The researchers have created a chain of thin insulating layers, so-called tunnel junctions, between metallic nano-islands, and integrated this chain with the most sensitive charge detector that exists. The charge of the electron is very small and difficult to detect.
– In an ordinary conductor, the electron transport is disordered, but in our chain, we force the electrons to line up in a train, and when this train moves along the chain by quantum mechanical tunnelling, we can detect every single electron. This phenomenon was predicted 20 years ago by Russian scientists, but no one has been able to demonstrate it experimentally before, says Jonas Bylander, who is a student in the group.
Conventionally, a current, I, is measured as the voltage, V, across a resistor, R, with a known value, using Ohm’s law, I=V/R. Then the single electrons are not noticed. In modern precision measurements, however, people try to express physical quantities in the form of countable units, or quanta, and the quantum of electrical current is the electron charge, e. If the electrons pass one by one with the frequency f, then the current is given by the relation I=ef. Currents much smaller than one picoampere can be measured with this new method.
– Our method can be developed into a new, better definition of the unit ampere. Furthermore, the measurement is self-calibrated, since the only parameter involved is a natural constant, e. The method is totally free from measurement offset and drift, says Jonas Bylander.
The Nature article:
Current measurement by real-time counting of single electrons.
Jonas Bylander, Tim Duty and Per Delsing, Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology, Göteborg, Sweden
Nature vol. 434, pp. 361-364, 17 March 2005.