In the 1980s and 1990s a series of revolutionary developments in the foundations of quantum mechanics led to what would later become the thriving fields of quantum information, quantum computation, and quantum cryptography. The roots of this revolution lie in the debate between Albert Einstein and Niels Bohr on the interpretation of quantum mechanics, specifically in the notion of "entangled" quantum states at the heart of the Einstein-Podolsky-Rosen argument for the incompleteness of quantum mechanics. What Einstein, Podolsky, and Rosen showed in their 1935 paper "Can Quantum-Mechanical Description of Physical Reality be Considered Complete?" was that composite quantum systems, consisting of spatially separated subsystems, could exist in certain states with peculiar nonclassical correlations between the outcomes of measurements on the subsystems. They argued that these correlations are incompatible with the assumption that the quantum state is a complete description of the system.

In...