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Posted: Jul 05, 2016
Record speed and accuracy achieved with single-electron pumps
(Nanowerk News) The National Physical Laboratory (NPL), in collaboration with NTT (Nippon Telegraph and Telephone Corporation) in Japan, has measured silicon single-electron pumps with the highest speed and accuracy ever achieved, paving the way towards practical primary standards for electric current.
NTT's silicon single-electron device chip mounted on a sample holder for NPL's high-precision current measurement system.
Single-electron pumps are tiny electronic devices that generate an electric current by moving individual electrons. These devices could be used as primary standards for the SI unit of electric current, the ampere. Presently, the definition of the ampere links it to the artefact kilogram, and there is no practical method to directly realise the ampere with the accuracy required for present-day electrical measurements.
The two key requirements of single-electron pumps are high accuracy and high speed. Because the electrical charge of each electron is very small, a huge number of electrons need to be pumped within a given time to produce a usable current. At the same time, the exact number of electrons pumped in each cycle needs to be known to obtain an accurate value of the current. The difficulty is that high-speed pumping tends to make devices operate less accurately. One way to overcome this and achieve more robust operation is to make the device very small, as this minimises detrimental effects arising from energy fluctuations.
In the state-of-the-art silicon device fabrication facility at the NTT Basic Research Laboratories in Atsugi, single-electron devices were made with a 10-nanometre-scale silicon wire. These devices were found to operate at well over 1 GHz, a barrier that conventional single-electron pumps made from gallium arsenide-based materials have not been able to break without a significant loss in accuracy.
The test performed in a high-precision small-electrical-current measurement system, developed by NPL, confirms that these silicon devices can operate at 1 GHz with an accuracy better than one part per million. Even at 2 GHz, the accuracy was maintained at a level of 3 parts per million.
This is the first time that silicon single-electron pumps have been tested at such accuracy levels. In 2018, the worldwide metrology community plans to redefine four of the seven SI base units, including the ampere, in terms of fixed fundamental constants.
Ahead of the redefinition, single-electron pumps must be reliably tested to ensure that the current produced does not depend on the precise details of the experiment - for example, what material the device is made from. Instead, it should depend only on the elementary charge, one of fundamental constants of nature, and the frequency of operation, which can be determined to a very high level of accuracy using atomic clocks. Highly-accurate techniques to verify the performance of single-electron pumps, such as those being developed at NPL, are crucial in the international effort to prepare for the new SI ampere.