Quantum Darwinism

Quantum Darwinism is a theory explaining the emergence of the classical world from the quantum world as due to a process of Darwinian selection. It is proposed by Wojciech Zurek and a group of collaborators including Ollivier, Poulin, Paz and Blume-Kohout. The development of the theory is due to the integration of a number of Zurek’s research topics pursued over the course of twenty-five years including: pointer states, einselection and decoherence.

Implications

Along with Zurek’s related theory of envariance, quantum Darwinism explains how the classical world emerges from the quantum world and proposes to answer the quantum measurement problem, the main interpretational challenge for quantum theory. The measurement problem arises because the quantum state vector, the source of all knowledge concerning quantum systems, evolves according to the Schrödinger equation into a linear superposition of different states, predicting paradoxical situations such as “Schrödinger's cat”; situations never experienced in our classical world. Quantum theory has traditionally treated this problem as being resolved by a non-unitary transformation of the state vector at the time of measurement into a definite state. It provides an extremely accurate means of predicting the value of the definite state that will be measured in the form of a probability for each possible measurement value. The physical nature of the transition from the quantum superposition of states to the definite classical state measured is not explained by the traditional theory but is usually assumed as an axiom and was at the basis of the debate between Bohr and Einstein concerning the completeness of quantum theory; perhaps the most famous debate in the history of physics.

Quantum Darwinism explains the transition of quantum systems from the vast potentiality of superposed states to the greatly reduced set of pointer statesWojciech Hubert Zurek "Decoherence, einselection, and the quantum origins of the classical" [http://hubcap.clemson.edu/~daw/D_PHYS455/RevModPhys.v75p715y03.pdf Reviews of Modern Physics, Vol. 75, July 2003] ] as a selection process, einselection, imposed on the quantum system through its continuous interactions with the environment. All quantum interactions, including measurements, but much more typically interactions with the environment such as with the sea of photons in which all quantum systems are immersed, lead to decoherence or the manifestation of the quantum system in a particular basis dictated by the nature of the interaction in which the quantum system is involved. In the case of interactions with its environment Zurek and his collaborators have shown that a preferred basis into which a quantum system will decohere is the pointer basis underlying predictable classical states. It is in this sense that the pointer states of classical reality are selected from quantum reality and exist in the macroscopic realm in a state able to undergo further evolution.

As a quantum system’s interactions with its environment results in the recording of many redundant copies of information regarding its pointer states, this information is available to numerous observers able to achieve consensual agreement concerning their information of the quantum state. This aspect of einselection, called by Zurek ‘Environment as a Witness’, results in the potential for objective knowledge.

Darwinian significance

Perhaps of equal significance to the light this theory shines on quantum explanations is its identification of a Darwinian process operating as the selective mechanism establishing our classical reality. As numerous researchers have made clear any system employing a Darwinian process will evolve. Darwinian processes are not confined to biology but are any following the simple Darwinian algorithm:

# Reproduction; the ability to make copies and thereby produce descendants.
# Heredity; the ability to pass traits to copies.
# Variation; differences in heritable traits that affect "Fitness” or the ability to survive and reproduce leading to differential survival.

Quantum Darwinism appears to conform to this algorithm and thus is aptly named:
# Numerous copies are made of pointer states
# Pointer states evolve in a continuous, predictable manner, that is descendants inherit many of their traits from ancestor states.
# Successive interactions between pointer states and their environment reveal them to evolve and those states to survive which conform to the predictions of classical physics within the macroscopic world.

From this view quantum Darwinism provides a Darwinian explanation at the basis of our reality, explaining the unfolding or evolution of our classical macroscopic world. It is perhaps surprising that this same mechanism operating at the emergence of classical reality from quantum reality is also seen by scientific theories to be operating at the emergence of biology from chemistry and at the emergence of culture from biology.

Notes and references

Published papers

* [http://arxiv.org/pdf/quant-ph/0308163 Quantum Darwinism and Envariance]
* [http://arxiv.org/pdf/quant-ph/0505031 Quantum Darwinism: Entanglement, Branches, and the Emergent Classicality of Redundantly Stored Quantum Information]
* [http://arxiv.org/pdf/quant-ph/0408125 Environment as a Witness: Selective Proliferation of Information and Emergence of Objectivity in a Quantum Universe]
* [http://arxiv.org/pdf/quant-ph/0405161 PROBABILITIES FROM ENTANGLEMENT, BORN’S RULE FROM ENVARIANCE]
* [http://arxiv.org/pdf/quant-ph/0306072 Decoherence and the Transition from Quantum to Classical—Revisited]
* [http://arxiv.org/abs/0707.2832 Relative States and the Environment: Einselection, Envariance, Quantum Darwinism, and the Existential Interpretation]
* [http://xstructure.inr.ac.ru/x-bin/theme3.py?level=1&index1=368126 Quantum Darwinism on arxiv.org]

External links

* [http://www.universaldarwinism.com/quantum%20darwinism.htm Universal Darwinism: Quantum Darwinism]
* [http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000093000022220401000001&idtype=cvips&gifs=yes American Physical Society: Quantum Darwinism Published]
* [http://www.nature.com/news/2004/041220/full/041220-12.html Nature.com: Natural selection acts on the quantum world]