Editing Quantum decoherence (section)

==Concept==
In [[quantum mechanics]], physical systems are described by a mathematical representation called a [[quantum state]]. Probabilities for the outcomes of experiments upon a system are calculated by applying the [[Born rule]] to the quantum state describing that system. Quantum states are either ''pure'' or ''mixed''; pure states are also known as ''wavefunctions''. Assigning a pure state to a quantum system implies certainty about the outcome of some [[measurement in quantum mechanics|measurement]] on that system, i.e., that there exists a measurement for which one of the possible outcomes will occur with probability 1. In the absence of outside forces or interactions, a quantum state evolves [[unitarity (physics)|unitarily]] over time. Consequently, a pure quantum state remains pure. However, if the system is not perfectly isolated, for example during a measurement, [[quantum coherence|coherence]] is shared with the environment and appears to be lost with time ─ a process called quantum decoherence or environmental decoherence. The quantum coherence is not lost but rather mixed with many more degrees of freedom in the environment, analogous to the way energy appears to be lost during friction in classical mechanics when it actually has produced heat in the environment.

[[Coherence (physics)|Decoherence]] can be viewed as the loss of information from a system into the environment (often modeled as a [[heat bath]]),<ref name="Bacon">{{cite arXiv |last=Bacon |first=D. |year=2001 |title=Decoherence, control, and symmetry in quantum computers |eprint=quant-ph/0305025 }}</ref> since every system is loosely coupled with the energetic state of its surroundings. Viewed in isolation, the system's dynamics are non-unitary (although the combined system plus environment evolves in a unitary fashion).<ref name="Lidar and Whaley">{{cite book |last1=Lidar |first1=Daniel A. |first2=K. Birgitta |last2=Whaley |title=Irreversible Quantum Dynamics |chapter=Decoherence-Free Subspaces and Subsystems |editor1-first=F. |editor1-last=Benatti |editor2-first=R. |editor2-last=Floreanini |pages=83–120 |series=Springer Lecture Notes in Physics |volume=622 |location=Berlin |year=2003 |arxiv=quant-ph/0301032 |isbn= 978-3-540-40223-7|bibcode=2003LNP...622...83L |doi=10.1007/3-540-44874-8_5 |s2cid=117748831 }}</ref> Thus the dynamics of the system alone are [[Irreversibility|irreversible]]. As with any coupling, [[Quantum entanglement|entanglements]] are generated between the system and environment. These have the effect of sharing [[quantum information]] with—or transferring it to—the surroundings.