  One of the biggest challenges of nanotechnology is the fabrication of
nano-objects with perfectly controlled properties. Here we employ a focused
laser beam both to characterize and to {\it in-situ} modify single
semiconductor structures by heating them from cryogenic to high temperatures.
The heat treatment allows us to blue-shift, in a broad range and with
resolution-limited accuracy, the quantized energy levels of light and charge
carriers confined in optical microcavities and self-assembled quantum dots
(QDs). We demonstrate the approach by tuning an optical mode into resonance
with the emission of a single QD and by bringing different QDs in mutual
resonance. This processing method may open the way to a full control of
nanostructures at the quantum level.
