The idea of quantum entanglement (QE) actually emerged about 90 years ago, but was implicitly presented in the late 1920s. However, QE became a significant tool in theoretical physics only in the mid-1970s, and the process of its transformation into a working tool occurred quite slowly. The first experiments confirming the reality of QE were conducted in the 1970s, but decisive experiments were performed only in the 1980s. At that time, QE attracted the attention of only a narrow circle of scientists who tried to better understand how quantum mechanics changed our ideas about physical reality. Recently, interest in QE has significantly increased because it has become the fundamental basis for the development of quantum computers and quantum communication networks. Now, reports that experimental physicists create entangled states of different particle configurations often appear not only in scientific journals but also in the media. Currently, there is no way to test these assumptions and hypotheses, because our technologies are not sufficiently developed for this. Perhaps in the near future, scientists will definitely find out. For now, we must assume that there is such a possibility. Key words: quantum physics, quantum spontaneity, physical reality, paradox, EPR experiment, Bell’s inequality.
The idea of quantum entanglement (QE) actually emerged about 90 years ago, but was implicitly presented in the late 1920s. However, QE became a significant tool in theoretical physics only in the mid-1970s, and the process of its transformation into a working tool occurred quite slowly. The first experiments confirming the reality of QE were conducted in the 1970s, but decisive experiments were performed only in the 1980s. At that time, QE attracted the attention of only a narrow circle of scientists who tried to better understand how quantum mechanics changed our ideas about physical reality. Recently, interest in QE has significantly increased because it has become the fundamental basis for the development of quantum computers and quantum communication networks. Now, reports that experimental physicists create entangled states of different particle configurations often appear not only in scientific journals but also in the media. Currently, there is no way to test these assumptions and hypotheses, because our technologies are not sufficiently developed for this. Perhaps in the near future, scientists will definitely find out. For now, we must assume that there is such a possibility. Key words: quantum physics, quantum spontaneity, physical reality, paradox, EPR experiment, Bell’s inequality.
In 1935, Einstein, Podolsky, and Rosen (EPR) argued that (then new) quantum mechanics was incomplete. At the heart of EPR’s paradox was the strange non-local nature of an entangled state which allowed EPR to simultaneously assign local values for position and momentum of a particle. In 1964, John Bell proposed an inequality (Bell’s inequality) that must be satisfied by any local realistic theory. The experimental confirmation of quantum physics, i.e., the experimental test of Bell’s inequality, required a pair of particles in an entangled state. The experiments first performed in the early 1970s, followed by a series of experiments until now, have confirmed the violation of Bell’s inequality, strongly implying that local realistic descriptions of nature are not consistent with experimental observations. The early simplistic view of entanglement has now been significantly expanded to include mixed states and multiple particles. Moreover, theoretical and experimental studies on the nature of entanglement have led to quantum information science where entanglement is an essential resource. In this article, we briefly review the early experiments on Bell’s inequality and experimental attempts to close “loopholes” as well as some key experiments on quantum information.
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.”
Quantum entanglement is an essential principle in quantum mechanics, illustrating the relationship between two or more spatially separated particles.This correlation has been experimentally proven through various techniques, including the entangled photon experiment. Furthermore, the breach of Bell's inequality has confirmed that entanglement represents a genuine physical occurrence, which cannot be accounted for by local hidden variables, thus highlighting the peculiar characteristics of quantum mechanics. This article aims to introduce quantum entanglement and Bell's inequality, and to discuss the experiments conducted by Alain Aspect, John Clauser, and Anton Zeilinger, which paved the way for the development of quantum information science. Additionally, this paper will explore the potential applications of quantum technology in communication, cryptography, and computing, which rely on the exceptional properties of entanglement.
The “spooky action at a distance” and consequently quantum entanglement of physical systems/particles has been debated ever since the famous paper of Einstein, Podolsky and Rosen-EPR Paradox (Einstein et al., 1935). In spite of being considered of highly controversial, there has been a considerable amount of work so far addressing the issue and many scientifically successful experiments were recently carried out proving that the entanglement is a physical reality (Horodecki et al., 2009). The influence of quantum mechanics and entanglement has been far from the imagination of most scientists, eventually reaching to the ultimate concept of consciousness/mind. The entanglement theory, its connection to the consciousness/mind and finally the entanglement theory of consciousness/mind therefore demands pure scientific interest. In order to maintain the requirements, the present work firstly summarizes the quantum entanglement theory together with its relations with Bell’s inequality and finally describes an “entanglement model of consciousness/mind”.
Entanglement, according to the Austrian physicist Erwin Schrödinger the Essence of Quantum Mechanics, has been known for a long time now to be the source of a number of paradoxical and counterintuitive phenomena. Of those the most remarkable one is usually called non-locality and it is at the heart of the Einstein-Podolsky-Rosen Paradox and of the fact that Quantum Mechanics violates Bell’s inequalities. Recent years saw an emergence of novel ideas in entanglement of three or more particles. Most recently it turned out that entanglement is an important concept in the development of quantum communication, quantum cryptography and quantum computation. First explicit experimental realizations with two or more photons include quantum dense coding and quantum teleportation.
Quantum entanglement is a key feature of quantum mechanics. It describes connections between quantum systems that classical theories cannot explain. The idea of entanglement started with the Einstein–Podolsky–Rosen (EPR) argument. This argument questioned whether quantum theory was complete and showed the non-classical nature of quantum correlations. Schrödinger later defined the concept and introduced the term "entanglement" to describe composite quantum states that cannot be separated. Bell then created a framework that showed the predictions of quantum mechanics clash with local hidden-variable theories, leading to Bell’s inequality. Experiments by Aspect and others provided strong evidence for the violation of Bell’s inequalities and supported the reality of entanglement. Building on these theories and experiments, quantum teleportation became an important protocol in quantum information science. Bennett and his team proposed a scheme that showed how an unknown quantum state can be sent between distant parties using shared entanglement and classical communication, without sending the quantum system itself. Later experiments confirmed that this protocol works across different physical platforms. This paper reviews the historical development of quantum entanglement and its role in the emergence of quantum teleportation. In addition to the theoretical discussion, the teleportation protocol is implemented using IBM’s Qiskit framework. The simulation results demonstrate the successful transfer of quantum states within the circuit model, illustrating how fundamental quantum communication concepts can be explored using quantum computing simulation tools.