Quantum realm is a term of art in physics referring to scales where quantum mechanical effects become important when studied as an isolated system. Typically, this means distances of 100 nanometers (10−9 meters) or less or at very low temperature. More precisely, it is where the action orangular momentum is quantized.
While originating on the nanometer scale, such effects can operate on a macro level generating some paradoxes like in the Schrödinger’s cat thought experiment. Two classical examples are electron tunneling and the double-slit experiment. Most fundamental processes in molecular electronics, organic electronics and organic semiconductors also originate in the quantum realm.
The quantum realm can also sometimes paradoxically involve actions at long distances. A well-known example is David Bohm’s (1951) version of the famous thought experiment that Einstein, Podolsky and Rosen proposed in 1935 (EPR paradox). Pairs of particles are emitted from a source in the so-called spin singlet state and rush in opposite directions. When the particles are widely separated from each other, they each encounter a measuring apparatus that can be set to measure their spin components along various directions. Although the measurement events are distant from each other, so that no slower-than-light or light signal can travel between them, the measurement outcomes are curiously correlated.