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...