"Spooky" Quantum Data Encryption

Hardy writes "Imagine an encrypted communications channel that immediately notifies the parties if they are being bugged. The American Institute of Physics site is running an article about exploiting what Einstein described as the "spooky" action at a distance properties of quantum entangled particles. The entanglement process can generate a completely random sequence of 0s and 1s distributed exclusively to two users at remote locations. Any eavesdropper's attempt to intercept this sequence will alter the message in a detectable way and enabling the users to discard the appropriate parts of the data. This random sequence of digits is then used to scramble the message. This approach solves the problem of distributing a shared key to both parties without it falling into the wrong hands. This diagram might help. "

Comments

You can't just change the laws of physics!

[awhoward]

By "impossible" I mean that within the framework of quantum electrodynamics it can *never* happen. Just like within the framework of arithematic you can never add one and one and get three. That's just how it works.

So you might say: "well, the laws of physics are changing so rapidly these days that this will soon be a possibility." But revolutions in physics are rarely, if ever, of the sort where all of the old theory is thrown out and a completely new theory is developed. Instead, discrepencies are discovered in some corner of a theory and new a theory is discovered which is a superset of both the old theory and the new data.

Also, "spooky action at a distance" in the form of quantum entanglement was never "impossible," it was just philosophically objectionable to some people, including Einstein. If you mean that "information can never travel faster than the speed of light in vacuum" when you say "faster than light (FTL)" travel, then you are incorrect if Maxwell's equations are to hold. All know examples of FTL (which are trivial and miss the point) violate some aspect of my previous statement in quotations. As for heavier-than-air flight, no rational scientist in any age who has observed a bird would tell you that it's impossible.

Quantum key theory

[Drone-X]

It has probably been said a lot before on /. but this is how (I understand that) Quantum Encryption works:

First of all it doesn't send encrypted data. It's just used to send random bits from Alice to Bob. Alice sends for every bit that's 1 a vertical polorised foton and a foton that's turned clockwise 45 for every bit that's 0.

Bob chooses one of two filters for every bit he receives. At random he uses a filter that can either receive a 1 (a filter that's turned counter-clockwise 45) or a filter that can receive a 0 (a filter that's horizontally polorised).

Bob will not receive a foton if he uses the wrong filter, which he will do aproximately half the time. This is because the polarisation direction of the bit and the filter would differ 90.

The interesting thing is that if Bob uses the correct filter, he has only 50 chance that he'll see the foton (can you say 'Quantum effects').

So far Bob knows that:
- he did not receive the bit (because he used the wrong filter or because he had 'bad luck')
- the bit is 1 (by using the correct filter)
- the bit is 0 (by using the correct filter)

Bob should, if knows the value of enough bits (which should be the length of the file to be transimitted), send back the numbers of the bits he received over an unsecure channel.

Alice will then know what Bob is using as a key and she can encrypt the file using XOR. Alice then sends the file over an unsecure channel and Bob can decrypt it.

But what if someone is listening? Let's say that Claude is receiving the bits that Alice send. But Bob will know that Claude is listening because he doesn't receive any bits. The solution would seem that Claude resends the bits to Bob. But there is a problem for Claude here, (s)he did only receive 1/4 of the bits correctly. 37.5% (approximately) will thus be incorrect. In stead of receiving 1/4 of the bits correctly, Bob will only receive 36.5% of 1/4 = 16% of the bits correctly.

But how could Bob and Alice know that not all the bits were received correctly? This is currently solved by sending part of the bits over a quality line (on which Claude could be listening though).

Another problem, letting Bob know that a polorized foton has been send could be solved by sending a pulse of non-polarized light an instance before the polorized foton.

Current results are 48km through optic fiber and 50 meter through the air (3km would do for satelites).