Breakthrough in multiple electron pumping in the Coldest Place in Africa

08 Nov 2023
Electron Pump Device
08 Nov 2023
Electron Pump Device
Electron pump device

Associate Professor Mark Blumenthal and his team from the NanoElectronics Research Centre in the Department of Physics at UCT have had a major breakthrough in their research detailed in a newly published paper in the journal EPJ Quantum Technology.  The paper “Multiple electron pumping” with contributions from students in the group, highlights the new ability to pump a multiple integer number of single electrons per cycle with a far higher degree of accuracy and fidelity than any previous on demand source of single electrons. Previous pumping mechanisms showed that the pumping of more than one electron per cycle degrades the quantisation of the measured current. Through extensive research endeavours the group is able to move as many as 7 individual electrons per cycle with unprecedented accuracy and reproducibility.  

Structure of the Fridge
Structure of the Dilution Fridge

Where did this research happen?
This all took place in the NanoElectronics laboratory – the coldest place in Africa – utilising a Cryogen Free He3/He4 Dilution Fridge which has a cooling power of up to 1400 μW at 120 mK and a base temperature of 7mK, making it one of the most powerful fridges of its kind on the market. The researchers using cleanroom techniques similar to what is employed in the design and development of transistor-based semiconductor chips, fabricated electron pumps at the nano scale, able to isolate, capture and manipulate individual electrons.  With high-speed electronics the researchers are able to move packets of single electrons 180 million times a second, a technique only a handful of research groups around the world can attempt.

What are electrons?
Electrons which are found in all atoms are so small they are regarded as dimensionless point particles, the smallest particle known.  Under classical physics a diameter has been attributed to the electron. If the electron is then scaled up such that its diameter was the length of an average human, 1.7m, the same scaling then applied to a human would result in the average human height being around 368 000 times larger than the diameter of the sun!  Electrons have a negative charge that causes them to repel each other like magnets with the same polarity.  This makes manipulating a fixed number in a reliable and reproducible way challenging and is why this finding of moving multiple electrons with such accuracies for the first time, is such a significant achievement. 

SEM golden gates
SEM golden gates

How has this been possible?
This breakthrough has been possible because of the change in design of the electron pumps designed in the semiconductor coupled with modified electronic signals that are operating the pump.  Using clean room fabrication technology the smallest structures comprising the electron pump are 100 nanometre gold finger gates which are around the size of a virus. In the NanoElectronics lab, the pumps are operated at around 30mK which is 30 times colder that the coldest know naturally occurring place in the universe, known as the Boomerang Nebula sitting at a balmy 1K. 

What is the impact of this breakthrough?
This work will find potential application in quantum metrology in the development of a new standard for electrical current.  It will also allow for the development of techniques in quantum cryptograph (using the quantum nature of particles to crypt), quantum computing as well as quantum information processing, where the requirement for a tuneable on-demand source of single electrons are needed. 

This significant discovery has been the result of collaboration between the University College London and Cambridge University.  It is a big achievement to have done this work and made the discovery here at the bottom end of Africa, where all the measurements were carried out. 

Being published in a Springer-Nature, open access, competitive journal is a huge achievement for the group. Associate Professor Blumenthal’s students assisted with the modelling of data and contributed to the writing of the paper.  Manipulating and tracking something as incredibly small as electrons is directly in the sphere of quantum mechanics. This is a demanding and exciting field to be working in, which comes with multiple frustrations and challenges – but a win like this spurs the team on to keeping going, pushing the barriers of science in the quantum world. 

Story:  Katherine Wilson