Illustration depicting quantum entanglement (mesh) between two particles (round). Their properties (spin, polarisation and momentum) are linked, or entangled, in such a way that the measurement of a property of one particle instantly correlates with the property of the second particle. This correlation occurs even when the particles are separated by large distances. Einstein described quantum entanglement as 'spooky action at a distance'. Quantum entanglement has been demonstrated experimentally with photons, neutrinos, electrons and carbon molecules such as buckyballs and small diamonds.