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Oxford University Press eBooks

What is Quantum Entanglement?

Gershon Kurizki, Goren Gordon

Published Apr 16, 2020• 5 citations
Publisher

Abstract

In this adventure, two quantum characters interact, because Henry has constructed a quantum suit for his cat, Schred. Henry and Schred end up in a quantum-entangled state. Remarkably, by measuring one of two entangled systems, the state of the other system is immediately collapsed, even if they are far apart. This bizarre feature of entanglement implies non-locality—synchronization or “collusion” between quantum objects, regardless of their distance. Cosmology provides an explanation: the universe emerged from a unified state describable as a quantum-entangled “hologram”. This notion resonates with the ancient Hindu view that the common essence of all things, the Brahman, can be revealed at every level of the natural hierarchy. The appendix to this chapter discusses operators that create entanglement.

DOI
10.1093/oso/9780198787464.003.0007

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In this adventure, two quantum characters interact, because Henry has constructed a quantum suit for his cat, Schred. Henry and Schred end up in a quantum-entangled state. Remarkably, by measuring one of two entangled systems, the state of the other system is immediately collapsed, even if they are far apart. This bizarre feature of entanglement implies non-locality—synchronization or “collusion” between quantum objects, regardless of their distance. Cosmology provides an explanation: the universe emerged from a unified state describable as a quantum-entangled “hologram”. This notion resonates with the ancient Hindu view that the common essence of all things, the Brahman, can be revealed at every level of the natural hierarchy. The appendix to this chapter discusses operators that create entanglement.
Perhaps the deepest mystery in all of physics is quantum non-locality – the ability of two distant systems to instantaneously know about each other. Non-locality means that two particles in a quantum system can be "entangled" such that the state of one particle affects the state of the other, regardless of how far apart they are. As well as challenging our common sense, non-locality suggests that the ultimate speed limit – the speed of light – is violated. Einstein spent many years worrying about this problem because such "superluminal" interactions seem to be incompatible with special relativity.
Introduction In this chapter, we will focus on the phenomenon of quantum entanglement . Entanglement is a special property that some states of two or more quantum particles may possess. When particles are in an entangled state they are correlated. But entanglement is far more than mere correlation . Entanglement is a type of correlation that macroscopic objects cannot have, even in principle; it is what we call a ‘non-local’ correlation. Entanglement, and the non-locality that derives from it, is by now considered to be, arguably, the most important aspect of quantum mechanics. Philosophically it leads to many puzzles and to a complete change in our view of nature. At the same time, the practical implications of this type of correlation are very significant. In particular, they can lead to the dramatically enhanced information processing capabilities that quantum systems possess. The whole field of quantum information processing (Steane, 1998; Bennett and DiVincenzo, 2000; Zoller et al. , 2005) has entanglement at its core. Given the above, it is natural to ask whether biological systems make use and take advantage of the enhanced possibilities that entanglement offers. It seems natural to expect this, especially at biological molecular level, where the laws of quantum mechanics apply. Entanglement between two isolated particles is easy to obtain: almost every time two particles interact they get entangled. But entanglement is very fragile. When two entangled quantum systems interact with a third, they easily become entangled in a tri-partite way; however, any entanglement between any two of them is reduced or disappears altogether.
Eve (E) entangles herself to Schred (S) before taking off to a conference. There she barely escapes an assault, and calls Henry. Is he too far away to help? Henry (H) resorts to quantum teleportation: a split-second later, H and E, aided by S, have traded places despite being thousands of miles apart! Schematically, the unknown state of H is teleported to E upon measuring the entangled state of E and S and communicating the result to E, who then trades states with H. Since the measured result is communicated at the speed of light, there is no faster-than-light signaling involved. In the distant future, space travel may be partly replaced by large-scale teleportation. Teleportation has intriguing philosophical implications, as it separates the object essence, its quantum information, from its non-essential material substance. The appendix to this chapter presents a quantum teleportation protocol that involves qubits.
Entanglement can be understood as an extraordinary degree of correlation between states of quantum systems – a correlation that cannot be given an explanation in terms of something like a common cause. Entanglement can occur between two or more quantum systems, and the most interesting case is when these correlations occur between systems that are space-like separated, meaning that changes made to one system are immediately correlated with changes in a distant system even though there is no time for a signal to travel between them. In this case one says that quantum entanglement leads to non-local correlations, or non-locality. More precisely, entanglement can be defined in the following way. Consider two particles, A and B, whose (pure) states can be represented by the state vectors ψA and ψB. Instead of representing the state of each particle individually, one can represent the composite two-particle system by another wavefunction, Ψ AB. If the two particles are unentangled , then the composite state is just the tensor product of the states of the components: Ψ AB = ψ A ⊗ ψ B; the state is then said to be factorable (or separable). If the particles are entangled, however, then the state of the composite system cannot be written as such a product of a definite state for A and a definite state for B. This is how an entangled state is defined for pure states: a state is entangled if and only if it cannot be factored: Ψ AB ≠ ψ A ⊗ ψ B.
Henry has successfully tested the Split button of his quantum suit that enables a quantum object to be in several places at the same time. This bizarre phenomenon is explained by the superposition principle, which reflects Henry’s wavelike properties. Yet Henry remains a single entity as he splits into two superposed versions. In order to understand such behavior, the emergence of the relevant concepts, from classical wave theory, through the quanta hypothesis and the subsequent development of QM, are reviewed. The possibility of such momentous changes in human thinking as the QM scientific revolution is disconcerting if one seeks the truth of the world through physics. A retrospective of the views on the rapport of physics with reality and its roots in mythical or symbolic pictures of the world suggests that the QM revolution eludes straightforward explanation. The QM superposition principle is mathematically formulated in the appendix to this chapter.
Quantum Entanglement-one of the weirdest and most exciting aspects of quantum mechanics. Two or more particles become entangled in a way that if the state of one of the particles is measured, then instantaneously the state of another particle is determined even if they are far apart from one another. The content of this paper explores the details of quantum entanglement: where it came from, what the basic principles are, what experiments support this notion, and what experiments we use to study entanglement. The trends of the data are laid out and what it signifies. From here, I then move to what effects quantum entanglement will have on quantum computing, cryptography and communication. I will attempt to justify my arguments through thirteen published academic sources prior to 2019.