I know this is probably not possible to use to communicate via EPR, so my question is why?

I create electron entangled pairs using pair production or some other method, (each color pair is an entangled pair) we send millions of those pairs toward two double slits separated far from each other. The person on the right side can decide to measure which slit the electron went through using detector D (which ruins the interference pattern) or they can decide to not measure which retains the interference pattern. This would seemingly also ruin or maintain the interference pattern for the person on the left side. enter image description here If the source of electrons is streaming continuously, the person on the right could send a message by using dot dash for "interference on" or "interference off" during short 1 second intervals.

Again, I presume this would not work, so why exactly? And please don't say "because you can't communicate faster than light." What would specifically go wrong in this set up that would not make it work as described? Thanks!

  • $\begingroup$ Possible duplicate: physics.stackexchange.com/questions/55028/… $\endgroup$ Oct 13, 2021 at 17:46
  • $\begingroup$ I saw that post, but the most voted answer doesn't address his particular setup (Lubos just says "No" effectively). And the green checked answer has a cumulative vote of -1. Therefore I feel my question here is justified to exist. $\endgroup$ Oct 13, 2021 at 18:23
  • $\begingroup$ Can you please specify the precise entangled state you want to produce these electrons in and show your calculation of the pattern on the screen under various experimental choices? It's hard to guess where your mistake is if you don't tell us what you did. $\endgroup$
    – WillO
    Oct 13, 2021 at 21:30
  • $\begingroup$ My question is more along the lines of whether there is an entangled state that would allow this. But to be more specific, why wouldn't the original EPR paper pair production of electron state work? $\endgroup$ Oct 13, 2021 at 22:31
  • $\begingroup$ Why do you think making the interference pattern on the right go away also makes the interference pattern on the left go away? It doesn't; that's not how entanglement works. $\endgroup$ Oct 14, 2021 at 1:08

1 Answer 1


Look at this double slit experiment one electron at a time:


The few electrons at a time, top frame, look random, there is no interference. The interference appears with the accumulations of same boundary conditions scatters ( double slits given width given distances) and verifies that the wave part in the wave-particle duality is a probability wave given by the $Ψ^*Ψ$ of the wavefunction $Ψ$, the solution of the quantum mechanical problem.

So whether there are two entangled particles in opposite direction, they will show no interference pattern. It is in the accumulation of events that the destruction of interference will appear when one tries to see which slit the particle went through, which is completely unsuitable for signaling anything. See the single photon at a time double slit experiment here.

  • $\begingroup$ Yes, I understand this, but imagine you are sending millions of electrons steaming out at a time. A 1 second burst of a million of them can create an entire interference pattern. $\endgroup$ Oct 13, 2021 at 18:24
  • $\begingroup$ Sure, but you cannot control the pattern with any kind of switch because QM probabilities happen as accumulation needing time, and a switch happens in a delta(t) which will be random to the phase of the probability interference patterns. $\endgroup$
    – anna v
    Oct 14, 2021 at 3:21
  • $\begingroup$ OK, well how long do you need to develop an interference pattern then? Simply wait that long between switches (i.e. detector D on or detector D off). Make sure the double slits are far enough away that even this long of a time scale will be faster than light speed communication between double slits. $\endgroup$ Oct 14, 2021 at 14:32
  • $\begingroup$ @DavidSantoPietro Look at the single electron plot. If you kept the exact geometry and each electron came at random times, you will still get the same interference pattern. It is in probability, not in space. $\endgroup$
    – anna v
    Oct 14, 2021 at 14:42
  • $\begingroup$ What if we use entangled momentum eigenstates, where the momentum is always equal and opposite and the same each time? I guess the electrons would potentially be detected at different times still since their position is unknown? So the info would be scrambled? $\endgroup$ Oct 14, 2021 at 18:39

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