# Delayed choice quantum eraser without randomization?

My understanding of delayed choice quantum eraser experiments roughly corresponds to the Wikipedia article about them, so please forgive if this question is stupid.

In all delayed choice experiments I read about, the choice whether to erase the path information is made in a random fashion (e.g. using a beam splitter). Why is such randomization necessary?

What would happen if instead of making random decisions here, I would try to send information by deliberately keeping or erasing the path information of the idler photons? Wouldn't this cause the interference pattern of signal photons to disappear and reappear according to the bits of data I am trying to send?

Why cannot this method (causing the interference pattern of signal photons to disappear/reappear by deliberately observing or erasing the path information of idler photons) be used for superluminal communications?

• the interference pattern is the result of many photons, not just one. So for practical purposes it does not matter if your desicions are random or deliberate, the receiver will not see an interference pattern until you send it the information about which photons were chosen to know the path. – Wolphram jonny Jan 15 '19 at 23:08
• Maybe I wasn't clear enough. I wasn't expecting to send one bit of information per photon. I would just send lots of photons and reconfigure the receiver of idler photons e.g. every second to make the interference pattern appear or disappear. – cuckoo Jan 15 '19 at 23:28
• It seems that the existence of the entangled iddle photons somehow changes the wavefunction of the photons going to D0 in a way that no interference pattern will ever appear at D0, no matter what you do with those iddle photons. I find it hard to swallow, but see one answer here physics.stackexchange.com/questions/292115/… – Wolphram jonny Jan 17 '19 at 14:32

Instead, a quantum eraser experiment allows you to restore the interference patterns by taking coincidence measurements that depend on the results of the quantum eraser's measurement. Taking the Wikipedia notation as a reference, even if you take away the $$\mathrm{BS}_a$$ and $$\mathrm{BS}_b$$ splitters as you propose, the pattern observed at $$D_0$$ will still remain as a shapeless blob, though now this shapeless blob can be decomposed into two complementary interference patterns when you consider independently the $$D_0$$ hits that coincide with $$D_1$$ hits and with $$D_2$$ hits, respectively.
And, of course, if you want to do that, you need to know which of the quantum-eraser detectors ($$D_1$$ or $$D_2$$) got a hit, and you need to have classical communication between the two parties for that to be feasible.