quantum mechanics: are entanglement and decoherence opposing processes?

Question

i get the impression they're the time reverse of each other, on the other hand my impression is probably wrong. any thoughts?

Answer 1

You are kind of on the right track.

"Quantum entanglement" is the term that applies when two (or more) quantum systems, although separated spatially from each other, are actually involved in the same quantum state. That is to say, that the "state of the world" is such that a measurement of one system will show a correlation to the other system; or mathematically, that the state of the total system is a sum of products of states of the two sub-systems.

In principle, any two systems that have ever interacted are quantum-entangled, but because they go on to further interact with more systems, the quantum states of those systems get tossed into the mix, and you gets sums of products of sums of products of quantum states with all of these systems. So what happens is that it becomes impossible to sort them all out, especially since as time goes on, some of these sub-systems entailed in this will be actually measured and have their quantum state determined.

Over time, then, the entanglement becomes so confused that it's simplest to assume that that the phase relationship among the product states vanishes. This is quantum decoherence.

So, to sum up:
- Quantum entanglement is the natural result of two quantum states interacting and then separating.
- Quantum decoherence is the result of the rest of the universe interacting with the two separated systems and introducing so many complications that the original relationship, although existent, can never be seen again.

It's just like Romeo & Juliet, isn't it?