I am the captain of a spaceship at the moon of Tellus, and my colleague Trillian is the captain of another spaceship at Proxima Centauri. I need to deliver a package to Trillian, who is going to meet me half way between. The problem is, there is a space pirate ship, whose captain is also interested of the contents of the package.

Luckily, we have two different routes to select from. We have also two entangled particles, one for me and one for Trillian. We have agreed beforehand that the state 0 of my particle (meaning state 1 of Trillian's particle) will take us to the first route, and the opposite states will lead us to the second route. We have also agreed to measure the states with parallel detectors, I first, and Trillian (sufficient time considering relativity and other fancy stuff) after me.

Question 1: Will Trillian know immediately after her measurement, which route I shall take, provided she can trust me completely?

Question 2: Will I know Trillian's route before her measurement (after mine), provided I can trust she follows our procedure?

Question 3: Will pirates be able to capture this info mid-space?

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    $\begingroup$ Regarding the (im)possibility of using entanglement for faster-than-light communication, see physics.stackexchange.com/a/440691/206691 $\endgroup$ – Chiral Anomaly Nov 19 '18 at 2:06
  • $\begingroup$ Thanks for the link. The concept of Quantum Key Distribution was what I was looking for. I just didn't know the correct term. $\endgroup$ – Iridium Knight Nov 21 '18 at 0:19

This method will "work," because no information actually needs to be transmitted between the two ships. The whole procedure works equally well classically. You could simply flip a coin before the two ships separate; each captain snaps a picture of the result of the flip but does not look it until it is time to meet up. This is secure as well, but so is simply agreeing which way to go before you separate; if the pirates do not have access to the captains' information, they have no way to known which route to guard.

What you cannot do—because it would require instantaneous communication—would be for one of the ships to locate the pirate vessel, then make a measurement on the entangled particle in their possession in such a way as to inform the other ship.

  • $\begingroup$ So it is not possible to measure the state of my particle sequentially with detectors of, say 45 or 90 degree offset from each other, until I get the desired (by me) state measured with the original detector (parallel with the Trillian's), and after that Trillian would measure the opposite state? $\endgroup$ – Iridium Knight Nov 20 '18 at 23:59

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