# B Delayed choice quantum communication - "Berlin riddle"

#### Bob Ziegler

At 12.00h a specific event is being expected in Berlin, whether the wall ist teared down, or not. The sender is in Berlin and wants to send the information about the event to the recipient on the moon. Both sides know about the possibly upcoming event. As the light takes 1 second to the moon, the sender sends 0.8 seconds before 12:00h a photon through a double slit in direction of the moon. The photon (slit inormation) is being measured and stored at the double slit, through what slit it went, that's why it is assumed the photon goes straight towards the moon, without effect of the wave-function. The recipient expects 0.2 seconds after 12:00h an incoming photon or no signal, and knows before: incoming photon means, that the "wall was teared" down, a missing photon means "wall still standing". At 12:00h it turns out, that the wall is still standing. The double-slit is immediately being removed at the sender in Berlin, so that the path-information is being destroyed, in expectation, that the photon in superposition does not longer fly straight, but now corresponding to the wave function, so that it misses the detector on the moon very likely. On the moon no signal is measured 0.2 after 12:00, with the interpretation that the wall is still standing. That is how the moon knew whithin 0.2 seconds about the event on earth, whereas a usual communication would have taken 1 second.

This idea of communication requires prior information (how to act) at both spots, and the light goes in usual speed towards the moon. So it appears to be no ftl communication.

But does it work that way?

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#### DrClaude

Mentor
Your scenario is very difficult to understand. Can you state clearly what is the expected result in all cases?

The double-slit is immediately being removed at the sender in Berlin, so that the path-information is being destroyed, in expectation, that the photon in superposition does not longer fly straight, but now corresponding to the wave function, so that it misses the detector on the moon very likely.
There is nothing you can do to the double-slit setup after the photon has passed through the slits that will affect that photon.

Note also that one photon does not give an interference pattern. It is the cumulative effect of many photons that will do that.

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