The most famous quantum phenomenon is quantum entanglement, which states that particles can be separated but linked so that they share information instantly. Quantum entanglement has been used to generate unbreakable encryption but has a maximum distance at which it is useful. Another example of a quantum phenomenon is atomic clocks, which use the principles of quantum theory to precisely measure time. Examples of quantum phenomena on the macroscopic scale are superfluidity, superconductivity, and the behavior of the Bose-Einstein condensate. A Bose-Einstein condensate is a state of matter in which separate atoms or subatomic particles, when cooled to near absolute zero, behave like a single particle.