Whereas entanglement and nonlocality belong to the fundamental findings of quantum theory, possible extensions to macroscopic systems outside the quantum laboratory are scarcely studied. This paper analyses conditions for entanglement to occur on a macrophysical level. An empirical basis is given by historic episodes and modern quantitative data. Theoretical understanding can start from the concept of perspective notions; the crucial new term is "common prearranged context", which characterizes the preparation to be made in advance (or naturally given conditions) in order to enable entanglement. A mathematical formalization is possible and gives some insight about how to handle perspective notions. 1. Entanglement in Quantum Theory and on a Macrophysical Level Entanglement and nonlocality belong to the eminent findings of modern quantum theory, confirmed experimentally beyond any doubt, and also by the functioning of quantum communication. Some years after the famous experiments by Aspect et al., 3 the idea was sketched that EPR correlatedness could be generalized beyond the original domain. 4 Along this line, series of fascinating experiments were performed inside the quantum laboratories. 5
Whereas entanglement and nonlocality belong to the fundamental findings of quantum theory, possible extensions to macroscopic systems outside the quantum laboratory are scarcely studied. This paper analyses conditions for entanglement to occur on a macrophysical level. An empirical basis is given by historic episodes and modern quantitative data. Theoretical understanding can start from the concept of perspective notions; the crucial new term is "common prearranged context", which characterizes the preparation to be made in advance (or naturally given conditions) in order to enable entanglement. A mathematical formalization is possible and gives some insight about how to handle perspective notions. 1. Entanglement in Quantum Theory and on a Macrophysical Level Entanglement and nonlocality belong to the eminent findings of modern quantum theory, confirmed experimentally beyond any doubt, and also by the functioning of quantum communication. Some years after the famous experiments by Aspect et al., 3 the idea was sketched that EPR correlatedness could be generalized beyond the original domain. 4 Along this line, series of fascinating experiments were performed inside the quantum laboratories. 5
Nonlocality and entanglement are now generally accepted as a fundamental finding of quantum theory, empirically confirmed beyond any doubt. Quite to the contrary, extensions to macroscopic systems (macro‐entanglement) have scarcely been analysed. It is the purpose of this paper to study conditions under which macro‐entanglement may occur, but also situations in which it can be jeopardized. The crucial new term for a theoretical understanding is “common prearranged context.” Two entangled entities may have a common historic, biographic or organizational context—which means that they are both involved in the same organized process. An existing macro‐entanglement cannot simply be annihilated, but it can be disturbed by competing effects at the same level. Practical consequences for special tests (randomized double‐blind controlled tests) are discussed as an example.
We consider the characterization of entanglement from the perspective of a Heisenberg formalism. We derive a two-party generalized separability criterion, and from this describe a physical understanding of entanglement. We find that entanglement may be considered as fundamentally a local effect, and therefore as a separate computational resource from nonlocality. We show how entanglement differs from correlation physically, and explore the implications of this concept of entanglement for the notion of classicality. We find that this understanding of entanglement extends naturally to multipartite cases.
Introduction The first realization that the validity of the quantum superposition principle in the Hilbert space describing a composite quantum system may give rise to fundamentally new correlations between the constituent subsystems came in the landmark 1935 paper by Einstein, Podolsky, and Rosen (EPR), where it was shown how the measurement statistics of observables in certain quantum states could not be reproduced by assigning definite wavefunctions to individual subsystems. It was in response to the EPR paper that Schrödinger, in the same year, coined the term entanglement ( Verschränkung ) to acknowledge the failure of classical intuition in describing the relationship between the “parts” and the “whole” in the quantum world: Whenever one has a complete expectation catalog – a maximum total knowledge – a ψ function – for two completely separated bodies, …then one obviously has it also for the two bodies together. But the converse is not true. The best possible knowledge of a total system does not necessarily include total knowledge of all its parts, not even when these are fully separated from each other and at the moment are not influencing each other at all. While Bell's strengthening of the original EPR-paradox setting and the subsequent experimental verification of Bell inequalities irreversibly changed the perception of entanglement from a property of counterintuitive “spookiness” to (beyond reasonable doubt) an experimental reality, the concept and implications of entanglement continue to be associated with a host of physical, mathematical, and philosophical challenges. In particular, investigation of entanglement in both its qualitative and quantitative aspects has intensified under the impetus of quantum information science (QIS).
How it all began In 1935, a decade after the invention of quantum theory by Heisenberg (1925) and Schrödinger (1926), three papers appeared, setting the stage for a new concept, quantum entanglement. The previous decade had already seen momentous discussions on the meaning of the new theory, for example by Niels Bohr (see his contribution to Albert Einstein: Philosopher-Scientist in Further reading); but these new papers opened up the gates for a new, very deep philosophical debate about the nature of reality, about the role of knowledge, and about their relation. Since the 1970s, it has become possible for experimenters to observe entanglement directly in the laboratory and thus test the counter-intuitive predictions of quantum mechanics whose confirmation today is beyond reasonable doubt. Since the 1990s, the conceptual and experimental developments have led to new concepts in information technology, including such topics as quantum cryptography, quantum teleportation, and quantum computation, in which entanglement plays a key role. In the first of the three papers, Einstein, Podolsky, and Rosen (EPR) realized that, if two particles had interacted in their past, their properties will remain connected in the future in a novel way, namely, observation on one determines the quantum state of the other, no matter how far away. Their conclusion was that quantum mechanics is incomplete and they expressed their belief that a more complete theory may be found. Niels Bohr replied that the two systems may never be considered independent of each other, and observation on one changes the possible predictions on the other. Finally, Erwin Schrödinger, in the paper which also proposed his famous cat paradox, coined the term “entanglement” (in German Verschränkung ) for the new situation and he called it “not one, but the essential trait of the new theory, the one which forces a complete departure from all classical concepts.”
The surprising aspects of quantum information are due to two distinctly non-classical features of the quantum world: first, different quantum states need not be orthogonal and, second, quantum states may be entangled. Non-orthogonality leads to the blurring of classical distinctions. On the other hand, entanglement leads via non-locality to teleportation and other ``entanglement-assisted'' forms of communication that go beyond what is classically possible. In this article we attempt to understand these new possibilities via an analysis of the significance of entanglement for the basic physical concepts of a ``particle'' and a ``localized physical system''. Classical particles can be individuated on the basis of qualitative differences in their sets of properties. But in entangled states the ``particle labels'' of the quantum formalism usually do not pick out such sets of individuating particle properties. It is sometimes nevertheless possible to think in terms of individual particles, which may be localized; but we argue that in general the structure of quantum mechanics is at odds with such a particle interpretation. This finally leads us to the conclusion that quantum mechanics is best seen as not belonging to the category of space-time theories, in which physical quantities are functions on space-time points. The resulting picture of the quantum world is relevant for our understanding of the way in which quantum theory is non-local, and it sheds light on the novel aspects of quantum information.
Quantum entanglement and coherence, traditionally studied in microscopic systems, have recently been observed and manipulated in macroscopic platforms. These phenomena play a critical role in understanding the boundary between classical and quantum physics and in developing quantum technologies such as superconducting qubits, optomechanical resonators, and atomic ensembles. This article reviews theoretical models, numerical simulations, and experimental techniques used to probe entanglement and coherence in large-scale systems. Five thematic outlines are presented, with one outline including two graphs illustrating (i) time evolution of entanglement in a macroscopic oscillator system, and (ii) coherence decay under environmental coupling. Applications, limitations, and future directions are discussed to provide a comprehensive perspective.