# How long does it take to detect entanglement?

I was just curious. Does it take a few minutes, hours or days? Has anyone here carried out an entanglement experiment? From what I understand, the results of the measurement of entangled particles go to a coincidence counter and that's where they check the results. How long is the period before they know the particles are correlated?

Thanks

mfb
Mentor
As fast as your electronics is, or as fast as your signal can propagate in the cables (limited by the speed of light) - nanoseconds for typical setup sizes.

Can one INFER that the particles were entangled at a point in time AFTER Bob and Alice made the made the measurements , but BEFORE they were compared by ALICE and Bob or anybody for that matter.

The way I see it, IF they are never compared , there is no way of knowing, so no argument can be made.

But If they are compared some time after they were recorded , why can we not ASSUME/INFER that they were correlated just after the point in time of the last measurement.

If we cannot assume this , what is the reason ??

I know Bell's inequality shows that the particles could not have initial hidden variable(s) , when created , but I'm more interested in the time between
the last measurement taken and the data compared.

mfb
Mentor
But If they are compared some time after they were recorded , why can we not ASSUME/INFER that they were correlated just after the point in time of the last measurement.
Of course we can do that. When we compare the results we know the measurement results. We can calculate the correlation.

As fast as your electronics is, or as fast as your signal can propagate in the cables (limited by the speed of light) - nanoseconds for typical setup sizes.

Really, that's fast. I thought it would take a lot longer like a few hours. Thanks for the info.

mfb
Mentor
What do you expect to happen in these hours?

What do you expect to happen in these hours?

I honestly had no idea. I just thought it would take more time to compare the results but I can see why this would happen in nanoseconds.

jfizzix
Gold Member
I was just curious. Does it take a few minutes, hours or days? Has anyone here carried out an entanglement experiment? From what I understand, the results of the measurement of entangled particles go to a coincidence counter and that's where they check the results. How long is the period before they know the particles are correlated?

Thanks
The current record of 7.1 ebits worth of entanglement per particle pair took about 30 hours or so to measure :)
https://arxiv.org/abs/1804.04515
Detectors may be fast, but you need enough statistics from many detection events to show there is entanglement beyond reasonable doubt.

That said, some systems may be way faster, if all you want to do is witness that there is entanglement instead of knowing how much there is. The simple case of the polarization of an entangled pair of photons can be measured much more quickly, because there's a lot fewer total outcomes to the entire probability distribution, so the statistics fill out a lot faster than the position-momentum statistics measured in this paper, which have many more possible outcomes (to put it mildly).