Attempting to measure electron charge accurately
The single electron tunnelling (SET) transistor has been devised by researchers seeking to find out whether the quantum nature of electrons will determine how devices are built. Single electron tunnelling transistors consist of a small metallic island connected to two separate electrodes by tunnel junctions for the entrance and exit of electrons. By applying voltage on a gate electrode capacitively coupled to the island, the energy needed to charge electrons on the island is controlled. The SET transisitors' sensitivity to charge makes them ideal for high-precision electrometers that could measure the delicate superpositions of charge states in superconducting islands. On the other hand, superconducting islands could provide a means for implementing the quantum bits needed for quantum computers. Therefore the focus of research efforts undertaken by the SQUBIT-2 project partners was focused on eliminating the undesirable measurement noise. More specifically, a radical approach to eliminating noise coming from the substrate was explored at the laboratories of the University of Jyväskylä in Finland. Aluminium single electron transistors were fabricated with the island out of contact with the silicon nitride (SiN2) substrate. Careful investigations have demonstrated that noise caused by trapped charge fluctuators in the substrate was minimised. The way has therefore been opened for the application of these devices in solid state systems suitable for quantum computation.