Kinda like how watching an 'NSync video burns your eyes out.

 Reply

If I understand correctly. The information you want to send is encrypted classically. It's the key that is quantumly generated. In one quantum key protocal you use photons. Photons can have several different states. Using random polarisation you can send the photon in a certain, random, state. In this way you can send a string of 1's and 0's as up-down or diagonal left states,etc.

Now, sender and receiver measure the photons using a randomly selected basis, between 2 options, (they don't tell each other which they're using to begin with). Once they have measured each photon they compare which measurements they used. This can be done over the phone. So you say ' i measured the first one using the X basis. Did you also use the X basis for the first photon?' If you didn't then you discard that bit. If you did use the same basis then you keep it. You this for all of the photons. BUT you don't discuss the results of the measurement for each photon. This is your key.

Then, you take a sample of the ones you kept to test for eavesdropping. You're gonna discard this sample later (so it isn't part of the key and thus the key is still safe). Then you compare your results for the sample...

If you get the same result for each photon (having used the same basis to measure) then you know the key is secure. If, when you're both using the same basis, you find even one photon (assuming noiseless channels) where you get a different result then you know that someone has eavesdropped.

The reason this works is that once you send a sufficiently long string of photons to make your key. The odds of the eavesdropper always picking the 'right' basis to measure in is vanishingly small. They'd need to be right every time for millions of bits to get away with eavesdropping.

Essentially, you're sending a pile of states at each other and whatever you measure the same you use as the key. For someone else to get the key by measuring is pretty much impossible given a suficently long key.

And once you have the secure key, you can encrypt the data classically and safely stick it up anywhere you like.

Note, this is just one protocol. There are a few.

 Reply

with only a sender and a reciever quantum cryptography works just like ordinary cryptography, shared random bit strings are used to encrypt data. the difference is not that it's harder to hack the difference is that intercepting the key and reading it is very very easy to detect and is therefor largely useless because as soon as the sender and reciever know that their key has been compromised they're just going to switch to a different key.

the data itself is encrypted and transmitted via normal channels and is not saved as a series of quantum states but without the key it would take a very very long time to decrypt.

 Reply

saving the key after reading it is really really easy, that's what the intended receptiant has to do in order to decode the message. the security lies in the fact that it's very very easy to tell that someone else has read the key if the basis in which each photon is read and sent is made public. the quantum key IS the private key, the public key is the basis in which each photon is read or sent. strictly speaking if the sender and the receiver were using quantum encryption and did not check with one another to detect an eavesdropper quantum encryption is no more secure than standard encryption.

 Reply