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If the axis of a spin is determined by the first measurement, would using that to tell humans on Earth whether a planet light-years away is habitable be FTL communication? Depending on measurement accuracy you could have the axes of the entangled particles represent a lot of information. Up/down is random so that should leave one hemisphere worth of data points per entangled pair, e.g. pointing at a projected map of needed or coming supplies.

Cees Timmerman
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  • That answer appears to say particles A and B would have equally random but inverted states, which are modified by the measurement after the entanglement is lost. – Cees Timmerman Aug 09 '22 at 14:11
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    Also related: https://physics.stackexchange.com/questions/100864/the-choice-of-measurement-basis-on-one-half-of-an-entangled-state-affects-the-ot – Emilio Pisanty Aug 09 '22 at 15:10

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Yes, that would be FTL communication, but no it is not possible to do.

To convince yourself, ask yourself: If person A measures first, which measurements can person B do, to tell which axis A measured along? Remember that after a single measurement by B, the state changes again, and unless the axis was already coincidentally the correct one, it will be different now.

If you have found something that seems to work, feel free to comment and I will discuss / give evidence as to what the issue is.

doublefelix
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  • So spin axis is set by measurement and only "up" when you happen to guess the same axis and orientation? So them saying "we need bread" and us measuring "ammo" at 50% instead of 100% is useless? As in "that axis wasn't X by this much but it is X now". – Cees Timmerman Aug 09 '22 at 13:04
  • @Cees What do you mean by 'measuring "ammo" at 50%'? When you measure spin there are only two possible outcomes. You first choose an axis direction, and then when you make a measurement the spin is either parallel to that direction or it's anti-parallel. BTW, the state space of a 2-level quantum system can be represented by the Bloch sphere. – PM 2Ring Aug 09 '22 at 13:26
  • Also a bit confused, but yes the first measurement sets the spin axis (assuming you're measuring a singlet state) but the second measurer has no way of knowing what that axis was set to.. without just getting a phone call from the first measurer. – doublefelix Aug 09 '22 at 13:35
  • @PM2Ring The projected supplies map part the axis was set to point to, but that the recipient could only guess at being the needed supplies. – Cees Timmerman Aug 09 '22 at 14:06
  • @Cees Sorry, you can't use entanglement to send a single bit FTL, and you certainly can't use it to send an axis direction FTL either. Any quantum communications protocol requires a classical channel as well as the quantum channel, so information transmission speed is restricted to the speed of light. – PM 2Ring Aug 09 '22 at 14:13
  • Hence pre-communicated maps. I guess both planets could produce complementary things based on how we know the states are complementary. – Cees Timmerman Aug 09 '22 at 14:22
  • Can you be more specific about what you mean by pre-communicated maps and how exactly the experiment would be done? Then we can help to address it – doublefelix Aug 09 '22 at 14:23
  • "Take this entangled particle and let me know by setting its axis what you need on this map. I'll check in a year." – Cees Timmerman Aug 09 '22 at 14:26
  • Sure, but then how does the person receiving the information actually read it out by measuring the particle? This is the part that doesn't work – doublefelix Aug 09 '22 at 14:28
  • http://electron6.phys.utk.edu/phys250/modules/module%203/spin.htm says the axis measures only "aligned? yes/no" and not degrees. I guess they could agree to sync the alignment of the measurement but the duplicate answer appears to say the measurement changes the state after the entangled state collapsed so can't even send a bit. – Cees Timmerman Aug 09 '22 at 14:31