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How does quantum entanglement work?

Updated on August 3, 2014

Strange world we live in...

Scotish scientist J. B. S. Haldane once said - "Universe is not only queerer than we suppose, but queerer than we could ever suppose." Not everyone could understand what professor had in his mind. Probably because not everyone knows how amazingly complex the world, we live in, is. And perhaps one of the best examples of its complexity is strangeness of quantum world. So in this article I'm going to introduce one of the biggest paradoxes in quantum mechanics - the quantum entanglement.

Quick introduction to quantum entanglement.

The counterintuitive predictions of quantum mechanics about strongly correlated systems were first discussed by Albert Einstein in 1935, in a joint paper with Boris Podolsky and Nathan Rosen. In this study, they formulated the EPR paradox (Einstein, Podolsky, Rosen paradox), a thought experiment that attempted to show that quantum mechanical theory was incomplete. However, they did not coin the word entanglement, nor did they generalize the special properties of the state they considered. Einstein was dissatisfied with the concept of entanglement, because it seemed to violate the speed limit on the transmission of information implicit in the theory of relativity. The EPR paper generated significant interest among physicists and inspired much discussion about the foundations of quantum mechanics, but relatively little other published work.

Many people relate quantum entanglement with excistense of "God". Do you ?

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How does quantum entanglement work ?

There are many (many!) mind-boggling happenings in the quantum level, but I want to briefly mention the most remarkable, which says that you can link together two quantum particles so that they effectively form a single entity, even though one could be triggering sight at your eye while the other is light years away in space. Often this link involves a particular characteristic of a particle, like a spin (Quantum spin - a digital measurement of a particle. That means that it can have only two values - up or down). Until you make a measurement, particle won't have a value for its spin (only a probability, say 50:50 for both up and down), because the particle isn't in one state or other - it's in what's known as a superposition of the states, both up and down at the same time!

Now imagine we link together two such quantum particles. We can entangle them in such a way that when we measure the spin of one, we know for certain that the other one will have the opposite spin. (There are a few ways to do this. The simplest is to create two photons from the same electron at the same time.) You can separate those two particles as far as you like - sending one to the opposite side of the universe if you wish - and when you check the spin on one particle and, say it's up(down), you know for certain that the other one is down(up).

Not much use from this remarkable property ?

In practice, though, there's no way to send useful information this way. The results of the spin measurement are random, so can't give any meaningful information. You can't chose if the spin is going to be up or down, it happens by chance.

Even so, the way entanglement transfers information can be used for some remarkable applications, from ways to keep data securely encrypted and computers that can solve problems that would take conventional machines the lifetime of the universe to solve, to quantum teleportation.

Why is quantum entanglement so important ?

When you make the entangled particles, neither of them has a value for spin. Each is both up and down with 50 per cent chance of being either when measured. The two particles are identical. It is only when you look at one and it randomly settles into the up (down) position that the other, instantly, however far the distance, becomes down (up). Some sort of a message has crossed the universe instantly! It's possible to test to see whether the particles already have the information secretly hidden away or come up with it when they are looked at - and there are no secret values. If you could use such a mechanism to send a message it would reach anywhere in the universe instantaneously.

More about quantum entanglement..

Scientists managed to entangle two particle that don’t exist at the same time!

Two pairs of particles were entangled (in our example these pairs are 1 & 2; 3 & 4), but before the entanglement of 3 & 4, particle 1 was measured and then destroyed... 1 and 4 particles aren't entangled yet, but they will be after some complicated manipulations with 2 and 3. When we measure a particle, some of its parts become stable and definite, say spin. First and third particles were measured after the entanglement and since we know that two entangled particles have different spins, we now know that:

Entanglement A:

  • 1 is up
  • 2 is down

Entanglement B:

  • 3 is up
  • 4 is down

Particles 2 and 3 are entangled and because of their congruent spin, the act of entanglement also destroys them. Now, what we are left with is particle 4 entangled with 1 because of the previous entanglement of particles 2 & 3. Because we measured the spin of particle one, we are able to confirm the spin readings from particle 4 as being entangled with particle 1.

Why this experiment is so important is because we managed to pair two particles that have never coexisted in one timeline.


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