I'm not an expert on this stuff, but I'm interested enough to chase
a few references...

Ivan Krstic <[EMAIL PROTECTED]> writes:
> The idea that observing modifies state is something to be approached with 
> caution. Read-only does make sense in quantum world; implementations of 
> early theoretical work by Elitzur and Vaidman achieved roughly 50% success 
> on interaction-free measurements.

Careful there--EV interaction-free measurements do *not* read the
internal state of the system measured.  The trick to the EV IFM is
that it determines the location (or existence) of a system without
interacting with the internal state of that system; a corollary is
that it derives no information about the internal state.

  "The meaning of the EV IFM is that if an object changes its internal
   state [...] due to the radiation, then the method allows detection
   of the location of the object without any change in its internal
   We should mention that the interaction-free measurements do not
   have vanishing interaction Hamiltonian. [...] the IFM can change
   very significantly the quantum state of the observed object and we
   still name it interaction free."
        Lev Vaidman, "Are Interaction-free Measurements Interaction
        Free?", http://arxiv.org/abs/quant-ph/0006077

Intercepting QC is all about determining the internal state (e.g.
photon polarization), and AFAIK that requires becoming entangled with
the state of the particle.  EV IFM doesn't appear to provide a way
around this.

and later...
> On Fri, 26 Sep 2003 09:10:05 -0400, Greg Troxel <[EMAIL PROTECTED]> wrote:
> > The current canoncial
> > paper on how to calculate the number of bits that must be hashed away
> > due to detected eavesdropping and the inferred amount of undetected
> > eavesdropping is "Defense frontier analysis of quantum cryptographic
> > systems" by Slutsky et al:
> >
> >   http://topaz.ucsd.edu/papers/defense.pdf
> Up-front disclaimer: I haven't had time to study this paper with the
> level of attention it likely deserves, so I apologize if the following
> contains incorrect logic. However, from glancing over it, it appears
> the assumptions on which the entire paper rests are undermined by work
> such as that of Elitzur and Vaidman (see the article I linked
> previously). Specifically, note the following:
> If we do away with the idea that there are no interaction-free
> measurements (which was, at least to me, convincingly shown by the
> Quantum seeing in the dark article), this paper becomes considerably
> less useful; the first claim's validity is completely nullified (no
> longer does interference with particles necessarily introduce
> transmission errors),

If Eve can measure the state of a particle without altering its state
at all, 100% of the time, then QC is dead--the defense function
becomes infinite.  But AFAICT the EV IFM techniques do not provide
this ability.

> while the effect on the second statement is
> evil: employing the proposed key distillation techniques, the user
> might be given a (very) false sense of security, as only a small
> percentage of the particles that Eve observes register as transmission
> errors (<=15%, according to the LANL figure).

Err...I think you've missed the point of the paper.  What they're
doing is deriving how many extra bits Alice and Bob have to transmit
given that Eve is intercepting their transmission, and only some
fraction (dependent on the interception technique) of those
interceptions are detectable.  They do not assume that all
interceptions appear as errors; they (initially) assume that all
errors are due to interceptions (they deal with the case of noisy
channels later in the paper).


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