This will not work.

If the photon starts in the vertical state, and is only rotated either by 0 or
90 degrees, the photon always is either perfectly vertically or horizontally
polarized. Hence the attacker can determine the state of the photon perfectly:
just measure in the standard (non rotated) basis. If you measure a horizontal
polarisation, the photon was indeed horizontally polarised, if you measure a
vertical polarisation, it was indeed vertically polarised.

That's why in the quantum key distribution protocols there are both
horizontally and diagonally polarised photons. Whatever the base you pick, the
result of a measurement on a diagonally polarised photon could sometimes also
have been obtained if the photon had been polarised horizontally. Hence, both
states cannot be distinguished with certainty.


On Mon, 15 Jan 2001 22:35:11 -0800 [EMAIL PROTECTED] writes:
> [... snip ...]
> If the cryptographer has nothing to say he leaves the photon alone.
> If he wants to say something he uses the next bit of his message to
> determine whether to rotate the photon 90 degrees or not.
> At the end, the photon polarization is measured by attempting to pass it
> through a vertical polarizer.  If it passes, the photon has not been
> rotated, while if it is absorbed, it was rotated.  In this way the
> message bit is recovered.
> Anonymity derives from the inability of an attacker to measure the photon
> without destroying it, unless he can guess its state.  The attacker can
> confirm a guess at the state, but as soon as he guesses wrong, the photon
> will be destroyed.  This will allow the attacker's presence to be detected
> as soon as he makes a wrong guess.  Unfortunately the attacker may be able
> to get lucky and acquire considerable information before he is detected.
> [... snip ...]

Jaap-Henk Hoepman             | Come sail your ships around me
Dept. of Computer Science     | And burn your bridges down
University of Twente          |       Nick Cave - "Ship Song"
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