Quantum entanglement

Quantum entanglement is a physical phenomenon that occurs when pairs or groups of particles are generated or interact in ways such that the quantum state of each particle cannot be described independently—instead, a quantum state may be given for the system as a whole.

Spontaneous parametric down-conversion process can split photons into type II photon pairs with mutually perpendicular polarization.

Quotes

• When the quantum system contains more than one particle, the superposition principle gives rise to the phenomenon of entanglement. It is now not just a particle interfering with itself—it is a system interfering with itself: an entangled system. Amazingly enough, Erwin Schrödinger himself realized that particles or photons produced in a process that links them together will be entangled, and he actually coined the term entanglement, both in his native German and in English. Schrödinger discovered the possibility of entanglement in 1926, when he did his pioneering work on the new quantum mechanics, but he first used the term entanglement in 1935, in his discussion of the Einstein, Podolsky, and Rosen (EPR) paper.
  • Amir D. Aczel, Entanglement: The Greatest Mystery in Physics (2002), Ch. 7 : Schrödinger and His Equation

• The two photons are entangled and according to local realism, their polarization planes should become independent... a typical EPR situation. Already in 1948, observations... agreed with quantum mechanics, not with local realism.
  • R. H. Dalitz and F. J. Duarte, John Clive Ward, Physics Today 53, 99-100 (2000).

• Quantum theory is now discussing instantaneous connections between two entangled quantum objects such as electrons. This phenomenon has been observed in laboratory experiments and scientists believe they have proven it takes place. They’re not talking about faster than the speed of light. Speed has nothing to do with it. The entangled objects somehow communicate instantaneously at a distance. If that is true, distance has no meaning. Light-years have no meaning. Space has no meaning. In a sense, the entangled objects are not even communicating. They are the same thing. At the “quantum level” (and I don’t know what that means), everything may be actually or theoretically linked. All is one. Sun, moon, stars, rain, you, me, everything. All one.

• I'd like to think that the moon still exists, even if I'm not looking at it.
  • Roger Ebert, in Life Itself : A Memoir (2011), Ch. 54 : How I Believe In God

• He [Albert Einstein] didn’t think the spooky action at a distance would be verified, but it was, He thought that was somehow unphysical. He presented this as an example of why quantum mechanics is probably wrong, but in fact it’s right.
  • Lawrence M. Krauss, quoted in Jonah Engel Bromwich, "When Einstein Was Wrong", The New York Times (Feb. 12, 2016)

• When two systems, of which we know the states by their respective representation, enter into a 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 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.
  • Erwin Schrödinger, “Discussion of Probability Relations Between Separated Systems,” Proceedings of the Cambridge Philosophical Society, 31: 555–563; 32 (1936)

• The phenomenon of entanglement is the essential fact of quantum mechanics, the fact that makes it so different from classical physics. It brings into question our entire understanding about what is real in the physical world. Our ordinary intuition about physical systems is that if we know everything about a system, that is, everything that can in principle be known, then we know everything about its parts. If we have complete knowledge of the condition of an automobile, then we know everything about its wheels, its engine, its transmission, right down to the screws that hold the upholstery in place. It would not make sense for a mechanic to say, “I know everything about your car but unfortunately I can’t tell you anything about any of its parts.”
  But that’s exactly what Einstein explained to Bohr — in quantum mechanics, one can know everything about a system and nothing about its individual parts — but Bohr failed to appreciate this fact. I might add that generations of quantum textbooks blithely ignored it.
  • Leonard Susskind, in the Preface of Quantum Mechanics: The Theoretical Minimum (2014) by Leonard Susskind and Art Friedman

• There is a troubling weirdness about quantum mechanics. Perhaps its weirdest feature is entanglement, the need to describe even systems that extend over macroscopic distances in ways that are inconsistent with classical ideas.
  • Steven Weinberg, Lectures on Quantum Mechanics (2nd ed., 2015), Ch. 12 : Entanglement

See also

• Bell's theorem
• EPR paradox