NPL's 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. Although this would seem to be an impediment to accurate pumping at high frequency, high magnetic fields can be used to suppress this effect by effectively stiffening the electronic wavefunction against external perturbation.