Feb 8, 2018

## There is nothing quantum about perfect correlation — so what makes entanglement special?

EDIT: Writing this response has encouraged me to explain entanglement in a longer post.

Hi Jason,

Thank you very much for this much needed article explaining what the basic principles behind a quantum computer are.

Unfortunately, you however simplify some of the concepts too much, admittedly along the lines of common misconceptions. I’m mainly referring to your explanation of entanglement. Actually, the effect you explain is perfect correlation and also possible in a completely classical world. An example which people often use is “Bertlmann’s socks”. Bertlmann is one of the pioneers of quantum information and has the rather eccentric habit of always wearing two differently colored socks, let’s say green and red ones. Now, imagine he could place his feet “lightyears apart” and you see one of them to be in green socks. You immediately know that the other one is red *without any entanglement involved*!

So what went wrong in your explanation? First of all, you omitted the probabilistic nature of quantum mechanics. You do not measure one qubit to be *always* up and the other one down, but rather measure 50% of the times up and down at both qubits. The second mistake was to omit the first effect you explained: superpositions. You can actually also measure in a basis of the superpositions (up+down) and (up-down) and you will still get the same result! 50% up-down and 50% up+down. That means that the information seemingly gained by this perfect correlation is *useless *unless you communicate your results to your measurement partner, but this you have to do over a classical communication channel, restricted to the speed of light.

This brings me to my last, but most important point:

**It is not possible to communicate information faster than the speed of light.**

And this also holds for entanglement. There is no *information* transported by measuring one of the qubits. But how can it be that they then always show opposite results? This is finally a point which you explained perfectly right: the entangled qubits behave as if they were one and the same object. And this was also experimentally proven by something called a test for bell’s inequality. It proved that our world cannot be real AND local at the same time. But this is maybe something for another time.