The UK’s National Physical Laboratory (NPL) holds an open day every two years, inviting members of the public into the laboratory to see the laboratories and talk to measurement scientists.

This year more than 3300 visitors were admitted to the site. The tour of the facilities included the laboratories where single electron devices are developed as part of the e-SI-Amp project.

In addition to seeing electron pump devices under optical microscopes and seeing the cryostats and instrumentation used to measure the device, several demonstrations were developed to help explain the principles behind single electron pumps. For instance, understanding the operation of the electron pump can be aided by a mechanical model (ours is not the first) where the length, time and energy scales are all scaled up, making the ‘pumping’ process easy to visualise.

Single electron pump fabricated on a GaAs wafer: This tiny device can trap and release single electrons at GHz frequencies.

Typically, electron pumps are fabricated on silicon or gallium arsenide semiconductor chips, with feature sizes of only 10s of nanometres. Operated at cryogenic temperatures (and often at high magnetic fields) and controlled by precision voltage sources and sophisticated RF synthesisers, they move electrons through the device one at a time. As each electron carries the same elementary charge and the rate of pumping is known (using an accurate time standard), these devices can generate a well-defined electrical current. The current (typically 100 pA) only depends on the elementary charge and the pump frequency and no other parameters.

The model pump is made from wood and 3D printed materials and is very much ‘macroscopic’! The ball-bearings are 15mm in diameter. An Arduino microcontroller controls the servo speed and counts the number of electrons passing a light gate. The rails are formed from 3.6 mm diameter semi-rigid coaxial cable.

The model is 100,000 times larger than a real pump, but illustrates the principle of controlling electrical current at its most granular level – single electrons. If each ball were an electron, at this pumping rate the current would only be 0.11 attoamperes (an aA is 10^-18 amperes). This would be undetectable by even the best current measuring instruments. You would need 1,000 000 000 000 000 000 of them operating in parallel to charge your mobile phone!

Our real electron pumps are a billion times faster than this, operating at a rate of ~billions of electrons per second. This still produces far too small a current to charge a phone, but is large enough and accurate enough to calibrate measuring instruments and to perform consistency tests of quantum electrical standards. In the revised SI (where the value of the elementary charge is fixed) operation of an electron can constitute the realisation of the the unit ampere. 

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