On Fri, Apr 26, 2002 at 09:36:22AM -0500, [EMAIL PROTECTED] wrote: > Would anybody with more knowledge care to comment on this? > > This article leads one to believe that one can eavesdrop without being > detected and with nearly 5/6ths confidence of the data on a quantum crypto > communication. This is in contrast to the claim to fame of quantum crypto > that the receiver will know if there is an eavesdropper. (This is what makes > quantum crypto work when all public key crypto gets broken.)
This is a result from the non-cloning theorem and is well studied in
Quantum Cryptography. Even though a practical realization which hits the
theoretical limit is quite impressive.
A simpliefied introduction to the consequences of imperfect cloning of quantum
states in QKD is found in a recent paper of Gisin et. al. called "Quantum Cryptography"
and was published in the "Review of Modern Physics" (sorry no exact citation - i have
only the pre-print).
Back to the problem:
The 16% in which the cloning is "wrong", introduce a QBER (equiv to the classical
Bit Error Rate (BER)) in the Quantum channel. Since a system without an eavesdropper
has QBERs of about a few percent this makes it possible to detect the eavesdropper.
[in the case the cloned photon is sent to Bob - in the other case, where the cloned
photon is used for a measurment, the information gathered by Eve is simply reduced to
84%]
If you use the Wigner protocol, this should also prevent a violation of the Bell
inequalities.
(is there a paper which shows the relation between QBER and the Bell inequalities ?)
I think it has been shown that if the information that Eve gathered is less than the
information
of Bob, then it is possible to create a secret key through privacy amplification. This
is
important since it tells us that even if our Eavesdropper reduces the cloning attempts
(which
reduces the QBER) it is not possible to reconstruct the key.
Summery: Eve either intruduces enough QBER to detect her, or she does not get enough
information
to reconstruct the key
Result: QC is not broken but it makes the job harder and the achievable QKD bitrates
lower
Hannes
--
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Hannes R. Boehm
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Institute of Experimental Physics
University of Vienna
Botzmanngasse 5
1090 Wien
Austria
web : http://www.quantum.univie.ac.at/
email: [EMAIL PROTECTED]
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email: [EMAIL PROTECTED]
web : http://hannes.boehm.org
PGP : http://hannes.boehm.org/hannes-pgp.asc
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