274 research outputs found
Quantum partial teleportation as optimal cloning at a distance
We propose a feasible scheme of conditional quantum partial teleportation of
a qubit as optimal asymmetric cloning at a distance. In this scheme, Alice
preserves one imperfect clone whereas other clone is teleported to Bob.
Fidelities of the clones can be simply controlled by an asymmetry in Bell-state
measurement. The optimality means that tightest inequality for the fidelities
in the asymmetric cloning is saturated. Further we design a conditional
teleportation as symmetric optimal N-> N+1 cloning from N Alice's replicas on
single distant clone. We shortly discussed two feasible experimental
implementations, first one for teleportation of polarization state of a photon
and second one, for teleportation of a time-bin qubit.Comment: 5 pages, 3 figure
Improved storage of coherent and squeezed states in imperfect ring cavity
We propose a method of an improving quality of a ring cavity which is
imperfect due to non-unit mirror reflectivity. The method is based on using
squeezed states of light pulses illuminating the mirror and gradual homodyne
detection of a radiation escaping from the cavity followed by single
displacement and single squeezing operation performed on the released state. We
discuss contribution of this method in process of storing unknown coherent and
known squeezed state and generation of squeezing in the optical ring cavities.Comment: 6 pages and 5 figure
Trusted Noise in Continuous-Variable Quantum Key Distribution: a Threat and a Defense
We address the role of the phase-insensitive trusted preparation and
detection noise in the security of a continuous-variable quantum key
distribution, considering the Gaussian protocols on the basis of coherent and
squeezed states and studying them in the conditions of Gaussian lossy and noisy
channels. The influence of such a noise on the security of Gaussian quantum
cryptography can be crucial, even despite the fact that a noise is trusted, due
to a strongly nonlinear behavior of the quantum entropies involved in the
security analysis. We recapitulate the known effect of the preparation noise in
both direct and reverse-reconciliation protocols, as well as the detection
noise in the reverse-reconciliation scenario. As a new result, we show the
negative role of the trusted detection noise in the direct-reconciliation
scheme. We also describe the role of the trusted preparation or detection noise
added at the reference side of the protocols in improving the robustness of the
protocols to the channel noise, confirming the positive effect for the
coherent-state reverse-reconciliation protocol. Finally, we address the
combined effect of trusted noise added both in the source and the detector.Comment: 25 pages, 9 figure
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