Quantum complementarity is a notion originated by one of the fathers of quantum mechanics, Niels Bohr, in an attempt to explain apparent inconsistencies in the structure of quantum theory. The general principle of complementarity states that a complete understanding of nature may require two (or more) mutually exclusive yet equally necessary descriptions of nature. The validity of either description of nature can be tested through specific experiments, but the observation of one particular description requires experiments that preclude the possibility of observing the other mutually exclusive, or complementary, descriptions. The fact that it is experimentally impossible to observe both descriptions at once is crucial in rendering the whole theory consistent.

The idea of wave-particle duality embodies the concept of complementarity, and indeed it was Bohr's attempts to understand wave-particle duality that gave rise to the idea of complementarity in general. Light, although thought of classically as...