BLOGS

The international system of units (SI) is made up of seven base units, which are the metre, kilogram, second, kelvin, ampere, mole and candela. The ampere, volt and ohm are the three fundamental units of electricity.

Ideally, a new definition of the ampere would be based on an extremely accurate source of electric current, capable of delivering one electron at a time. A single-electron pump (SEP) could be ideal in this respect because it produces a flow of individual electrons by shuttling them into a quantum dot and emitting them precisely one at a time. A good SEP also pumps the electrons quickly, so a sufficiently large current is generated.

Until recently, two types of SEP were promising contenders: tunable barrier pumps made from semiconductors, which are fast, and so-called hybrid turnstiles made from superconductors, which can be mounted in parallel to make the output current larger. Although the most accurate, a third type of pump usually made from metallic islands is too slow for making a practical current standard, but the UK researchers have now improved its performance by making it from graphene, which is a semi-metal. Graphene is a sheet of carbon just one atom thick that has a honeycomb lattice structure.

"Our experiments have shown that graphene is ideal for pumping large currents and its 2D crystal structure is just what is needed to make electrons pass through the SEP quickly," team leader Malcolm Connolly told physicsworld.com. The electron flow can reach near-gigahertz frequencies, very close to what is needed to create a current standard, he added.

If it proves accurate enough, the SEP could also help close the "quantum metrological triangle", which relates current, voltage and resistance. Voltage can be measured using the AC Josephson effect, while resistance can be related through the quantum Hall effect. Both these relationships include the same two fundamental constants – Planck’s constant, h, and the charge on the electron, e. A metrological current pump would allow physicists to directly relate current to frequency, and thus test whether e and h are as universal as we think.