Developing the latest devices and techniques to disseminate the SI units to industry
The SI units for current, voltage and resistance are all realised using quantum techniques. We are actively involved with the development of quantum devices and technologies, and how they can be used to support the realisation of the SI units and disseminate them to industry.
Single electron pumps
NPL is developing nano-scale devices for moving electrons one at a time around an electrical circuit. These devices may form the foundation of a future redefinition of the SI base unit for current, the ampere. 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.
Quantum spillage in single electron pump
Our single-electron pumps generate a current by repeatedly trapping and releasing individual electrons. For use in quantum current standards, operation at high speed is desirable. However, the fundamental laws of quantum mechanics (first predicted in the 1920s) dictate how fast an electronic wavefunction can be perturbed without disturbing the system from its ground state.
We have discovered that these 'nonadiabatic' effects can lead to 'quantum spillage' of electrons from our electron pumps. These electrons escape from our trap like water spilled from a coffee cup, which has a detrimental effect on the accuracy of the pump. Interestingly, this actually gives us a novel way of looking at the electronic states in the dot. Fortunately, we have also found that high magnetic fields can be used to control the spillage. At high magnetic fields we can effectively 'stiffen' the electronic wavefunction, which protects it from the external perturbation.