Credit: © 2007 Nature Physics

As electronic devices get smaller, researchers need new ways to measure quantities such as electric current. A novel approach to measuring current that is based on counting electrons as they 'surf' through a quantum wire could lead to a new definition of the ampere.

The present definition dates back to the nineteenth century, and, like several other units, it ultimately relies on the kilogram being defined as the mass of a platinum–iridium cylinder that is stored near Paris. A long-term goal in metrology, therefore, is to define units in terms of fundamental physical constants such as the charge on the electron.

Physicists have been attempting to redefine the ampere for decades, but have not been able to achieve the performance that is needed for a reliable standard. Now, Mark Blumenthal of Cambridge University and the National Physical Laboratory in the UK and co-workers1 have shown that it may be possible to define the ampere in terms of electron charge with a device that uses a series of gates to control the current through a quantum wire.

By applying sinusoidal voltages to the gates, the electrons surf through the device on a time-dependent potential, rather than tunnelling, as happens in most other approaches. Blumenthal and co-workers were able to measure currents at the nanoampere level, which is enough for a reliable standard, and believe they can improve the accuracy of their approach from 1 part in 10,000 to the 1 part in 10 million that is needed.