1,653 research outputs found
Quantum cloning without signaling
Perfect Quantum Cloning Machines (QCM) would allow to use quantum nonlocality
for arbitrary fast signaling. However perfect QCM cannot exist. We derive a
bound on the fidelity of QCM compatible with the no-signaling constraint. This
bound equals the fidelity of the Bu\v{z}ek-Hillery QCM
Sundays in a Quantum Engineer's Life
I am a Quantum Engineer, but on Sundays I have principles, John Bell opened
his "underground colloquium" in March 1983, words which I will never forget!
What! John Bell, the great John Bell, presented himself as an engineer!?! one
of those people who make things work without even understanding how they
function?!? whereas I thought of John Bell as one of the greatest theoretician.Comment: Talk presented at the Conference in Commemoration of John S. Bell,
Vienna 10-14 November 2000. 4 pages & 3 figure
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
Quantum Relative States
We study quantum state estimation problems where the reference system with
respect to which the state is measured should itself be treated quantum
mechanically. In this situation, the difference between the system and the
reference tends to fade. We investigate how the overlap between two pure
quantum states can be optimally estimated, in several scenarios, and we
re-visit homodyne detection.Comment: 10 page
EPR Test with Photons and Kaons: Analogies
We present a unified formalism describing EPR test using spin 1/2 particles,
photons and kaons. This facilitates the comparison between existing experiments
using photons and kaons. It underlines the similarities between birefringence
and polarization dependent losses that affects experiments using optical fibers
and mixing and decay that are intrinsic to the kaons. We also discuss the
limitation these two characteristics impose on the testing of Bell's
inequality.Comment: 12 pages, 5 figure
Heisenberg picture operators in the quantum state diffusion model
A stochastic simulation algorithm for the computation of multitime
correlation functions which is based on the quantum state diffusion model of
open systems is developed. The crucial point of the proposed scheme is a
suitable extension of the quantum master equation to a doubled Hilbert space
which is then unraveled by a stochastic differential equation.Comment: LaTeX2E, 6 pages, 3 figures, uses iopar
Coherent quantum measurement for the direct determination of the degree of polarization and Polarization Mode Dispersion compensation
An example of a coherent measurement for the direct evaluation of the degree
of polarization of a single-mode optical beam is presented. It is applied to
the case of great practical importance where depolarization is caused by
polarization mode dispersion. It is demonstrated that coherent measurement has
the potential of significantly increasing the information gain, compared to
standard incoherent measurements.Comment: 4 pages + 3 figure
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