Dynamic quantum dots with tunneling

In a QD, electrons are restricted to a region of space so tiny as to enforce a quantum regime; the electrons may only have certain discrete energies, which can be useful, depending on the circumstances, in producing laser light or in detectors and maybe even future computers. QDs are usually fabricated in a semiconductor and controlled with voltages applied to nearby electrodes. In recent years, though, fast-moving QDs have been fashioned by trapping electrons in the minima of surface acoustic waves that zip through a channel past static surface gates. (For more on SAWs, see Physics Today, March 2002, page 42 .) A team of physicists at the Cavendish Laboratory at the University of Cambridge has now determined the tunneling rate of electrons from such dynamic QDs, a necessary step for putting the traveling dots to practical use. The experimenters calculated the tunneling rates for the case of 1, 2, or 3 electrons trapped in each QD (blue dots in the artist’s rendition; the actual device is also shown) and the SAWs moving at about 2800 m/s past a tunneling barrier to a two-dimensional electron gas (the big blue blob). They found that the tunneling occurred on a 600-ps time scale and that its rate was controlled by both the number of electrons in each dot and the voltage on the barrier. (M. R. Astley et al., Phys. Rev. Lett. 99 , 156802, 2007 .)