Simple question concerning quantum entanglement

[Devilstower]

I realize I’m likely to have my hand slapped and see this thread closed down promptly, because it doesn’t meet the strict guidelines. However, I’m hoping to get an answer which I’ve had a hard time finding from web searches or Twitter queries to physicist friends.

Q: Is it possible to determine if a particle is part of an entangled pair without causing the collapse of that pair?

[Devilstower]

Thanks. As a science fiction author, I’m looking for a means of communication that holds up to cursory examination. It doesn’t have to be genuinely workable, but shouldn’t raise an “well, that’s ridiculous” from the casualty well-informed reader.

If it’s possible to detect that particles are still entangled, transmission of a message becomes trivial, with complexity limited only by the number of particle pairs, and the accuracy of your clock. That can happen without any particle exchange, and regardless of any actual measure of particle properties. So, in a fiction work at least, might convincingly happen without the exchange of “information” in the usual sense.

[mandrewa]

I realize I’m likely to have my hand slapped and see this thread closed down promptly, because it doesn’t meet the strict guidelines. However, I’m hoping to get an answer which I’ve had a hard time finding from web searches or Twitter queries to physicist friends.

Q: Is it possible to determine if a particle is part of an entangled pair without causing the collapse of that pair?

No.  But there are different levels of no.

For instance let's take the classic double slit experiment.  If you sent a set of particles, not just one, but a set, through the double slits, and you see an interference pattern on the screen behind, then I think (I emphasize the 'I think') this means there is no way that you as the observer are a part of a chain of entanglement with that set of particles.

But, and I'm not certain of this but just believe it is the case, it still could be that these particles, the ones that are not entangled with you, are entangled with other particles that you are also not entangled with. 

[1]

Q: Is it possible to determine if a particle is part of an entangled pair without causing the collapse of that pair?

No.

'Entangled' is not an inherent property of a particle; it's a simply a designator that is used when the properties of two or more particles are correlated at the quantum level rather than the classical level. As such, entanlgement itself is not something that can be detected. If it is not known a priori (e.g., a system specifically designed to create entangled particle pairs) then entanglemen can generally only be determined by comparing measurements of multiple particles. If statistcal correlations are found that cannot be explained by classical physics, but can be explained by QM, then the particles can be consideredto have been in an entangled state.

Because entanlgement is a correlation between two or more particles, it can never be detected by measuring a single particle alone. Furthermore, because comparing information from all relevant particles requires collecting that information at a single point, it can never be used for faster than light communication; despite the hopes of many. This is the 'classical channel' that you will see described in real world applications such as quantum encoding/encryption.

For ftl communication in a sci fi story, pick something hypothetical like tachyons, and then just don't go into too much detail on how it all works.