John Denker, <[EMAIL PROTECTED]>, writes:
> Eve need not limit herself to snooping "the signal".  What she really wants 
> to know is the "state of mind" of the participants, i.e. the settings of 
> their rotators.  If she knows that, she knows everything.  She can, as a 
> final step, synthesize a mockup of the final result and feed it to Arthur.
> Eve can mount a known-plaintext attack against each rotator.  That is, she 
> can send in a known photon, or if necessary multiple known photons, and see 
> what comes out.

Yes, this does seem to be a powerful attack.  I don't think she could
learn much with a single photon, but if she could send multiple photons
through while the rotator was still set up she could learn as much as
she wanted about the rotation angle.  Plus if she were using her own
photons, the circulating photon would not be affected and her attack
would not be detected.

> It would not be easy for the participants to detect such an attack 
> directly.  They could defend against it to some degree by pre-arranging 
> strict timing requirements on their signals... but they would need to keep 
> these arrangements secret from Eve.  At this point AFAICT the whole scheme 
> is in danger of losing its elegance, and perhaps of losing its raison d'etre.
> Or does somebody have a good defense against this hyper-active attack?

The only thing I can suggest would be if the rotation stations could
somehow count or limit the number of photons going through so that they
would know when there were extra.  I think this is possible in theory;
whether it can be done in practice is questionable.

One idea would be to use strict but public timing for the circulating
photon, only opening the gate for long enough to send that one through.
Eve knows when the gate opens, but to get hers through she has to send
them at the same time as the circulating one.  If we then use a nonlinear
material that can only handle one photon at a time, it might be noticeable
when two or more were present.

Another idea would be for the stations to actually absorb the photon
in some manner that preserved its polarization, and then to re-emit it.
These could be primed to pass only a single photon.

I'm sure both of these ideas have serious practical difficulties but
perhaps something along these lines could be made to work.


Reply via email to