First Bank Transfer via Quantum Cryptography

An anonymous reader writes with today's announcement that "the Austrian project for Quantum Cryptography made the world's first Bank Transfer via Quantum Cryptography Based on Entangled Photons; see also Einstein-Podolski-Rosen Paradoxon." (For more background, see the recent Slashdot post "Quantum Cryptography Leaving the Lab.")

Comments

Quantum Crypto Provably Flawed?

[theLOUDroom]

I'm asking this question again because it came a bit to late to the last discussion I posted it in

Is quantum crypto provably flawed?

I've seen tons of blurbs stating the the link is "absolutely" secure, but it seems that isn't really the case. (see the bottom of the page.)

What strikes me about all this is the following section:
"each pulse should be attenuated to an average of about .1 photon to reduce the probability of generating a two-photon pulse that could be split and eavesdropped undetectably."


What that says to me is that there is not way to 100% know you're transmitting just one photon.

It sounds like there's no device that is capable of transmitting one and only one photon with 100% reliability. If this is the case, a lot of the arguments about how secure this is are vastly overstated.

In the end QC would be vulnerable to a man-in-the-middle attack by watching for multi-photon emissions.

If this is the case, a lot of the noise surrounding QC could turn out to be hype. (The big plus for quantum crypto is that it's supposedly immune to this.) Is there a quantum physicist in the house?

Re:Quantum Crypto Provably Flawed?

[gunnk]

I think you're worried about something that happens, but isn't a useful eavesdropping technique. Suppose that you have a device for emitting single photons. Further suppose that the emitter accidentally emits two photons for a single bit 1% of the time.

If an eavesdropper successfully split the extra photons off, they have successfully captured 1% of the data stream. First off, that's not much data if you want to reconstruct something meaningful in the way of information carried by the stream.

Another problem, however, is the effect of the splitter on the rest of the stream. When a single photon passes the splitter, which path does it choose? If I'm not mistaken, that choice will be at random. If so, then the presence of the splitter becomes immediately detectable because half the single photon pulses never reach their destination. In fact, the number missing is likely to be so close to 50% that the presence of the splitter should be obvious to the bank.

Re:But...

[blincoln]

You know the two-slit experiment? Well, its just like that
-- standard explanation for weird quantum things when you don't know the right answer.

I was just reading about that last night in The Elegant Universe.

For those who haven't heard of it before, here's the experiment:

- take a wall with light shining on it from a projector.

- place a board in-between the wall and the projector that interrupts the beam of light. The board should have two vertical slits cut in it, which can be opened and closed independently of each other.

If you open just the left one, you get a vertical bar of light on the wall.

If you open just the right one, you also get a vertical bar of light on the wall, offset from the one that was there with the left one open.

Now, intuitively you would think that if you opened both at once, you would just get two vertical bars of light, but you don't. Wave interference means you get a whole bunch of light and dark vertical bars on the wall.

Here's the spooky quantum-mechanical part - the same interference effect happens even if the projector is designed to only emit one photon at a time, then wait until it has hit the wall (or the board) before sending another. You will still get the bands of dark and light.

Pretty weird, eh?