Discovery of Quantum Entanglement

The discovery of quantum entanglement occurred during the professional debate between Niels Bohr and Albert Einstein as they discussed the nature of quantum physics.

While the debate between them intensified, Einstein had a chance of conferring to Erwin Schrodinger on June 19, 1935 about the matter of discussion between him and Bohr and related his idea as follows: “My way of thinking is now this: properly considered, one cannot get at the talmudist if one does not make use of a supplementary principle: the “separation principle”. That is to say, “the second box, along with everything having to do with its contents, is independent of what happens with regard to the first box (separated partial systems).” If one adheres to the separation principle, then one thereby excludes the second point of view, and only the Born point of view remains, according to which the above state description is an incomplete description of reality, or the real states. After the collision, the real state of (AB) consists precisely of the real state A and the real state of B, which two states have nothing to do with one another. The real state cannot depend upon the kind of measurement I carry out on A. (”Separation hypothesis” from above.) But then for the same state of B there are two (in general arbitrarily many) equally justified Equation, which contradicts the hypothesis of a one-to-one or complete description of the real states”.

Einstein’s statement is validly in contrast to that of Niels Bohr’s which is as follows: “If the partial systems A and B form a total system which is described by its Equation, there is no reason why any mutually independent existence (state of reality) should be ascribed to the partial systems A and B viewed separately, not even if the partial systems are spatially separated from each other at the particular time under consideration. The assertion that, in this later case, the real situation of B could not be (directly) influenced by any measurement taken on A is therefore, within the framework of quantum theory, unfounded and (as the paradox shows) unacceptable”.

In the above presentations of opposing ideas it is clear that the idea of Niels Bohr is the one most favored by Erwin Schrodinger because by August 1935, Erwin Schrödinger (1887-1961) published an article entitled “Discussion of probability relations between separated systems” in the “Proceedings of the Cambridge Philosophical Society”. It was in this article that the word “entanglement“, which refers to the non-separability of quantum states of composite systems, was given its familiar meaning within the context of quantum mechanics.

The article appeared during a time of much activity and discussion on the issues of “physical reality” or “objective realism” of quantum mechanics, issues that the famous paper by Einstein, Podolsky, and Rosen (EPR) raised in May 1935. It was a time when the true mysteries and ghostly implications of quantum mechanics were beginning to sweep up widespread interest and controversy among the physics community. The conceptualization of quantum entanglement was the seed that eventually made important contributions towards the growth of new branches of research into the interplay between information theory and quantum mechanics - branches of research we now collectively call “Quantum Information Science”.

We quote the words of Schrödinger as he presents the concept of entanglement to the world: “When two systems, of which we know the states by their respective representatives, enter into temporary physical interaction due to known forces between them, and when after a time of mutual influence the systems separate again, then they can no longer be described in the same way as before, viz. by endowing each of them with a representative of its own. I would not call that one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought. By the interaction the two representatives [the quantum states] have become entangled.”

The development of quantum entanglement presents a very interesting and typical case how fundamental research leads to new technologically interesting concepts in which initially it was introduced by Einstein and Schroedinger because of its philosophical interest. This, together with Bell’s theorem, led to experiments beginning in the early 1970-s which also were only motivated by their importance for the foundations of physics. Most remarkably, in recent years people discovered that quantum entanglement can be useful in completely novel ways of transmitting and processing of information with no analog in classical physics. Here the most developed areas are quantum communication, quantum cryptography, quantum teleportation and quantum computation.

As what is already known today, entanglement is a quantum process that connects two particles (say, photons of light) in such a way that changes to one of the particles are reflected instantly in the other, even if they’re light-years apart.

The possibilities read like something out of science fiction: communications devices that could span the stars, codes that simply cannot be broken lest the very laws of physics be overturned, computers that dwarf todays machines in speed and power, teleportation and many more.