Re:Researchers Advance 'Quantum Teleportation'
by lisper on Saturday November 22, 2014 @06:34PM
(#8647083)
> Again, I promise you I understand the physics as well or better than you.
That's certainly possible, but I wouldn't take it as a foregone conclusion. Nonetheless, I apologize if I came across as strident.
> Why would Bob ever look at it if it's always made up?
Why would he not? He doesn't know whether or not the bits are made up. Let's call them "corrupted" because it doesn't have to be intentional duplicity on Alice's part. It could be simple experimental error, or a noisy communications channel, or a MITM attack. None of these are distinguishable by Bob from Alice having not made a measurement. In any event, Bob might look because he wants to determine if the bits are corrupted. Or he might look because he thinks he's participating in the quantum teleportation protocol, notwithstanding that he's actually not because the bits are corrupted. Note that for any given bit pair he has to choose one or the other: teleport, or check for corruption. He can't do both.
It is also worth noting that even the possibility of corruption is enough to debunk the idea that Alice's measurement results in a physical change on Bob's side. Bob can tell (probabilistically) that there is corruption happening, and he can put a probabilistic upper bound on the amount of corruption that is happening, but under no circumstances can he tell which measurements are corrupted and which are not. One in four corrupted measurements will appear to be genuine by pure chance, and there's no way for Bob to know, even probabilistically, which is which.
That's certainly possible, but I wouldn't take it as a foregone conclusion. Nonetheless, I apologize if I came across as strident.
> Why would Bob ever look at it if it's always made up?
Why would he not? He doesn't know whether or not the bits are made up. Let's call them "corrupted" because it doesn't have to be intentional duplicity on Alice's part. It could be simple experimental error, or a noisy communications channel, or a MITM attack. None of these are distinguishable by Bob from Alice having not made a measurement. In any event, Bob might look because he wants to determine if the bits are corrupted. Or he might look because he thinks he's participating in the quantum teleportation protocol, notwithstanding that he's actually not because the bits are corrupted. Note that for any given bit pair he has to choose one or the other: teleport, or check for corruption. He can't do both.
It is also worth noting that even the possibility of corruption is enough to debunk the idea that Alice's measurement results in a physical change on Bob's side. Bob can tell (probabilistically) that there is corruption happening, and he can put a probabilistic upper bound on the amount of corruption that is happening, but under no circumstances can he tell which measurements are corrupted and which are not. One in four corrupted measurements will appear to be genuine by pure chance, and there's no way for Bob to know, even probabilistically, which is which.