Current measurement by real-time counting of single charges

Licentiate thesis, 2005

The first real-time observation of time correlated single-electron tunnelling is reported in this thesis. This is a direct detection of charge discreteness in an electrical current. When a current, I, flows through a chain of metallic islands, connected by small tunnel junctions, a lattice of charges is formed due to the Coulomb repulsion. These charges propagate throughout the array by time correlated quantum mechanical tunnelling at the average frequency f=I/e, where e is the charge of the electron. This phenomenon is analogous to the AC Josephson effect. We have combined such a chain with an ultrasensitive charge sensor, a radio-frequency single-electron transistor (RF-SET), and injected the full charge into the SET island. By using the RF-SET to monitor the single charges as they pass by, we have measured currents in the range 5 fA–1 pA by counting electrons. This is a fundamentally new way to measure small currents, which is more accurate than established techniques and moreover has the advantage of being self-calibrated since the only parameter involved is the natural constant e. As a consequence, the method does not suffer from measurement offset or drift. In an optimized device, with higher absolute current and better accuracy, we see a possibility to use our electron counter in an experiment that closes the quantum metrological triangle relating current, voltage and frequency by fundamental constants. Direct extensions of this work would be to look for Bloch oscillations of frequency f=I/2e in a superconducting array similar to the one used. Furthermore, our method could shed new light on the statistical properties of mesoscopic charge transport.