The double quantum dot is realized in a 2DEG AlGaAs-GaAs semiconductor heterostructure, see Fig.(3.4). Focused ion beam implanted in-plane gates define a narrow channel of tunable width which connects source and drain (left and right electron reservoir). On top of it, three Schottky gates define tunable tunnel barriers for electrons moving through the channel. By applying negative voltages to the left, central, and right Schottky gate, two quantum dots (left and right ) are defined which are coupled to each other and to the source and to the drain. The tunneling of electrons through the structure is large enough to detect current but small enough to have a well-defined number of electrons ( and ) on the left and the right dot, respectively. The Coulomb charging energy ( meV and meV) for putting an additional electron onto the dots is the largest energy scale, see Fig.(3.4).
The main experimental trick is to keep the system within these states and to change only the energy difference of the dots without changing too much, e.g., the barrier transmissions. The measured average spacing between single-particle states ( and meV) is still a large energy scale compared to the scale on which is varied. The largest value of is determined by the source-drain voltage which is around meV. The main findings are the following:
1. At a low temperature of mK, the stationary tunnel current as a function of shows a peak at with a broad shoulder for that oscillates on a scale of eV, see Fig.(3.5).
2. For larger temperatures , the current increases stronger on the absorption side than on the emission side. The data for larger can be reconstructed from the mK data by multiplication with Einstein-Bose factors (the Planck radiation law) for emission and absorption.
3. The energy dependence of the current on the emission side is between and . For larger distance of the left and right barrier ( nm on a surface gate sample instead of nm for a focused ion beam sample), the period of the oscillations on the emission side appears to become shorter.
Those who are interested in more details on this fascinating experiment can read the article: T. Fujisawa, T. H. Oosterkamp, W. G. van der Wiel, B. W. Broer, R. Aguado, S. Tarucha, and L. P. Kouwenhoven, Science 282, 932 (1998).