Real-time detection of electron tunnelling in a quantum dot

Nanostructures in which strong (Coulomb) interactions exist between electrons are predicted to exhibit temporal electronic correlations1. Although there is ample experimental evidence that such correlations exist2, electron dynamics in engineered nanostructures have been observed directly only on long timescales3. The faster dynamics associated with electrical currents or charge fluctuations4 are usually inferred from direct (or quasi-direct) current measurements. Recently, interest in electron dynamics has risen, in part owing to the realization that additional information about electronic interactions can be found in the shot noise5 or higher statistical moments6,7 of a direct current. Furthermore, interest in quantum computation has stimulated investigation of quantum bit (qubit) readout techniques8,9, which for many condensed-matter systems ultimately reduces to single-shot measurements of individual electronic charges. Here we report real-time observation of individual electron tunnelling events in a quantum dot using an integrated radio-frequency single-electron transistor10,11. We use electron counting to measure directly the quantum dot's tunnelling rate and the occupational probabilities of its charge state. Our results provide evidence in favour of long (10 µs or more) inelastic scattering times in nearly isolated dots.