Quantum cryptography

[bascule]

As I understand it quantum cryptography presently depends on polarization. More specifically:

 

When blue light is pumped into a nonlinear crystal, entangled photon pairs (imaged here as a red beam with the aid of a diode laser) emerge at an angle of 3° to the blue beam, and the beams are sent into single-mode fibers to be detected. Because the entangled photons “know” each other, any interference will result in a mismatch when the two beams are compared.

 

However, recently, a method for snooping on the key without disturbing the polarization has been devised:

 

http://www.emergentchaos.com/archives/2007/04/quantum_cryptography_crac.html

 

My question is could something like a Bell test experiment be used to generate a shared random pad. Specifically, what if you had a central generator of entangled photons equidistant from two receiving stations, such that when either side measured a property, the property would be the same on both sides but random.

 

Wouldn't this be a more secure approach? In effect, wouldn't the random pad not even EXIST until the waveform of the entangled photons was collapsed?

[swansont]

Do you have a link to details of the actual experiment instead of a blog entry trashing QC? The news@nature link requires a subscription above and beyond access to the journal itself.

[bascule]

How's this?

 

http://www.truthera.com/2007/05/02/quantum-cryptography-is-cracked-no-longer-100-secure/

 

I think all of these are going to be boiled down for the general public. I can't find the paper on it yet either (if it's been published)

[swansont]

OK, so you have to use the same detector as the receiver of the information. That's an interesting interpretation of "hacked"

[bascule]

So here's where I'm confused:

 

To grab the information en-route would require a ‘quantum non-demolition box’ — a theoretically possible but as-yet-unbuilt device that could measure the photon and pass it along.

 

How does that not violate the Heisenberg Uncertainty Principle?

[swansont]

So here's where I'm confused:

 

 

 

How does that not violate the Heisenberg Uncertainty Principle?

 

 

I would need some details of how it's theoretically possible. From the gist of the article it seems that the polarization and momentum are entangled, so you measure the momentum without destroying the polarization entanglement. But I suspect that requires knowledge of how the photons were entangled in the first place.

[bascule]

So back to my original question:

 

Couldn't the same setup as a Bell test experiment be used to produce a shared random pad?

 

Wouldn't this approach make snooping literally impossible?

[SkanderH]

If anybody is still looking for the original paper, I remember something appearing in Phys Rev A some weeks ago. There's also a recent review of the result in Nature. Sorry I can't give more details.