Quantum Darwinism : Survival of the Fittest in the Quantum World

Quantum Darwinism is a fascinating theory that attempts to explain how the classical world emerges from the quantum world through the process of natural selection induced by the environment's interaction with the quantum system. This theory was proposed by Wojciech Zurek and a group of collaborators in 2003, including Ollivier, Poulin, Paz, and Blume-Kohout. It is the culmination of Zurek's research over twenty-five years, which includes pointer states, einselection, and decoherence. A 2010 study claimed to provide preliminary evidence in support of quantum Darwinism. The study observed scars of a quantum dot "becoming a family of mother-daughter states," suggesting that they could stabilize into multiple pointer states. Similar perturbation-induced scarring has been suggested in disordered quantum dots, as well (see scars). However, Ruth Kastner's circularity criticism implies that the claimed evidence may be subject to scrutiny. Essentially, the phenomenon of decoherence that underlies the claims of Quantum Darwinism may not arise in a unitary-only dynamics. Therefore, even if there is decoherence, it does not necessarily show that macroscopic pointer states emerge naturally without some form of collapse.

Quantum Darwinism and the related theory of envariance, both proposed by Wojciech Zurek, aim to explain the emergence of the classical world from the quantum world and offer a potential solution to the quantum measurement problem. The measurement problem arises from the fact that the quantum state vector evolves into a linear superposition of different states, which can lead to paradoxical situations such as Schrödinger's cat, which are not observed in the classical world. Traditionally, this problem has been resolved by assuming a non-unitary transformation of the state vector at the time of measurement into a definite state, which is then used to predict the probability of each possible measurement value. However, the physical nature of this transition remains unexplained and has been the subject of debate between scientists such as Niels Bohr and Albert Einstein. Quantum Darwinism proposes that the classical world emerges through a process of natural selection induced by the environment interacting with the quantum system. This process selects against the many possible quantum states in favor of a stable pointer state, which can be observed in the classical world. The theory of envariance posits that in entangled quantum systems, certain properties remain invariant and can be observed by multiple observers, leading to the emergence of classical reality. In 2010, a study claimed to provide preliminary evidence of quantum Darwinism through observations of quantum dots stabilizing into multiple pointer states. However, the evidence has been subject to criticism, including the circularity criticism by Ruth Kastner, who argues that the phenomenon of decoherence underlying quantum Darwinism may not arise in a unitary-only dynamics.

Quantum Darwinism proposes that the selection process, called einselection, is responsible for the transition of quantum systems from the vast potentiality of superposed states to the greatly reduced set of pointer states. This selection is induced by the quantum system's continuous interactions with the environment, including measurements and interactions with the surrounding photons. These interactions lead to decoherence, which is the manifestation of the quantum system in a particular basis dictated by the nature of the interaction. The preferred basis into which a quantum system will decohere is the pointer basis underlying predictable classical states, and it is in this sense that the pointer states of classical reality are selected from quantum reality.However, the success of the einselection program depends on assuming a particular division of the universal quantum state into 'system' + 'environment', with the different degrees of freedom of the environment posited as having mutually random phases. This phase randomness does not arise from within the quantum state of the universe on its own, which limits the explanatory power of the Quantum Darwinism program.As a quantum system's interactions with its environment result in the recording of many redundant copies of information regarding its pointer states, this information is available to numerous observers who can achieve consensual agreement concerning their information of the quantum state. This aspect of einselection, called 'Environment as a Witness' by Zurek, provides the potential for objective knowledge. Quantum Darwinism offers a groundbreaking perspective on the emergence of classical reality from the quantum world, providing a novel solution to the quantum measurement problem. The theory suggests that the transition of quantum systems from a superposition of states to a set of pointer states occurs through the process of einselection, which is a selection process induced by continuous interactions with the environment. The interactions lead to decoherence, which manifests the quantum system in a particular basis, dictated by the nature of the interaction with the environment. As a result, the pointer states underlying predictable classical states are selected from the vast potentiality of superposed quantum states and exist in the macroscopic realm, able to undergo further evolution. However, the einselection program assumes a particular division of the universal quantum state into a 'system' and 'environment,' limiting the explanatory power of the Quantum Darwinism program. One of the most significant contributions of the theory is its identification of a Darwinian process as the selective mechanism establishing our classical reality. This process conforms to the Darwinian algorithm of reproduction/heredity, selection, and variation. The process results in numerous copies of pointer states, with successive interactions between pointer states and their environment revealing the evolution of those states that conform to the predictions of classical physics within the macroscopic world. This happens in a continuous, predictable manner, and descendants inherit many of their traits from ancestor states. The pointer states do not mutate, and the selection by the environment is among the pointer states preferred by the environment, such as location states. Quantum Darwinism offers a compelling Darwinian explanation for the evolution of our classical macroscopic world, shedding light on the complex processes that underlie our reality.