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Nadia wanted entanglement to work as a wire . If two photons are entangled , she had read , a measurement here sets the photon there , so a chosen bit should appear at the far end with no ordinary signal in between . She wanted to know whether that sentence described a network , or only a correlation . It describes a correlation . When the pair is measured , the two results are tied together , and neither person chose the tie . You learn how the far result sits relative to yours . You do not decide it . Einstein's phrase , spooky action at a distance , names the discomfort of that agreement . It does not name a telegraph . Nothing in the measurement lets Nadia pick a zero or a one and have that choice show up , as her choice , at the other station . A faster - than - light message is not hiding in the pair . What the disturbance does buy is narrower , and real . In the key protocol called BB84 , the two parties send prepared photons and later compare a sample on a public channel . An eavesdropper who measured those photons leaves a mark , because the measurement changes what the check will show . The shared secret is drawn from the photons that pass the check . BB84 does not need entanglement to do this . Entanglement is a second route to a key , not the same machine as that protocol . The security is a claim about detecting disturbance under the assumptions of the proof . It is not a promise that every box labeled quantum is safe once it leaves the lab . A network is harder still , for a reason that has nothing to do with a pin on a map . The no - cloning rule says you cannot make a perfect copy of an unknown quantum state . A classical repeater copies a bit and sends the copy on . A quantum link cannot amplify itself that way . To stretch entanglement farther , a repeater has to join short entangled segments by swapping them , without making a copy of the unknown state . That device is a different object from a louder version of today's router . The Micius satellite showed one long hop of the entangled kind . It sent entangled photons down to the ground stations at Delingha and Lijiang , about 1 , 200 kilometers apart . The hop is evidence that a link can be long . It is not evidence that you can dial a quantum network the way you dial the web . Nadia left the paper able to refuse the wire story . Entanglement agrees . It does not carry a bit you chose . A quantum internet , if it is built , has to live with that , and with the ban on copying .
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