Fig. 1.

Two separate systems, 1 and 2, are said to be entangled if the properties of 1 and 2 cannot be defined independent of each other. Entanglement gives rise to correlations, regardless of the distance between 1 and 2. (A) In a 2D space, such as polarization (which can only be vertical or horizontal), individual measurement results on each system are random, but always correlated. Each system is in a superposition of the horizontal and vertical polarizations (B) In higher dimensions, one can imagine spinning a color wheel. When individually examined, the color where the wheel stops is random, but always the same for 1 and 2. Each system is in a superposition of the many colors in the wheel.