ABSTRACT
Quantum theory, developed by physicists in the early 1900s and spectacularly successful in its own field, also provides a more complete and natural arena for developing concepts of communication and computation. The central principle of quantum mechanics is the superposition principle. Quantum physicists often use the words observation and measurement as near synonyms, describing situations in which a system under study interacts with another system, animate or inanimate, so that the latter learns or records something about the former. A diagonal measurement distinguishes diagonal photons reliably but randomises rectilinear photons. A rectilinear measurement distinguishes vertical and horizontal photons reliably, but randomises diagonal photons. Consider a diagonal photon about to be subjected to a measurement of its vertical versus horizontal polarisation. Photons behave reliably if measured along an axis parallel or perpendicular to their original polarisation.
