So based on this thread : Why is quantum entanglement considered to be an active link between particles?

which was asked 7 years ago, the answer says

"Entanglement is being presented as an "active link" only because most people - including authors of popular (and sometimes even unpopular, using the very words of Sidney Coleman) books and articles - don't understand quantum mechanics"

if so, then how has china developed a technology where they transmitted data via Earth-to-space quantum entanglement?


also some other countries have said that they developed a technology where they transmitted data in a less of a distance

so am i missing something here?

also not sure how is this the duplicate of my previous question when my previous question is the duplicate of a thread that was posted 7 years ago and now I'm responding to that, basically if someone asks a question even after 7 years no one can ask a question about that thread?


marked as duplicate by Norbert Schuch, David Z Jul 26 '18 at 3:12

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.


China did not transmit information via quantum entanglement. Rather, they ensured that the same information existed at two places. There is a subtle difference between the two situations. Two entangled particles share a state. Observe the state of one, and you then know the state of the other -- but you do not change or affect the state of the other. The challenge in establishing quantum keys for encryption is to ensure that the transmitted particles: 1) are entangled with their partner particles that stay behind, and 2) retain their entanglement until their states are measured at their destination. Then, when the state of the transmitted particle and its stay-at-home partner are both measured, we know that the measured states will be the same (or opposite, depending on the type of entanglement).

  • $\begingroup$ No. I'm saying that no information is transmitted. But you're right that we can't change the state of one and thereby change the state of the other. Entangling, say, the spins of two particles A and B ensures that A and B have the same spin (or opposite spin depending on the type of entanglement), but does not determine what that spin will be. By measuring the spin of A or B, we learn the spin state of both A and B. $\endgroup$ – S. McGrew Jul 25 '18 at 16:12
  • $\begingroup$ All relevant theory and experiment supports the conclusion that it's impossible to transmit data using the method. But it's still valuable. Suppose you could tear a piece of paper roughly in half and send one piece to John and keep the other piece for yourself. Use a detailed description of the torn edge to produce an encryption key to encrypt a message. Broadcast the encrypted message to John. John can analyze his piece to get the key, and can decrypt the message; but nobody else can get the key unless they obtain one or both pieces of paper. $\endgroup$ – S. McGrew Jul 25 '18 at 16:57
  • $\begingroup$ Quantum entanglement provides a way for both parties to have the same encryption key, without predetermining what the key will be. It's as if you and John have "entangled paper": you each tear a piece of paper, and due to QM entanglement the tears turn out to be identical. You encrypt a message using a key derived from the tear, and John decrypts your message using his (identical) tear. Nobody else has a piece of paper entangled with yours and John's, so they can't decrypt the message. $\endgroup$ – S. McGrew Jul 25 '18 at 17:12

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