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Dr Joanna Waldie

Dr Joanna Waldie

Herchel Smith Postdoctoral Research Fellow

Office Phone: 01223 764172

Research Interests

Single Electron Pumps

An electron pump repeatedly transmits a single electron (or another whole number of electrons) into an electronic circuit. Electrons are fundamental particles and they all have exactly the same electronic charge, e = 1.60217662 × 10-19 coulombs. Electrical current is the rate of flow of charge. So, a perfect single electron pump will generate an exactly known current I = ef, where f is the pump repetition frequency. We want to use this to make a new definition of what we mean by the SI unit of current, the ampere, and a new way to calibrate current meters (ammeters). If our electron pumps can reliably pump single electrons more than 6.7 billion times per second, to give a current more than 1 nanoampere (nA), with an error rate less than 1 in 10 million, then the electron pump will be the most accurate way to define the ampere and calibrate ammeters. This could significantly improve the accuracy of all kinds of measurements that rely on detecting small currents, from monitoring air pollution levels to testing for radioactivity and measuring doses for radiotherapy.

I am researching electron pumps made from quantum dots in the compound semiconductor gallium arsenide (GaAs), in collaboration with the UK National Physical Laboratory. These pumps can already produce 0.15 nA of current with an error rate less than 1 in a million. To increase the current output and reduce the error rate we need to understand more about the physics of single electron transport, and use this understanding to develop better pumps. I am working on ways to combine pumps in parallel, to get more current with existing designs. We are also developing large arrays of pumps that can be tested one by one using an on-chip multiplexer, so that we can test new pump designs much more rapidly. This work is part of the European Metrology Programme for Innovation and Research project e-SI-Amp, which is working towards making the new electron pump ampere definition a reality. You can watch a video about this project here.

I am also involved in research into other low-dimensional electronic systems in GaAs-based devices, including real-time studies of charge dynamics in double quantum dots, interaction-driven states in electron-hole bilayers and charge mobility in 2D hole systems.

Key Publications

A complete laboratory for transport studies of electron-hole interactions in GaAs/AlGaAs ambipolar bilayers, Ugo Siciliani de Cumis, Joanna Waldie, Andrew F. Croxall, Deepyanti Taneja, Justin Llandro, Ian Farrer, Harvey E. Beere, and David A. Ritchie, Applied Physics Letters 110, 072105 (2017); doi: 10.1063/1.4976505 (Open access version here)

Switching between attractive and repulsive Coulomb-interaction-mediated drag in an ambipolar GaAs/AlGaAs bilayer device, B. Zheng, A. F. Croxall, J. Waldie, K. Das Gupta, F. Sfigakis, I. Farrer, H. E. Beere, and D. A. Ritchie, Applied Physics Letters 108, 062102 (2016); doi: 10.1063/1.4941760 (Open access version here)

Measurement and control of electron wave packets from a single-electron source, J. Waldie, P. See, V. Kashcheyevs, J. P. Griffiths, I. Farrer, G. A. C. Jones, D. A. Ritchie, T. J. B. M. Janssen, and M. Kataoka, Physical Review B 92, 125305 (2015); doi: 10.1103/PhysRevB.92.125305 (Open access version here)