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, yielded more and more precise results, in situations closer and closer to the ideal theoretical scheme. After explaining the debate and describing some experiments, we will show in this talk how this conceptual discussion has prompted the emergence of the new field of quantum information and quantum technologies.
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, yielded more and more precise results, in situations closer and closer to the ideal theoretical scheme. After explaining the debate and describing some experiments, we will show in this talk how this conceptual discussion has prompted the emergence of the new field of quantum information and quantum technologies.
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.
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.
In 1935, Einstein discovered entanglement, an amazing property of quantum mechanics which contradicted his world views. An intense debate with Niels Bohr followed, and was settled only after the discovery by John Bell of the famous inequalities named after him. Bell's theoretical work stimulated experiments, nowadays accessible to university laboratory classes, which demonstrate that entangled twin photons remain a single global object even when at a distance. We must take this conceptual revolution into account in our quantum description of the world. Applications of entanglement are also on the way, and a new field of research, Quantum Information, uses entanglement to develop applications as quantum cryptography and quantum computing. It may well be that we are living a second quantum revolution, which might change our society as strongly as the first quantum revolution did, when it gave us the laser and the transistor (and integrated circuits) at the root 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.”
Entanglement is one of the most surprising features of quantum mechanics. However, it was not until the last decades of the twentieth century that its full importance was understood and it was realized that it could lead to revolutionary applications in the area of quantum information. It was A. Einstein who discovered the extraordinary properties of non-factorizable two-particle states, when seeking to demonstrate that the formalism of quantum mechanics is incomplete. He presented his findings in 1935 in his famous article published jointly with B. Podolsky and N. Rosen, now referred to as the ‘EPR’ paper. Soon afterwards, Schrödinger coined the term ‘entangled states’ to emphasize the fact that the properties of the two particles are inextricably bound together. In the EPR article, Einstein and his colleagues used quantum predictions to conclude that the formalism of quantum mechanics was incomplete, in the sense that it did not account for the whole of physical reality, and that the task of physics was therefore to find a more complete theory. They did not contest the validity of the quantum formalism, but suggested that a further, more detailed level of description would have to be introduced, in which each particle of the EPR pair would have well-defined properties that were not taken into account in the quantum formalism.
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.