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Quantum world mysteries - Entanglement

In a recent paper published in Science, researchers demonstrate how the entanglement of electron spins of defects in silicon carbide can be prolonged up to 10 000 times. This is a huge step forward in a practical sense for the attempt to utilize quantum states (solid-state qubits) as information carriers. 
But what is entanglement, and what is so mysterious about it? The mystery is due to the behavior which is opposite to our everyday experience of the objects around us. It relies on one of the basic principles of quantum mechanics - superposition. Superposition is also known in classical mechanics, e.g. think of the waves in the water and how the number of waves can interact in a complex way, forming patterns. But in the quantum world, the superposition is in the nature of things – state of some quantum system is, while not disturbed, superposition of all possible combinations of n particle states at the same time – like in the famous Schrodinger example, the cat is both dead and alive. Some interesting consequences of this are demonstrated in Young’s experiment (read in the previous post). This fundamental difference leads to an enormous (exponential) increase in the information which can be stored in quantum systems in the state of specific superposition (coherence). 

Entanglement goes a step further-some quantum state can be generated such that it is not only the superposition of particle states but such superposition that cannot be broken down to individual particle states, meaning that the quantum state of each particle of the group cannot be described independently of the state of the others. Some measurable properties of these particles are entangled. Think for example of two electrons with entangled spins. Two remarkable consequences arise.  Measuring the spin of one particle gives a definite answer to the property of other (kind like you cheated by knowing the sum of the spins in the beginning); however, keep in mind that each spin has no definite value until the measurement happens, and only interference with the entangled system - measurement, in this case, collapse state of a spin into one definite value. Secondly, the distance between electrons in the moment of measurement is of no importance at all. If you think of the information about measurement traveling from one electron to the other to ‘inform’ it that the measurement happened, you will find it that the speed it had was much larger (over 10000 times higher) than the speed of light… so some other interpretation might be needed to reconcile quantum theory and Einstein's theory of relativity. 

Entanglement can be also viewed in light of the basic thermodynamic laws-this interpretation is seen in the interesting work of the physicist Vlatko Vedral:

Entanglement is actually a natural state of the quantum system and pure quantum states are typically almost maximally entangled...This striking observation was already made decades ago..., although it was initially phrased as ‘subsystem entropy typically being maximal’—this was before subsystem entropy became the canonical measure of entanglement for pure states ... For example, bearing the above in mind it is not surprising that the difficulty for an experimenter trying to perform e.g. quantum computing is not to generate entanglement but to control what is entangled with what, and in particular to avoid entanglement between the experiment and the environment, as that will increase the entropy of the system.

Markus P. Muller, Oscar C. O. Dahlsten and Vlatko Vedral in Unifying typical entanglement and coin tossing: on randomization in probabilistic theories, Communications in Mathematical Physics volume 316, pages 441–487(2012)

For the most intriguing story of quantum mechanics and thermodynamics written in popular science (by authors opinion) read Decoding Reality -The Universe as Quantum Information by Vlatko Vedral. If you want to read more on the subject of quantum entanglement, here are some interesting papers to start with:

Teleportation, Entanglement and Thermodynamics in the Quantum World, Martin B. Plenio and Vlatko Vedral 2008

A simple thermodynamical witness showing universality of macroscopic entanglement, Vlatko Vedral

The necessity of entanglement and the equivalency of Bell’s theorem with the second law of thermodynamics, Ian T. Durham