Surface acoustic waves (SAWs)

can excite artificial molecules. A lithographically fabricated quantum dot allows electrons only a restricted menu of energies. The same is true for a pair of quantum dots 200 nm apart, and with just the right voltage applied, electrons can tunnel from one dot to the other. In fact, an electron, viewed as a spread-out quantum wave, can be considered to reside in both dots at the same time, a property that makes the quantum-dot “molecule” potentially useful for quantum computation (see Physics Today, March 2006, page 16 ). Now, using SAWs excited in the substrate supporting a double quantum dot, a group of scientists has been able to probe and even change the dots’ quantum energy states. The piezoelectrically generated waves, less than 1 nm in amplitude, ripple over the surface for hundreds of microns. The SAW–dot system can operate in both directions: The quantum dots can be used to monitor the acoustic waves, which because of their minuscule energy are otherwise difficult to detect; or the acoustic waves can be used to interrogate the electronic status of the dots, which makes possible quantum information applications. The researchers are at the University of Twente and the Delft University of Technology in the Netherlands; NTT Corporation, the Tokyo Institute of Technology, and the University of Tokyo in Japan; and Jilin University in China. (W. J. M. Naber et al., Phys. Rev. Lett. 96, 136807, 2006 http://dx.doi.org/10.1103/PhysRevLett.96.136807 .)