879 research outputs found
Quantum Cloning, Eavesdropping and Bell's inequality
We analyze various eavesdropping strategies on a quantum cryptographic
channel. We present the optimal strategy for an eavesdropper restricted to a
two-dimensional probe, interacting on-line with each transmitted signal. The
link between safety of the transmission and the violation of Bell's inequality
is discussed. We also use a quantum copying machine for eavesdropping and for
broadcasting quantum information.Comment: LaTex, 13 pages, with 6 Postscript figure
Nonlinear quantum state transformation of spin-1/2
A non-linear quantum state transformation is presented. The transformation,
which operates on pairs of spin-1/2, can be used to distinguish optimally
between two non-orthogonal states. Similar transformations applied locally on
each component of an entangled pair of spin-1/2 can be used to transform a
mixed nonlocal state into a quasi-pure maximally entangled singlet state. In
both cases the transformation makes use of the basic building block of the
quantum computer, namely the quantum-XOR gate.Comment: 12 pages, LaTeX, amssym, epsfig (2 figures included
A local hidden variable model of quantum correlation exploiting the detection loophole
A local hidden variable model exploiting the detection loophole to reproduce
exactly the quantum correlation of the singlet state is presented. The model is
shown to be compatible with both the CHSH and the CH Bell inequalities.
Moreover, it bears the same rotational symmetry as spins. The reason why the
model can reproduce the quantum correlation without violating the Bell theorem
is that in the model the efficiency of the detectors depends on the local
hidden variable. On average the detector efficiency is limited to 75%.Comment: 6 pages + 1 figure. A software producing data violating Bell
inequality between two classical computers can be downloaded from
http://www.gapoptique.unige.ch/News/BellSoft.as
The Hatsopoulos-Gyftopoulos resolution of the Schroedinger-Park paradox about the concept of "state" in quantum statistical mechanics
A seldom recognized fundamental difficulty undermines the concept of
individual ``state'' in the present formulations of quantum statistical
mechanics (and in its quantum information theory interpretation as well). The
difficulty is an unavoidable consequence of an almost forgotten corollary
proved by E. Schroedinger in 1936 and perused by J.L. Park, Am. J. Phys., Vol.
36, 211 (1968). To resolve it, we must either reject as unsound the concept of
state, or else undertake a serious reformulation of quantum theory and the role
of statistics. We restate the difficulty and discuss a possible resolution
proposed in 1976 by G.N. Hatsopoulos and E.P. Gyftopoulos, Found. Phys., Vol.
6, 15, 127, 439, 561 (1976).Comment: RevTeX4, 7 pages, corrected a paragraph and added an example at page
3, to appear in Mod. Phys. Lett.
Quantum correlations and secret bits
It is shown that (i) all entangled states can be mapped by single-copy
measurements into probability distributions containing secret correlations, and
(ii) if a probability distribution obtained from a quantum state contains
secret correlations, then this state has to be entangled. These results prove
the existence of a two-way connection between secret and quantum correlations
in the process of preparation. They also imply that either it is possible to
map any bound entangled state into a distillable probability distribution or
bipartite bound information exists.Comment: 4 pages, published versio
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