Developing devices and techniques to disseminate the SI units
Quantum electrical metrology is an area that develops technologies to enable the realisation of SI electrical units. These technologies underpin the ubiquitous usage of electricity in our modern society. The expanding usage of electricity in the highly networked digital society, environmental regulations and space technologies generates the demand for electrical measurements that are more accurate, sensitive and affordable.
Primary electrical standards are used to produce electrical quantities such as current, voltage, and resistance following the definition of the SI units. The corresponding electrical units such as ampere, volt, and ohm, are disseminated through traceable chains of calibration, such as calibration laboratories, instrument manufacturers to end users.
Since the electrical units are defined by fundamental constants such as elementary charge and Planck constant, the act of unit realisation must be performed by quantum mechanical experiments by controlling individual electrons or by exploiting the behavior of superconducting devices.
Single electron pumps
NPL is developing nano-scale devices for moving electrons one at a time around an electrical circuit. We are attempting to perfect the control of electrical charge using nano-devices to trap and manipulate single electrons. These techniques can be used to generate very precise electrical currents, which have the potential to represent the unit of current, the ampere. Single electron devices may also be a building block in future quantum circuitry and can be used to test our understanding of the laws of quantum mechanics.
Josephson junction arrays
NPL is exploiting the success of Josephson-effect devices as quantum standards of voltage to realise a new capability for waveform metrology. Using delta-sigma modulation techniques, the voltage generated by the junctions is manipulated with high resolution and high speed to enable quantum-accurate measurements over the frequency range from dc to 1 MHz.
Quantum Hall effect
The SI unit of resistance at NPL is realised using a quantum Hall effect device. Recent research on graphene devices has enabled this effect to be realised at both lower magnetic fields and higher temperatures, whilst still retaining part per billion accuracy. We are developing a table-top primary standard of resistance incorporating both a graphene quantum Hall effect sample and a cryogenic current comparator in the same cryostat, in order to provide a compact and easy-to-operate system for metrology laboratories.