Why is quantum entanglement difficult to realize?

[Aidyan]

When I read about entanglement it is described as the singlet state two particles (or eventually even molecules) are in after interaction with each others or due to a decay. Put in that way it does not sound difficult to do. Just let two particles scatter with each others and we should have an entangled system. But when I read about the practical realizations we get to know about entanglement as something tremendously difficult to realize in laboratory. So, I'm probably missing something here... Can someone help?

 

[f95toli]

It is not actually that difficult to realize. The problem is maintaining it.
ANY coupling to the environment will lead to decoherence, which in turn destroys the entanglement.
This can be quantified using two (well, three but let's keep it simple) time constants known as T1 and T2. T1 is the relaxation time and T2 the dephasing time of a system.

In order for us to be able to perform any interesting experiments with entanglement we need to be able to manipulate the system faster than T1 and T2.
Now, T1 and T2 varies quite a lot between different systems, from nanoseconds (or even less) to minutes (e.g. isolated ions) but is of the order of maybe a few microseconds for most "interesting" systems, i.e. systems where there is some way for us to to manipulate it and perform various operations within the time-frame set by T1 and T2.

 

[Aidyan]

Ok, but why is this so difficult after all? Isn't it relatively easy to build with modern technology vacuum chambers which reach almost perfect empty space? What of the environment leads to the de-coherence, i.e. to small T1 and T2, once the particles are free to move throughout free space?

 

[f95toli]

Firstly, even when e.g. ions are measured in high vacuum they are not "free" as such; you always need some form of "confinement" to make sure the they stay in one place. Particles at room temperature move at high speeds (just set mv^2/2=3/2kBT and solve for v with T=300K) and would quickly collide with the walls of the vacuum chamber if they were not cooled and trapped using lasers.

Secondly, there is no such thing as an "isolated" system; the reason being that a system can always interact with the environment via electromagnetic interactions (i.e. by emitting/absorbing photons). This limits T1 in many systems, and the only way to mitigate it is by clever electromagnetic engineering.

Finally, an experiment where the particles were just sitting there would not be very interesting. We always need some way to control them and probe their state, but this means that you have to couple to them which inevitably also leads to decoherence.

 

[DrChinese]

When I read about entanglement it is described as the singlet state two particles (or eventually even molecules) are in after interaction with each others or due to a decay. Put in that way it does not sound difficult to do. Just let two particles scatter with each others and we should have an entangled system. But when I read about the practical realizations we get to know about entanglement as something tremendously difficult to realize in laboratory. So, I'm probably missing something here... Can someone help?

I will add to f95toli's comments.

In the past 2 decades, the favored technique of readily generating entangled particle pairs is via Parametric Down Conversion (PDC or sometimes SPDC). This is accomplished by shining a laser through one or two nonliear crystals (often made of barium borate - BBo) which generates a steady stream of pairs of photons. These are relatively easy to collect because their path out of the crystal can be predicted following certain conservation rules. Once collected, they can be moved around using optical fiber without affecting their entangled state much. Further, there are a lot of them to test, perhaps 100 pairs per second.

So my point is there are more convenient techniques than starting with particles in a singlet state.