Question: how quantum entanglement actually works and can we use it in technology,i mean eg. for transferring data at infinite speed?

David Freeborn answered on 17 Jun 2013:

Hi shobhit,

This is a really difficult question, but I’ll try to answer it!

In quantum mechanics, a particle only takes a “fixed” value when we measure it. Before then, they have all the possible values, and only choose one when we look at it.

Let’s consider a quantum property called “spin”. A particle can have spin-up, spin-down or no spin at all. Until we measure a particle’s spin, we don’t know what it is, so the particle takes on all possible spins.

Let’s say we have a particle, and we measure it to have no overall spin. Then the particle decays into two smaller particles, and they both fly away in different directions. We don’t know what the spin of either particle is, so they take on the mixture of all 3 types of spin. But we know the total spin must still be zero.

So, if we measure the spin of one of these particles, it must take choose a particular spin, say spin-up. But then the other particle needs to choose the opposite spin, say spin-down. So by measuring one particle, we can also change the properties of the other particle.

We say these two particles are in an “entangled state”. Changing one particle’s properties will also change the other.

We cannot use it to transfer data at infinite speed: because Einstein’s law of relativity says no information can travel faster than light. We still don’t quite understand how this works though, because the effects of quantum entanglement should be instant. This is one of the big mysteries of physics at the moment.

But we can use quantum entanglement to build quantum computers. Because all the particles in the computer take every possible state at once, the quantum computer can perform millions of calculations, all at the same time.

Anyway, I hope that was slightly clear! It’s a really complicated topic!