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

Linking Measures for Macroscopic Quantum States via Photon-Spin Mapping

We review and compare several measures that identify quantum states that are "macroscopically quantum". These measures were initially formulated either for photonic systems or spin ensembles. Here, we compare them through a simple model which maps photonic states to spin ensembles. On the one hand, we reveal problems for some spin measures to handle correctly photonic states that typically are considered to be macroscopically quantum. On the other hand, we find significant similarities between other measures even though they were differently motivated.Comment: 12 pages, 1 figure; published in a special issue of Optics Communications: "Macroscopic quantumness: theory and applications in optical sciences"; v2: minor change