Quantum decoherence | Wikipedia audio article
Channel: wikipedia tts
Date: 2018-12-24

This is an audio version of the Wikipedia Article:
https://en.wikipedia.org/wiki/Quantum_decoherence


00:02:54 1 Mechanisms
00:03:30 1.1 Phase-space picture
00:08:28 1.2 Dirac notation
00:10:26 1.2.1 System absorbed by environment
00:12:06 1.2.2 System not disturbed by environment
00:15:29 1.3 Loss of interference and the transition from quantum to classical probabilities
00:23:03 1.4 Density-matrix approach
00:32:48 1.5 Operator-sum representation
00:41:57 1.6 Semigroup approach
00:47:07 2 Examples of non-unitary modelling of decoherence
00:48:27 2.1 Rotational decoherence
00:53:50 2.2 Depolarizing
00:54:37 2.3 Dissipation
00:55:23 3 Timescales
00:56:11 4 Measurement
00:57:53 5 Mathematical details
01:04:33 6 Experimental observations
01:04:43 6.1 Quantitative measurement
01:05:49 6.2 Reducing environmental decoherence
01:06:18 7 Criticism
01:07:10 8 In interpretations of quantum mechanics



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SUMMARY
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Quantum decoherence is the loss of quantum coherence. In quantum mechanics, particles such as electrons  are described by a wave function, a mathematical description of the quantum state of a system; the probabilistic nature of the wave function gives rise to various quantum effects. As long as there exists a definite phase relation between different states, the system is said to be coherent. This coherence is a fundamental property of quantum mechanics and is necessary for the functioning of quantum computers. However, when a quantum system is not perfectly isolated, but is in contact with its surroundings, coherence decays with time, a process called quantum decoherence. As a result of this process, the relevant quantum behavior is lost.
Decoherence was first introduced in 1970 by the German physicist H. Dieter Zeh and has been a subject of active research since the 1980s. Decoherence has been developed into a complete framework, which is said to solve the measurement problem.Decoherence can be viewed as the loss of information from a system into the environment (often modeled as a heat bath), 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). Thus the dynamics of the system alone are irreversible. As with any coupling, entanglements are generated between the system and environment.  These have the effect of sharing quantum information with—or transferring it to—the surroundings.
Decoherence has been used to understand the collapse of the wave function in quantum mechanics. Decoherence does not generate actual wave-function collapse.  It only provides an explanation for the observation of wave-function collapse, as the quantum nature of the system "leaks" into the environment.  That is, components of the wave function are decoupled from a coherent system and acquire phases from their immediate surroundings.  A total superposition of the global or universal wavefunction still exists (and remains coherent at the global level), but its ultimate fate remains an interpretational issue.  Specifically, decoherence does not attempt to explain the measurement problem.  Rather, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive.  Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble".
Decoherence represents a challenge for the practical realization of quantum computers, since such machines are expected to rely heavily on the undisturbed evolution of quantum coherences.  Simply put, they require that coherent states be preserved and that decoherence is managed, in order to actually perform quantum computation.