3,374 research outputs found
Dynamical CPA approach to an itinerant fermionic spin glass model
We study a fermionic version of the Sherrington-Kirkpatrick model including
nearest-neighbor hopping on a -dimensional simple cubic lattices. The
problem is reduced to one of free fermions moving in a dynamical effective
random medium. By means of a CPA method we derive a set of self-consistency
equations for the spin glass order parameter and for the Fourier components of
the local spin susceptibility. In order to solve these equations numerically we
employ an approximation scheme which restricts the dynamics to a feasible
number of the leading Fourier components. From a sequence of systematically
improved dynamical approximations we estimate the location of the quantum
critical point.Comment: 9 pages, 6 figures, revised versio
Insecurity Of Imperfect Quantum Bit Seal
Quantum bit seal is a way to encode a classical bit quantum mechanically so
that everyone can obtain non-zero information on the value of the bit.
Moreover, such an attempt should have a high chance of being detected by an
authorized verifier. Surely, a reader looks for a way to get the maximum amount
of information on the sealed bit and at the same time to minimize her chance of
being caught. And a verifier picks a sealing scheme that maximizes his chance
of detecting any measurement of the sealed bit. Here, I report a strategy that
passes all measurement detection procedures at least half of the time for all
quantum bit sealing schemes. This strategy also minimizes a reader's chance of
being caught under a certain scheme. In this way, I extend the result of
Bechmann-Pasquinucci et al. by proving that quantum seal is insecure in the
case of imperfect sealed bit recovery.Comment: 4 pages, title changed to better reflect what is exactly proven, to
appear in Phys.Lett.
Eavesdropping without quantum memory
In quantum cryptography the optimal eavesdropping strategy requires that the
eavesdropper uses quantum memories in order to optimize her information. What
happens if the eavesdropper has no quantum memory? It is shown that the best
strategy is actually to adopt the simple intercept/resend strategy.Comment: 9 pages LaTeX, 3 figure
Reply To "Comment on 'Quantum String Seal Is Insecure' "
In Phys. Rev. A. 76, 056301 (2007), He claimed that the proof in my earlier
paper [Phys. Rev. A 75, 012327 (2007)] is insufficient to conclude the
insecurity of all quantum string seals because my measurement strategy cannot
obtain non-trivial information on the sealed string and escape detection at the
same time. Here, I clarify that our disagreement comes from our adoption of two
different criteria on the minimum amount of information a quantum string seal
can reveal to members of the public. I also point out that He did not follow my
measurement strategy correctly.Comment: 2 page
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