# Can quantum entanglement be used to coordinate actions at "FTL speeds" without breaking causality or actual faster-than-light communication?

I know there are a lot of similar question but I don't believe this to be a copy. I understand that if two people lived far away they could not transfer information through quantum entangled particles because forcing a particle into a particular spin breaks the entanglement and simply observing the particle to collapse the other part of the pair will give a perfectly random result. But what about using entanglement to sort of indirectly coordinate plans from far away:

I know this is wrong somehow and uses a childish interpretation of the idea of communicating information but this is just to make this as clear as possible: lets say the year is 3050, there are 2 leaders of an allied war who want to attack a planet, they are currently on opposite sides of the planet and have 2 plans they can decide upon, 1) both attack from the east and west at once or north and south at the same time. Using an atomic clock the leaders coordinate to check the state of quantum entangled particles (or a qubit, doesn't matter) at 12pm. If the qubit collapses as a (1/0) they go with plan A while (0/1) means plan B.

I believe that this does not constitute faster than light communication because both plans were conceived ahead of time and the particle or quantum state was just used as a random number generator, but it still seems as though the plan of attack was being transferred.

My questions are:

1. could this scheme actually be used or is there something I'm missing.
2. Why does this not constitute faster than light communication?
3. I would also just like to hear people who are smarter than me's thoughts on the physics around this hypothetical.
• Congratulations! You have independently discovered (a weaker variant of) Quantum Key Distribution. Commented Aug 19, 2020 at 9:03
• As a general rule of thumb, any time you see the words "quantum entanglement" and "FTL" in a question, the answer is "no".
– Mark
Commented Aug 19, 2020 at 20:03
• "I don't believe this to be a copy" And according to QM, it can't be. Commented Aug 19, 2020 at 23:56
• Just to make a point. Let's assume the planet in question is the Earth. Diameter = 12,742 km. Speed of light: 299,792 km/s. For the information to be transferred FTL, the transfer would need to take place in < 42.5 ms. No matter how precise the atomic clocks are, I'm not sure two humans could be trusted to check the qubit together to that level of precision. But it's an interesting question!
– JBH
Commented Aug 20, 2020 at 0:25
• You have the Mermin Peres game: en.wikipedia.org/wiki/Quantum_pseudo-telepathy Commented Aug 20, 2020 at 20:10

Going off of WillO's answer, while this scheme would work it would be no more effective than using a printer and two pieces of paper. Yes, your scheme is different in that it involves quantum nonlocality, but nevertheless it does not constitute faster-than-light communication because no information is being transferred between the two leaders. Their respective observations are correlated, but are nevertheless random. Hence, there's no problem. Is it weird? Yes. Is it a threat to causality? No. :)

• I disagree -- the proposed scheme is stronger than the classical variables described here, so long as the qubits are kept safely guarded. The value does not exist until measured so eavesdroppers would impact the measurement results, which would not be the case for the classical variables. (Of course, the proposed protocol does not actually guard against eavesdroppers, so in practice it is weaker, but the core idea can be expanded into a full QKD scheme that can probably achieve stronger robustness against eavesdroppers.) Commented Aug 19, 2020 at 9:05
• @EmilioPisanty since the eavesdropper knows in which base the qubit will be measured, they can measure in that base without affecting any future measurement. I don't believe there are ways to detect an eavesdropper without communication after the eavesdropping act (do correct me if I'm wrong, please) Commented Aug 19, 2020 at 10:42
• Or, if they measure in the wrong base, they may cause the generals to use different plans and cause the attack to fail. That's even better than knowing which plan will be used! Commented Aug 19, 2020 at 11:03
• @EmilioPisanty You can also prevent eavesdropping on paper if the paper is safely guarded. Nothing new here. Commented Aug 19, 2020 at 11:17
• How does SR affect the ability to measure simultaneously at 12pm? Commented Aug 19, 2020 at 14:43

Alternatively, they could have a computer print out two copies of the same random number, stuff the copies in their pockets, consult them when it's time to attack, and attack from one direction or another depending on whether the number is odd or even.

Why does this not constitute FTL communication? If the answer to that is clear to you, then you've answered your own question.

• But I don't think that's an equivalent analogy. Comparing it to printing out a random number implies that the particles were predetermined which would be an example of the hidden variable theory which was proven incorrect. The particles or qubits aren't determined until exactly 12pm (in the example) where both leaders know how to act at that exact moment and couldn't have determine how to act before hand. Maybe I'm wrong about this but it doesn't seem as simple as being equivalent to printing a random number ahead of time. Commented Aug 19, 2020 at 2:52
• @Corey HVTs are indeed incorrect -- but they are perfectly valid models if your experiment only ever involves a measurement of a single observable (instead of multiple incompatible ones), as your protocol does. Commented Aug 19, 2020 at 9:00
• @Corey The point is that if you look at this from a relativity-but-not-quantum point of view, there's a way of accomplishing the same thing. Thus, doing it doesn't violate relativity's FTL rules, even if the mechanism does involve quantum. Basically, quantum particles act as if they have hidden variables with respect to this thought experiment. The absence of hidden variable doesn't prohibit some phenomena being observably identical hidden variables. Just because QM is weird doesn't mean it has to be weird all the time. Commented Aug 19, 2020 at 23:53
• @Corey The "print ahead of time" is taken to be more metaphorically. The core idea of the quantum proposal in the OP is that nobody can tell before 12 o'clock which attack will take place. In order to achieve the analogon with a printout you would have to create two non-leaking printouts of a true random number, for example from a noise generating diode true RNG. These are the requirements because any information leak would result in the agreement to become knowable ahead of time. (ctd) Commented Aug 20, 2020 at 13:32
• @Peter-ReinstateMonica How is this not knowable ahead of time? Couldn't one party measure the quantum state early on their side as if they read the random number early? Commented Aug 20, 2020 at 15:18

# STL

The information is inherently contained in the particle pair themselves. Therefore, information only moves as quickly as the particles themselves do. What is not included in the description of your scenario is how the entangled particles originate. The particles cannot become entangled unless they are in very close proximity to begin with. Thereafter, they must move at slower-than-light speeds to their respective destinations. Even though the final state of the observable is not known until the observation occurs, I think it is fair to say that the information about the final state is implicitly encoded in the entanglement, rather than the observation.

In this sense, I think the paper printing scenario really is apt. Here, we would say that the same printer must print both pieces of paper, and the paper has the special property that once you look at it, the information is lost. This is exactly equivalent to saying that the information is not there until you look at it, from the perspective of an observer. That is, an observer cannot tell the difference between the printer deciding the outcome at the time of printing vs. the papers magically manifesting the same value at the time of observation. Even so, the printer analogy makes it clear that the information is created at "entanglement time" (i.e., print time), and thus, the information speed is simply the speed at which the papers move about, not the speed at which they are observed.

• But that is definitively incorrect. What you proposed, that the final state information is implicitly encoded in the entanglement, rather than the observation is the Hidden variable theory which was proven wrong. I think, after reading all the responses, the reason that this does not constitute FTL communication is because the actual information, to use plan A or B, was developed ahead of time, and the entanglement is just being used to coordinate the plan, not actually send information between the two locations. Commented Aug 19, 2020 at 23:49
• Local Hidden Variables are forbidden by the Bell Inequality, but I'm not claiming that the the actual state of the final observation is created at entanglement. I'm merely pointing out that the entangled outcome is what is created there and "carried" with the particles as they separate. That is, the entanglement carries the information: "observable A == observable B", not A = (1/0) or A = (0/1). Two particles not entangled do not carry this info. Commented Aug 19, 2020 at 23:59

You can replace the atomic clock with two boxes of lunch packed in a dark room.

The modified story:

The leaders mum cooked lunch in the previous day, chicken and salad. However, when packing, the lights went off, so she doesn't know which pot is which. She makes sure she gives her children lunch from the same pot. Using an the lunch boxes the leaders coordinate to check the state of their lunch (or the lunchbox, doesn't matter) at 12pm. If the lunch is chicken (1/0) they go with plan A while salad (0/1) means plan B.

Does that sounds like a plan? It does. You can even implement the above version of it now, hopefully not in a planet-attacking way, without quantum equipment.

I think the most simple answer is that the leaders told each other that they would measure the particles at 12 pm. Hence, information was transmitted ahead and there is no communication (information exchange) at the time of the measurement.