Shadow states and shaded states
Ching Tsung Lee · Quantum and Semiclassical Optics Journal of the European Optical Society Part B · 1996
For every real state with photon-number distribution , a virtual state with distribution can be constructed and called a shadow state because it cannot stand by itself. The real state mixed with its shadow state is called a shaded state. This is a systematic way to construct nonclassical states. The introduction of shadow states makes it much easier to identify, or to isolate, the origin of the nonclassical effects in shaded states. The usefulness of this concept is illustrated by several examples.