Nano-Earthquakes: Acoustic Waves Excite Artificial Molecules

By absorbing photons from a laser, an atom can be excited to any of various discrete energy levels allowed by quantum mechanics. What about artificial atoms? A quantum dot, created by the same lithographic methods used to prepare electronic chips, is nearly a zero-dimensional zone of semiconducting material; as with electrons inside atoms, electrons inside the confinement of a quantum dot will also possess only a restricted menu of allowed energies.

The same is true for a pair of quantum dots 200 nanometer apart; with just the right voltage applied, electrons can tunnel from one dot to the other. In fact, an electron, considered as a spread-out quantum wave phenomenon, can be considered to reside in both dots at the same time, a property which makes the quantum-dot "molecule" potentially useful for carrying out quantum computing operations.

Now, a group of scientists have been able to probe, and to change, the quantum energy states of a double quantum dot with sound waves, or more particularly surface acoustic waves excited in the substrate supporting the dots.

The acoustic waves, less than 1 nanometer in amplitude, ripple through the surface for distances as long as hundreds of microns as a sort of nano-earthquake, are created through the process of piezoelectricity; a small voltage is sent into a series of tiny electrodes painted onto the surface. This excites the faint acoustic waves (see figure at Physics News Graphics).

The acoustic-dot arrangement, mediated by the delicate interactions between electrons and phonons, can work in both directions: The quantum dots can be used to monitor the acoustic waves -- otherwise difficult to detect because of their tiny energy -- or the acoustic waves can be used to interrogate the electronic status of the dots, which makes possible the aforesaid quantum-information applications.

The researchers involved work at the University of Twente and the Delft University of Technology (Netherlands), NTT Corporation, Tokyo Institute of Technology, and University of Tokyo (Japan), and Jilin University (China).

Naber et al., Physical Review Letters, 7 April 2006
Contact Wouter Naber, w.j.m.naber@utwente.nl
Image at Physics News Graphics