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.
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).
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.
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
Naber et al.,
Physical Review Letters, 7 April 2006
Contact Wouter Naber, email@example.com
Image at Physics News Graphics