Large-scale (macroscopic) quantum coherence leads to novel phenomena. For instance, the laser, superconductivity, and superfluidity are examples of highly coherent quantum systems, whose effects are evident at the macroscopic scale. These examples of quantum coherence are Bose–Einstein condensates. Here, all the particles that make up the condensate are in-phase; they are thus necessarily all described by a single quantum wavefunction.

On the other hand, the Schrodinger’s cat thought experiment, highlights the fact that quantum coherence is not typically seen at the macroscopic scale but has been observed in the motion of a mechanical resonator (see Quantum machine).

— Wikipedia on Quantum coherence

2011.10.02 Sunday ACHK