So What is This Quantum Entanglement ?

These days we can constantly hear term "quantum entanglement". But what really is quantum entanglement? 

Stanford Encyclopaedia of Philosophy gives the following definition: "Quantum entanglement is a physical resource, like energy, associated with the peculiar nonclassical correlations that are possible between separated quantum systems. Entanglement can be measured, transformed, and purified."

In human language quantum entanglement means an inescapable relationship between quantum systems, such that if one part of the system is measured for some quantity (for example spin or polarization), other part instantaneously changes to give the same result.  

Another example of QE:

Source

Perhaps the best example of quantum entanglement would be the original mind experiment by Albert Einstein, which introduced quantum entanglement. Let's imagine two photons were entangled and then shot off in different directions, each traveling at the speed of light, and you were to observe and measure the spin of one of the photons. Would the other photon “know” instantaneously and change its spin accordingly? Einstein argued that it wouldn't happen, as it would violate special theory of relativity, as it would be a form of faster-than-light communication.

Back in the early 1980s science became advanced enough to test the famous Einstein's thought experiment. French physicist Alain Aspect conducted a series of experiments which tested the nature of entangled photon pairs and how they might share information. What he found was amazing! In fact, the measurement of one photon did affect the state of its entangled partner, instantaneously!  

Today scientists keep on experimenting with quantum entanglement. In May of 2010, Chinese scientists successfully achieved quantum teleportation of information over a distance of 10 miles. In January of 2011, physicists S. Jay Olson and Timothy Ralph of Australia’s University of Queensland produced the mathematics to support the quantum teleportation of information through time, from the past to the future.

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