Editing Quantum decoherence (section)

===Concept===
[[Quantum computing#Decoherence|Decoherence]] causes the system to lose its quantumness, which invalidates the superposition principle and turns 'quantum' to 'classical'.<ref>{{Citation |last=Zurek |first=Wojciech H. |title=Decoherence and the transition from quantum to classical -- REVISITED |date=2003-06-10 |arxiv=quant-ph/0306072 |bibcode=2003quant.ph..6072Z }}</ref> It is a major challenge in quantum computing.

A real quantum system inevitably meets the surrounding environment, the interaction shows up as noise in physical process.  It's extremely sensitive to environmental noise. Electromagnetic fields, temperature fluctuations, and other external perturbations, as well as measurement, lead to decoherence.

Decoherence is a challenge for the practical realization of [[Quantum computer|quantum computers]], since such machines are expected to rely heavily on the undisturbed evolution of quantum coherences. They require that the coherence of states be preserved and that decoherence be managed, in order to actually perform quantum computation. Because of decoherence, it is necessary to finish the quantum process before the qubit state is decayed.<ref>{{Cite book |last1=Zhang |first1=Yu |last2=Deng |first2=Haowei |last3=Li |first3=Quanxi |last4=Song |first4=Haoze |last5=Nie |first5=Leihai |chapter=Optimizing Quantum Programs Against Decoherence: Delaying Qubits into Quantum Superposition |date=July 2019 |title=2019 International Symposium on Theoretical Aspects of Software Engineering (TASE) |pages=184–191 |doi=10.1109/TASE.2019.000-2|arxiv=1904.09041 |isbn=978-1-7281-3342-3 }}</ref>

The physical quantity coherence time is defined as the time that the quantum state holds its superposition principle.

Preventing decoherence and thus extending the coherence time of quantum systems serves to improve the stability of the computation.<ref>{{Cite book |last1=Zhang |first1=Yu |last2=Deng |first2=Haowei |last3=Li |first3=Quanxi |last4=Song |first4=Haoze |last5=Nie |first5=Leihai |chapter=Optimizing Quantum Programs Against Decoherence: Delaying Qubits into Quantum Superposition |date=July 2019 |title=2019 International Symposium on Theoretical Aspects of Software Engineering (TASE) |pages=184–191 |doi=10.1109/TASE.2019.000-2|arxiv=1904.09041 |isbn=978-1-7281-3342-3 }}</ref>