# Entanglement, ftl communication and destruction of the entangled state

#### mix609

my question is about the no-communication theorem in quantum mechanics:
Assume that i have a pair of entangled photons which are entangled on their polarization. also assume that i send one to alice and another to bob. and alice wants to send a message to bob. no communication theorem states that communication is not possible because the polarization when measured by the sender(alice) will be random and while alice can know what result bob will get if he measures the polarization of his photon, it does not imply communication because alice would have to use a classical channel to communicate what result she got upon measurement, to bob. that defeats the very purpose of the experiment

my question is, what if its possible to control the polarization on alice's side by some means(assuming physicists find out some new novel way to do this 5 or 10 years from now. i know this is a very BIG assumption but perhaps possible after a decade more of QM research). can the no communication theorem be said to be violated now? my other question is: even if it were possible to control the polarization, would it be possible to reuse the photon pairs for communicating more than once?(cause any attempt to measure the entanglement will also destroy it. hence only 1 bit of data can be sent and the entanglement is subsequently and immediately destroyed). so can it be said that: not only is the randomness a problem, the other problem is that the entanglement can be used only once per photon pair and in order to communicate 1 byte(1 byte=8 bits) of data, one would require 8 entangled photon pairs?

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#### bhobba

Mentor
my question is, what if its possible to control the polarization on alice's side by some means(assuming physicists find out some new novel way to do this 5 or 10 years from now. i know this is a very BIG assumption but perhaps possible after a decade more of QM research).
So your question is why isn't it possible to send information FTL if some way was discovered on how to do it?

That's tautological and as such not really saying anything.

If such was possible it would violate a very basic principle of physics called the cluster decomposition property and would be a revolution in physics earning its discoverer an instant Nobel prize. You cant predict the future, but such seems highly unlikely because it would mean violating causality which in effect is predicting the future.

Thanks
Bill

#### mix609

So your question is why isn't it possible to send information FTL if some way was discovered on how to do it?

and my other question was: can an entangled photon pair be reused to send information or is it possible not more than once coz when alice measures, the entangled state is destroyed due to the act of measuring?

#### Nugatory

Mentor
and my other question was: can an entangled photon pair be reused to send information or is it possible not more than once coz when alice measures, the entangled state is destroyed due to the act of measuring?
The measurement ends the entanglement.

#### mix609

The measurement ends the entanglement.
which suggests that its pretty useless even if it were possible to communicate ftl coz its like having a cell phone which lets u say "hello" and then self destructs itself and now a brand new phone has to be purchased to say the next word after which it again self destructs and so on.....

So if one day if its somehow possible to overcome the randomness barrier, an entangled photon pair can still be used to send only 1 bit and then it expires. yeah, one cud use several such pairs to send longer messages but all of them would be "exhausted" rapidly

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