion times smaller than the current used when boiling a kettle), the team were able to measure the current with an accuracy of one part-per-million, confirming that the electron pump was accurate at this level. This result is a milestone in the precise, fast, manipulation of single electrons and an important step towards a re-definition of the unit ampere.
As reported in Nature Communications, the team used a nano-scale semiconductor device called a 'quantum dot' to pump electrons through a circuit. The quantum dot is a tiny electrostatic trap less than 0.0001 mm wide. The shape of the quantum dot is controlled by voltages applied to nearby electrodes.
The dot can be filled with electrons and then raised in energy. By a process known as 'back-tunneling', all but one of the electrons fall out of the quantum dot back into the source lead. Ideally, just one electron remains trapped in the dot, which is ejected into the output lead by tilting the trap. When this is repeated rapidly this gives a current determined solely by the repetition rate and the charge on each electron – a universal constant of nature and the same for all electrons.
The research makes significant steps towards redefining the ampere by developing the application of an electron pump which improves accuracy rates in primary electrical measurement.
National Physical Laboratory: http://www.npl.co.uk/
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