3,441 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
Quantum state diffusion, measurement and second quantization
Realistic dynamical theories of measurement based on the diffusion of quantum
states are nonunitary, whereas quantum field theory and its generalizations are
unitary. This problem in the quantum field theory of quantum state diffusion
(QSD) appears already in the Lagrangian formulation of QSD as a classical
equation of motion, where Liouville's theorem does not apply to the usual field
theory formulation. This problem is resolved here by doubling the number of
freedoms used to represent a quantum field. The space of quantum fields is then
a classical configuration space, for which volume need not be conserved,
instead of the usual phase space, to which Liouville's theorem applies. The
creation operator for the quantized field satisfies the QSD equations, but the
annihilation operator does not satisfy the conjugate eqation. It appears only
in a formal role.Comment: 10 page
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
A local variable model for entanglement swapping exploiting the detection loophole
In an entanglement swapping process two initially uncorrelated qubits become
entangled, without any direct interaction. We present a model using local
variables aiming at reproducing this remarkable process, under the realistic
assumption of finite detection efficiencies. The model assumes that the local
variables describing the two qubits are initially completely uncorrelated.
Nevertheless, we show that once conditioned on the Bell measurement result, the
local variables bear enough correlation to simulate quantum measurement results
with correlation very close to the quantum prediction. When only a partial Bell
measurement is simulated, as carried out is all experiments so far, then the
model recovers analytically the quantum prediction.Comment: 5 pages, 5 figure
Linear quantum state diffusion for non-Markovian open quantum systems
We demonstrate the relevance of complex Gaussian stochastic processes to the
stochastic state vector description of non-Markovian open quantum systems.
These processes express the general Feynman-Vernon path integral propagator for
open quantum systems as the classical ensemble average over stochastic pure
state propagators in a natural way. They are the coloured generalization of
complex Wiener processes in quantum state diffusion stochastic Schrodinger
equations.Comment: 9 pages, RevTeX, appears in Physics Letters
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
Entanglement 25 years after Quantum Teleportation: testing joint measurements in quantum networks
Twenty-five years after the invention of quantum teleportation, the concept
of entanglement gained enormous popularity. This is especially nice to those
who remember that entanglement was not even taught at universities until the
1990's. Today, entanglement is often presented as a resource, the resource of
quantum information science and technology. However, entanglement is exploited
twice in quantum teleportation. First, entanglement is the `quantum
teleportation channel', i.e. entanglement between distant systems. Second,
entanglement appears in the eigenvectors of the joint measurement that Alice,
the sender, has to perform jointly on the quantum state to be teleported and
her half of the `quantum teleportation channel', i.e. entanglement enabling
entirely new kinds of quantum measurements. I emphasize how poorely this second
kind of entanglement is understood. In particular, I use quantum networks in
which each party connected to several nodes performs a joint measurement to
illustrate that the quantumness of such joint measurements remains elusive,
escaping today's available tools to detect and quantify it.Comment: Feature paper, Celebrating the Silver Jubilee of Teleportation (7
pages). V2 (March'19): Many typos corrected (sorry) and a few comments adde
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