Abstract “Photon Interference: The Foundation of Quantum Communication” moves the story of interferometry into the quantum realm, beginning with the Einstein–Podolsky–Rosen paradox and the principle of quantum entanglement that was refined by David Bohm who tried to banish uncertainty from quantum theory. John Bell and John Clauser pushed the limits of what can be known from quantum measurement as Clauser tested Bell’s inequalities, confirming the fundamental nonlocal character of quantum systems. Leonard Mandel pushed quantum interference into the single-photon regime, discovering two-photon interference fringes that illustrated deep concepts of quantum coherence. Quantum communication began with quantum cryptography and developed into quantum teleportation that can provide the data bus of future quantum computers.
Abstract “Photon Interference: The Foundation of Quantum Communication” moves the story of interferometry into the quantum realm, beginning with the Einstein–Podolsky–Rosen paradox and the principle of quantum entanglement that was refined by David Bohm who tried to banish uncertainty from quantum theory. John Bell and John Clauser pushed the limits of what can be known from quantum measurement as Clauser tested Bell’s inequalities, confirming the fundamental nonlocal character of quantum systems. Leonard Mandel pushed quantum interference into the single-photon regime, discovering two-photon interference fringes that illustrated deep concepts of quantum coherence. Quantum communication began with quantum cryptography and developed into quantum teleportation that can provide the data bus of future quantum computers.
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 represents one of the most profound and counterintuitive phenomena in modern physics, fundamentally challenging classical notions of locality, causality, and separability. This comprehensive review examines the theoretical foundations of quantum entanglement, tracing its conceptual development from Einstein's critique of quantum mechanics through Bell's theorem to contemporary experimental verification. The paper synthesizes current understanding of entanglement's mathematical formalism, its role in violating Bell inequalities, and the resolution of the EPR paradox. Extensive discussion is devoted to landmark experiments that have conclusively demonstrated non-local correlations, including photon polarization studies, satellite-based quantum communication, and loophole-free Bell tests. The work further explores emerging technological applications in quantum cryptography, quantum computing, and quantum teleportation, while addressing fundamental questions regarding the nature of reality, information, and the relationship between quantum mechanics and general relativity. Finally, the paper examines open questions and speculative frameworks suggesting that entanglement may represent a foundational principle underlying spacetime geometry itself. Video documentation of this analysis is available at: https://www.youtube.com/watch?v=YDGKuXYRPpI.
Expository paper providing a historical survey of the gradual transformation of the "philosophical discussions" between Bohr, Einstein and Schrödinger on foundational issues in quantum mechanics into a quantitative prediction of a new quantum effect, its experimental verification and its proposed (and loudly advertised) applications. The basic idea of the 1935 paper of Einstein-Podolsky-Rosen (EPR) was reformulated by David Bohm for a finite dimensional spin system. This allowed John Bell to derive his inequalities that separate the prediction of quantum entanglement from its possible classical interpretation. We reproduce here their later (1971) version, reviewing on the way the generalization (and mathematical derivation) of Heisenberg's uncertainty relations (due to Weyl and Schrödinger) needed for the passage from EPR to Bell. We also provide an improved derivation of the quantum theoretic violation of Bell's inequalities. Soon after the experimental confirmation of the quantum entanglement (culminating with the work of Alain Aspect) it was Feynman who made public the idea of a quantum computer based on the observed effect.
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 was once a subject of intense philosophical debate and regarded as a puzzling theoretical anomaly. However, it has now been empirically validated and is widely recognized as a fundamental feature of quantum mechanics. This review traces its development in chronological order, beginning with the Einstein–Podolsky–Rosen (EPR) paradox, which raised profound questions about the completeness of quantum theory, and John Stewart Bell’s subsequent theorem, which established a measurable framework to verify entanglement’s non-local nature. The discussion then turns to contemporary experimental and applied domains where entanglement serves as a critical resource. Notable examples include quantum teleportation, entanglement-enhanced metrology, quantum radar, and emerging quantum energy devices such as quantum batteries. The transition from theoretical dispute to practical application underscores how entanglement now enables tangible technologies—ranging from ultra-secure communication networks to highly precise sensors. Furthermore, potential interdisciplinary applications in fields such as quantum biology are also explored, suggesting new pathways for innovation. This review aims to bridge foundational concepts with current technological advances, illustrating the continuing impact of quantum entanglement on both scientific understanding and real-world engineering.
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.