prepare a quantum system), we are converting it into an information-driven system with a given purpose (to achieve a change that would not happen naturally without our intervention). All laboratory experiments, whether a simple classroom demonstration or a sophisticated table-top quantum experiment, fall into this category.

It will be useful for our discussion of quantum systems to identify common features in a classical measurement process. First of all, note that it necessarily involves force-driven interactions, but they are controlled by a human being or a preplanned artifact. For instance, when you measure the size of an object with a caliper, the instrument interacts elastically with the object; if you measure it with a ruler, you need to submerge everything in a “bath of photons” whose scattering or reflection is what your optical system uses to extract the wanted information on “object–apparatus” interaction. Note that in the latter example, *the environment* has become an integral part of the instrument (this will be important for the discussion of quantum measurements). In summary, the measurement process represents an information-driven interaction between the object or system to be measured and a measurement apparatus or device (which contains a reference which we call “the unit”). 