# I Sending a cryptographic key faster than the speed of light

#### Lars278

Summary
Bell's theorem, EPR and quantum entanglement
Quantum entanglement does not imply that you can send information faster than the speed of light since you cannot manipulate what your sending. You don't know what you have until you have measured it. But you do know that you have the corresponding photon at the other location in space no matter if that place is 4 inches away or 1 billion light years away. If your satisfied with what you get as long as you know that you have the corresponding particle at the other end; this means that you can send a cryptographic key faster than the speed of light. So you are able to send a cryptographic key into a black hole, though you can't use it for communication. (cryptographic keys come in corresponding pairs)

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Staff Emeritus
No, yu can't send a cryptographic key faster than light. Nobody is sending anyone anything. You can have two people receive a key at the speed of light, but nothing in that requires entanglement or even QM. You could put up a poster saying "The secret word is 'crackpot'" and accomplish the same thing.

#### Nugatory

Mentor
Googling for "quantum key distribution" will find much information on the various ways that entanglement can be used to create and share cryptographic keys.

All practical techniques require a classical channel as well, so there's no way of exchanging keys at faster than the light speed. Even if it were possible, there's little value in doing so because any message encrypted with the key must travel at less than the speed of light.

#### Lars278

No, yu can't send a cryptographic key faster than light. Nobody is sending anyone anything. You can have two people receive a key at the speed of light, but nothing in that requires entanglement or even QM. You could put up a poster saying "The secret word is 'crackpot'" and accomplish the same thing.
You're oversimplifying! There's a difference between travelling 1 billion light years, then writing "crackpot" and travelling 1 billion light years, then writing the complement of a sequence you do not know until you get there.

#### DrChinese

Gold Member
Quantum entanglement does not imply that you can send information faster than the speed of light since you cannot manipulate what your sending. You don't know what you have until you have measured it. But you do know that you have the corresponding photon at the other location in space no matter if that place is 4 inches away or 1 billion light years away. If your satisfied with what you get as long as you know that you have the corresponding particle at the other end; this means that you can send a cryptographic key faster than the speed of light. So you are able to send a cryptographic key into a black hole, though you can't use it for communication. (cryptographic keys come in corresponding pairs)
You should also be aware that there is no sense in which your "key" is traveling in any particular direction. From here to the black hole, or from the black hole to here, there is no way to distinguish these scenarios. Note that time is not a factor at all, i.e. first measurement vs. second measurement is simply an arbitrary label in QM when you are considering an entangled system.

#### Nugatory

Mentor
You're oversimplifying! There's a difference between travelling 1 billion light years, then writing "crackpot" and travelling 1 billion light years, then writing the complement of a sequence you do not know until you get there.
But not a useful distinction. Either you and your counterpart carried the entangled particles on your outbound journeys and only looked at them when you arrive, in which case you might just well have carried the shared key along instead; or (as in all practical implementations) the particles are transmitted to both you and your counterpart after the fact, in which case there's no FTL possibility.

Quantum key distribution is a powerful and useful technique, but the value doesn't come from any FTL possibilities; it comes from the cryptographic randomness (required to maintain the unbreakability of one-time-pad methods) and its resilience to third parties covertly acquiring the keys.

To make a convincing counterargument, you would have to describe your proposed protocol in detail, similarly to the way that the currently accepted quantum key distribution protocols have been described.

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