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Chinese Satellite Breaks Distance Record For Quantum-Key Exchange (sciencemag.org)
slew writes: Science Magazine reports a team of physicists using the Chinese Micius satellite (launched back in August 2016) have sent quantum-entangled photons from a satellite to ground stations separated by 1200 kilometers, smashing the previous world record. Sending entangled photons through space instead of optical fiber networks with repeaters has long been the dream of those promoting quantum-key exchange for modern cryptography. Don't hold your breath yet, as this is only an experiment. They were only able to recover about 1000 photons out of about 6 billion sent and the two receiving stations were on Tibetan mountains to reduce the amount of air that needed to be traversed. Also the experiment was done at night to minimize interference from the sun. Still, baby steps... Next steps for the program: a bigger satellite for more power and moving to quantum teleportation instead of simple key exchange. The results of the experiment were published in the journal Science.
So they sent entangled photons to ground, and there was a second signal, one that says "entanglement successful, photon is valid", and that was send how exactly? The signal they use to filter out all the other photons detected. That signal, the one that's actually carrying the real information here.... the one any attacker would attack if this was ever used in main stream use.
Look, I have a space based machine that fires ping pong balls at the earth, it paints them with random paint, and spins them in random ways. There is no stastical correlation between spin and paint color and each parameter is totally randomly selected.
Sometimes a machine ejects two at the same time, I call these "entangled". They have the same paint as a consequence of being ejected at the same time from the same machine, and they have related spins because they were ejected at the same time from the same machine imparting the same spin.
I send Alice below ping pong balls from a group of these ball firing machines, if I get two at the same time of the same color. (My entangled ones). I keep one and tell her below that this one is a valid entangled ping pong ball. I tell her this "valid" signal via a more reliable link. She uses it to select the subset of valid ping-pong balls from invalid ones.
My ping pong ball machines pass a nice Bells test, as long as I prefitler so we only consider "entangled" ping pong balls.
If I go check my spin for these "valid" balls, I know that stastically significantly the spin on hers will correlate.
There is no quantum magic in my ping pong ball machine. It is doing exactly what you expect it to do. The act of filtering the subset of pingpong balls by properties likely to select ones ejected from the same machine at the same time, is what lifts the few events and makes them stastically significant.