Quantum coherence
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Quantum coherence refers to the condition of a quantum system whose constituents are in-phase. That is, the various states that make up the combined wavefunction have a fixed phase relationship with one another, making them correlated to one another as described by quantum mechanics. This should be contrasted with quantum decoherence, the process whereby coherence is lost. In a two-particle system, if there is a distinct phase relationship between the two component states, it is called a pure state. The correlation between the two particles exceeds what would be predicted for classical correlation alone (see Bell's inequalities). If this two-particle system is decohered (which would occur in a measurement via Einselection), then there is no longer any phase relationship between the two states. The system is now said to be mixed, and is entirely analogous to a classical system of mixed probabilities (the correlations are classical).
Large-scale (macroscopic) quantum coherence leads to very amazing phenomena. For instance, the laser, superconductivity, and superfluidity are examples of highly coherent quantum systems. One example that shows the amazing possibilities of macroscopic quantum coherence is the Schrödinger's cat thought experiment. Another example of quantum coherence is in a Bose-Einstein condensate. Here, all the atoms that make up the condensate are in-phase. They are thus all described by a single quantum wavefunction. Their behavior is communal and inseparable until the coherence is destroyed.
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