Acoustic Quantum Dots.

A new experiment at the Cavendish Lab at the University of Cambridge is the first to controllably shuttle electrons around a chip and observe their quantum properties. A quantum dot restricts electrons to a region of space in a semiconductor so tiny as to be essentially zero-dimensional. This in turn enforces a quantum regime; the electron may only have certain discrete energies, which can be useful, depending on the circumstances, for producing laser light or for use in detectors and maybe even future computers.

A quantum dot is usually made not by carving the semiconductor into a tiny grain but rather by imposing restrictions on the electron’s possible motions by the application of voltages to nearby electrodes. This would be a static quantum dot. It is also possible to make dynamic quantum dots-that is, moving dots that are created by the passage of surface acoustic waves (SAWs) moving through a narrow channel across the plane of a specially designed circuit chip (see figure at http://www.aip.org/png/2007/289.htm). The acoustic wave itself is generated by applying microwaves to interleaved fingered electrodes atop a piezoelectric material like GaAs. The applied electric fields between finger-electrodes induce a sound wave to propagate along the surface of the material.

These acoustic waves have the ability to scoop electrons and chauffeur them along the surface.

The tiny region confining the electron even as it moves is in effect a quantum dot. Such acoustic-based dynamic quantum dots have made before, but according to Cambridge researcher Michael Astley (mra28@cam.ac.uk), this is the first time the tunneling of the electrons (even single electrons) into and out of the quantum dots has been observed. This is an important part of the whole electron-shuttling process since one wants control over the electron motions and spins. If, moreover, electrons in two very close acoustic wave channels could be entangled, then this would present the chance to make a sort of flying qubit, which could be at the heart of a quantum computer. (Astley et al., Physical Review Letters, upcoming article)