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 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

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

Incoherent and Coherent Eavesdropping in the 6-state Protocol of Quantum Cryptography

All incoherent as well as 2- and 3-qubit coherent eavesdropping strategies on the 6 state protocol of quantum cryptography are classified. For a disturbance of 1/6, the optimal incoherent eavesdropping strategy reduces to the universal quantum cloning machine. Coherent eavesdropping cannot increase Eve's Shannon information, neither on the entire string of bits, nor on the set of bits received undisturbed by Bob. However, coherent eavesdropping can increase as well Eve's Renyi information as her probability of guessing correctly all bits. The case that Eve delays the measurement of her probe until after the public discussion on error correction and privacy amplification is also considered. It is argued that by doing so, Eve gains only a negligibly small additional information.Comment: 17 pages, 4 figures: Updated version, discussion on error correction and privacy amplification adde