Quantum decoherence

Summary

Quantum decoherence is the loss of interference between the parts of a superposition when a system interacts with its environment. In the reduced density matrix of the system, the off-diagonal entries in a preferred basis shrink towards zero, while the diagonal entries stay. It explains why an open quantum system in contact with many degrees of freedom shows classical statistics in that basis. It is a main obstacle to building quantum computers, and it is central in discussions of the measurement problem.