Quantum mechanics gives probabilistic predictions, it is very abstract, and leads to paradoxes. It was discovered in 1925 in two acts of ingenious creativity. By Heisenberg, at the Helgoland island, and by Schrödinger half a year later in Alps. It allows to explain and predict an enormous range of phenomena, but almost immediately met an opposition. Einstein fully recognized it as a practical tool, but criticized its non-deterministic nature. The Einstein-Bohr debate began. Clauser, Aspect and Zeilinger ended it.
Quantum mechanics gives probabilistic predictions, it is very abstract, and leads to paradoxes. It was discovered in 1925 in two acts of ingenious creativity. By Heisenberg, at the Helgoland island, and by Schrödinger half a year later in Alps. It allows to explain and predict an enormous range of phenomena, but almost immediately met an opposition. Einstein fully recognized it as a practical tool, but criticized its non-deterministic nature. The Einstein-Bohr debate began. Clauser, Aspect and Zeilinger ended it.
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.
In 1935, the Einstein-Podolsky-Rosen paper introduced a new kind of two-particles quantum state, which was named 'entangled' by Schrödinger.Showing that measurements on both particles were strongly correlated even if the particles were widely separated, Einstein argued that the formalism did not describe all the elements of the physical reality, and he concluded that Quantum Mechanics was not complete.Bohr immediately opposed this reasoning, claiming that Quantum Mechanics was consistent as it was, and that it did not need to be completed, and even that it should not be completed.The discussion between Einstein and Bohr lasted until the death of the two giants, leaving the question unsettled.It was considered by most of the physicists only as an epistemological debate, holding on questions of interpretation, but without any practical consequence on how to use Quantum Mechanics.The situation changed in 1964, when Bell discovered that taking Einstein's point of view led to consequences in contradiction with the predictions of Quantum Mechanics in some (rare) situations.The debate was then displaced from the realm of epistemology to the one of physics, since it could be settled by experiments.After a series of experiments closer and closer to the ideal Gedankenexperiment, more and more physicists realized that entanglement was definitely weirder than any previous concept, and that it might be used for new ways of processing and transmitting information.This prompted the development of a new field, quantum information, which brings us into a new quantum age, and may change our society as dramatically as the first quantum revolution, which gave us integrated circuits and lasers, responsible for the development of the information and communication society.
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.”
In 1935, with co-authors Podolsky and Rosen, Einstein discovered a weird quantum situation, in which particles in a pair are so strongly correlated that Schrödinger called them “entangled”. By analyzing that situation, Einstein concluded that the quantum formalism is incomplete. Niels Bohr immediately opposed that conclusion, and the debate lasted until the death of these two giants of physics. In 1964, John Bell discovered that it is possible to settle the debate experimentally, by testing the now celebrated "Bell's inequalities", and to show directly that the revolutionary concept of entanglement is indeed a reality. A long series of experiments, started in 1972, have produced more and more precise results, in situations closer and closer to the ideal theoretical scheme.After explaining the debate, and describing some experiments, I will show how this conceptual discussion has prompted the emergence of the new field of quantum information, at the heart of the second quantum revolution.
Abstract Following the path by which humanity learned quantum mechanics can lead to an improved understanding of the theory and the origins of its perceived limitations. Our goal is to retrace the development by investigating primary sources, including original published papers and letters, with attention to their timing and influence. Quantum mechanics began in 1900 with the introduction of Planck’s constant, which lead to the “old” quantum theory and Bohr’s model of the atom. With the introduction of matter waves in 1924, a second more intense period began. By 1928 the complete fundamental structure of quantum mechanics was established with the introduction of the Schrödinger equation, the proof of its equivalence to Heisenberg’s matrix mechanics, and the introduction of the relativistic Dirac equation. The theory retains the same essential form today, fueling the technological revolution that began in the last century. We culminate with an introduction to quantum information and computing. This completely new physical scientific paradigm was developed largely in Germany during a period of enormous social, economic, and political upheaval—in a hostile intellectual environment—that parallels the overturning of classical physics. It is striking that all the problems debated today were immediately recognized by the founders of the theory and instructive to see how they dealt with the various issues. We emphasize the centrality of the often neglected second quantization form of the theory for questions of interpretation and give a detailed examination of von Neumann’s widely misunderstood hidden variable theorem.
Abstract Einstein receive a letter from Bose and developed his idea into Bose–Einstein statistics. He also used De Broglie’s idea to clarify wave-particle duality. De Broglie also inspired Schrödinger to develop his equation in quantum mechanics. Bothe and Geiger ruled out Bohr’s BKS theory experimentally. Heisenberg came up with wave mechanics on the island of Heligoland. Born and Jordan realised that matrices were the mathematical key to quantum mechanics. Schrödinger developed an equivalent formalism. Heisenberg formulated his uncertainty principle. The Solvay Conference in Brussels in October 1927 was the scene for the famous Einstein–Bohr showdown, in which Bohr refuted Einstein’s thought experiment calculated to show that quantum mechanics was incomplete, a subject to which Einstein would return years later in the EPR paper. In Brussels, Einstein met Elisabeth Queen of the Belgians, a violinist with whom he would play chamber music many times. Einstein collaborated with Emil Rupp on experiments to explore quantum effects. Rupp turned out to be a charlatan who duped Einstein with fabricated results. Einstein became involved with the academic politics of the Hebrew University of Jerusalem, and clashed with its head, Judah Magnes.