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.
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.
The quantum revolutions: from concepts to technology The development of quantum mechanics in the beginning of the twentieth century was a unique intellectual adventure, which obliged scientists and philosophers to change radically the concepts they used to describe the world. After these heroic efforts, it became possible to understand the stability of matter, the mechanical and thermal properties of materials, the interaction between radiation and matter, and many other properties of the microscopic world that had been impossible to understand with classical physics. A few decades later, that conceptual revolution enabled a technological revolution , at the root of our information-based society. It is indeed with the quantum mechanical understanding of the structure and properties of matter that physicists and engineers were able to invent and develop the transistor and the laser – two key technologies that now permit the high-bandwidth circulation of information, as well as many other scientific and commercial applications. After such an accumulation of conceptual – and eventually technological – successes, one might think that by 1960 all the interesting questions about quantum mechanics had been raised and answered. However, in his now-famous paper of 1964 – one of the most remarkable papers in the history of physics – John Bell drew the attention of physicists to the extraordinary features of entanglement: quantum mechanics describes a pair of entangled objects as a single global quantum system, impossible to be thought of as two individual objects, even if the two components are far apart.
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, 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, 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.
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 work of John Bell in the mid 1960s and experiments carried out to test his famous inequalities in the following decades have led to a detailed re-examination of the concepts of quantum mechanics, and revealed the full importance of the notion of entanglement. This reconsideration helped to generate the new and extremely rich field of research known as quantum information in the 1980s. The guiding idea behind this field of activity is that, by exploiting the specific rules of quantum physics, one can conceive of new ways of calculating and communicating, in which the rules of play are no longer the well-known classical rules. One can thus develop new methods of cryptography in which the message is protected by the basic principles of quantum mechanics, and new computation methods that can be exponentially more efficient than classical algorithms. Quantum information is not therefore a mere sideline for physics, but concerns information theory, algorithmics and the mathematics of complexity theory. This research has already led to proposals for new algorithms and new computation architectures based on quantum logic gates with no classical equivalent. Still on the fundamental level, the meeting of information theory and quantum mechanics which lies at the heart of quantum information has led to a very stimulating regeneration of the theoretical tools used on both sides.