Entanglement

When you can tell something about one object by looking at another, we call those objects correlated. Quantum particles can share a special sort of correlation called entanglement, where the two objects are so strongly correlated that the properties of one cannot be described without considering the properties of another. Instead of being described by two separate wavefunctions, entangled objects are described by a single joint wavefunction. Entanglement describes a superposition state of multiple quantum particles, such as two electrons. While the properties of the electrons may be individually highly uncertain, their properties when measured together may be incredibly predictable. For example, two position-entangled electrons may have wavefunctions that spread over a large space, but when we measure one electron, we instantly know exactly where the other one is. Entanglement can be seen between quantum particles even if we separate them by vast distances, with some experiments showing entanglement surviving over hundreds of kilometres.

Source: Institute of Quantum Computing  – University of Waterloo