Very-High Energy Gamma-Ray Flux Limits for Nearby Active Galactic Nuclei

Combining the results of targeted observations, H.E.S.S. has accumulated a large amount of extra-galactic exposure at TeV energies. Due to its large field of view a considerable part of the sky (0.6 sr) has been observed with high sensitivity outside the targeted observation positions. Since this exposure region contains little inherent bias, it is well suited for studies of extra-galactic source populations. Given the correlation between ultra-high energy cosmic rays and nearby extra-galactic objects recently claimed by the Auger collaboration, this unbiased sky sample by H.E.S.S. is of interest since it includes (besides the targeted sources) 63 AGN within 100 Mpc, for which very-high energy gamma-ray flux limits are derived.Comment: 4 pages, 5 figures, Submitted to Proceedings of "4th International Symposium on High Energy Gamma-Ray Astronomy 2008

Optimal cloning of single photon polarization by coherent feedback of beam splitter losses

Light fields can be amplified by measuring the field amplitude reflected at a beam splitter of reflectivity R and adding a coherent amplitude proportional to the measurement result to the transmitted field. By applying the quantum optical realization of this amplification scheme to single photon inputs, it is possible to clone the polarization states of photons. We show that optimal cloning of single photon polarization is possible when the gain factor of the amplification is equal to the inverse squareroot of 1-R.Comment: 10 pages, including 1 figure, extended from letter to full paper, to be published in New Journal of Physic

Generation of highly non-classical n-photon polarization states by super-bunching at a photon bottleneck

It is shown that coherent superpositions of two oppositely polarized n-photon states can be created by post-selecting the transmission of n independently generated photons into a single mode transmission line. It is thus possible to generate highly non-classical n-photon polarization states using only the bunching effects associated with the bosonic nature of photons. The effects of mode-matching errors are discussed and the possibility of creating n-photon entanglement by redistributing the photons into n separate modes is considered.Comment: 8 pages, including 4 figures, extended version of the original letter paper, includes discussion of linear polarization statistic

Cherenkov Telescope Array: The next-generation ground-based gamma-ray observatory

High energy gamma-ray astronomy is a newly emerging and very successful branch of astronomy and astrophysics. Exciting results have been obtained by the current generation Cherenkov telescope systems such as H.E.S.S., MAGIC, VERITAS and CANGAROO. The H.E.S.S. survey of the galactic plane has revealed a large number of sources and addresses issues such as the question about the origin of cosmic rays. The detection of very high energy emission from extragalactic sources at large distances has provided insights in the star formation during the history of the universe and in the understanding of active galactic nuclei. The development of the very large Cherenkov telescope array system (CTA) with a sensitivity about an order of magnitude better than current instruments and significantly improved sensitivity is under intense discussion. This observatory will reveal an order of magnitude more sources and due to its higher sensitivity and angular resolution it will be able to detect new classes of objects and phenomena that have not been visible until now. A combination of different telescope types will provide the sensitivity needed in different energy ranges.Comment: 4 pages, 3 figures, to appear in the proceedings of the 30th International Cosmic Ray Conference, Merida, July 200

Triggering of Imaging Air Cherenkov Telescopes: PMT trigger rates due to night-sky photons

Imaging air Cherenkov telescopes are usually triggered on a coincidence of two or sometimes more pixels, with discriminator thresholds in excess of 20 photoelectrons applied for each pixel. These thresholds required to suppress night-sky background are significantly higher than expected on the basis of a Poisson distribution in the number of night-sky photoelectrons generated during the characteristic signal integration time. We studied noise trigger rates under controlled conditions using an artificial background light source. Large tails in the PMT amplitude response to single photoelectrons are identified as a dominant contribution to noise triggers. The rate of such events is very sensitive to PMT operating parameters.Comment: 19 pages, latex,epsf, 7 figures appended as uuencoded file, submitted to Journal of Physics

A phantom force induced by the tunneling current, characterized on Si(111)

Simultaneous measurements of tunneling currents and atomic forces on surfaces and adsorbates provide new insights into the electronic and structural properties of matter on the atomic scale. We report on experimental observations and calculations of a strong impact the tunneling current can have on the measured force, which arises when the resistivity of the sample cannot be neglected. We present a study on Si(111)-7\times7 with various doping levels, but this effect is expected to occur on other low-conductance samples like adsorbed molecules, and is likely to strongly affect Kelvin probe measurements on the atomic scale.Comment: 4 pages, 4 figures, submitte