Electron Pumps and Re-Definition of the SI Unit Ampere

The upcoming 2011 general conference on weights and measures (CGPM) will consider a proposal to re-define the SI unit ampere by assigning a fixed value to the electron charge e, while simultaneously releasing the permeability of free space μ₀ to become a measured parameter. This review provides the background to the proposed re-definition and describes research into electron pumps aimed at supporting it. Particular emphasis is given to recent results on semiconductor electron pumps, which are currently the most promising candidates for metrological current standards.