KJW said:
The Rev Dodgson said:
Well as New Scientist and I understand it (and we may well be both completely wrong), the satellite sends of a pair of entangled particles, and by comparing their measurements on these particles the two people wanting to communicate can tell if anyone else is listening in or not, but what happens exactly if there is an eavesdropper, I’m not sure.
In simple terms, the two-particle wavefunction is collapsed by an eavesdropper, and by determining whether or not the wavefunction has collapsed, the presence of an eavesdropper can be revealed.
Yes. As Rev D pointed out in the original post, if an eavesdropper is present then the message is blocked. An encryption system that allows messages to be easily blocked doesn’t seem to be much use. I know no more than Rev D and KJW on this one. Perhaps if I read up on the technical literature … I’ll start with CN’s link.
Already there is a problem because high bandwidth data is traditionally sent using methods where the polarisation is used for multiplexing, so to send a secure signal you have to switch off some of the multiplexing, limiting bandwidth. Possible with a dedicated 1 to 1 optical fibre line but not on general communications networks.
There’s also the problem that it is now possible to devise schemes to read a secured data line by interfering just enough with the signal to read it without destroying the entanglement – so it’s not completely safe against eavesdropping. A particular method I have in mind is to read the photon my measuring the force on a clear block of glass placed in the path of the signal – remember the recent thread about reading the pressure exerted by light using an extremely accurate force gauge.
In a thought experiment, Suppose for instance you eavesdrop by reading out from second optical fibre placed parallel to the optical fibre containing the message – the signal leakage is going to occur whether or not the second optical fibre is used to eavesdrop. Quantum encryption may cover that possibility, but I can’t see how it can tell if there was a genuine eavesdropper or just a normal inevitable signal loss.
“Bob measures some photons correctly and others incorrectly. At this point, Alice and Bob establish a channel of communication that is insecure – that is, other people can listen in. Alice then proceeds to advise Bob as to which polarizer she used to send each photon bit – but not how she polarized each photon. So she could say that photon number 8597 (theoretically) was sent using the rectilinear scheme, but she will not say whether she sent an UP/DOWN or LEFT/RIGHT. Bob then confirms if he used the correct polarizer to receive each particular photon. Alice and Bob then discard all the photon measurements that he used the wrong polarizer to check. What they have, is, on average, a sequence of 0s and 1s that is half the length of the original transmission.”
“Now, suppose we have an eavesdropper, Eve, … This is useless to Eve, as half the time she used the wrong detector and will misinterpret some of the photons that will form that final key, rendering it useless.”
Not useless. A key with half the elements missing could be cracked by computer later, for example by trial and error.
“To discover Eve’s nefarious doings, they must perform the above procedures, with which they will arrive at an identical key sequence of 0s and 1s – unless someone has been eavesdropping, whereupon there will be some discrepancies.”
Or unless there are natural faults somewhere along the line, such as the inevitable signal leakage out of the optical fibre.
To my small brain, it seems to be a good system so long as the optical fibre cable remains unknown to any enemy. Once the cable is known, the use of a sharp pair of scissors could result in a permanent loss of communication.