Experimental Test of Two-way Quantum Key Distribution in Presence of Controlled Noise

We describe the experimental test of a quantum key distribution performed with a two-way protocol without using entanglement. An individual incoherent eavesdropping is simulated and induces a variable amount of noise on the communication channel. This allows a direct verification of the agreement between theory and practice.Comment: 4 pages, 3 figure

Football history: a German perspective on current research fields

In the last 25 years, German football has experienced several turning points. First, the collapse of communism in 1989 rendered necessary a complete reorganisation of the sport and football landscape in eastern Germany. Second, in parallel with the award in the year 2000 of the right to host the 2006 FIFA World Cup in the Federal Republic of Germany, a significant upturn took place in academic research on football. The reappraisal of the National Socialist era, in particular, has experienced a noticeable upswing since the year 2000. The present article focuses on several key thematic areas that shape current research and will shape research in the future. They include, first, the question of the character of the Vereine (here: football clubs) under the conditions of a communist dictatorship, and second, an increased interest in the biographical component of the history of the development of football. At the same time, it is clear that by now the question of a memory culture in football is no longer limited to national perspectives such as the reappraisal of the "Miracle of Bern". Rather, the dynamic development of European football renders possible the emergence of European realms of shared memory. At the same time, a boom in public engagement with football history can be observed in Germany and elsewhere. This holds true for both the social commemoration of footballers who were victims of war and tyranny and for popular cultural publications. In recent years, therefore, there has not only been an increase in the “museumisation” but also in the media marketing of football history. Thus, football history itself has become a market that is served by various stakeholders, such as clubs, companies, and the media

Bright filter-free source of indistinguishable photon pairs

We demonstrate a high-brightness source of pairs of indistinguishable photons based on a type-II phase-matched doubly-resonant optical parametric oscillator operated far below threshold. The cavity-enhanced down-conversion output of a PPKTP crystal is coupled into two single-mode fibers with a mode coupling efficiency of 58%. The high degree of indistinguishability between the photons of a pair is demonstrated by a Hong-Ou-Mandel interference visibility of higher than 90% without any filtering at an instantaneous coincidence rate of 450 000 pairs/s per mW of pump power per nm of down-conversion bandwidth. For the degenerate spectral mode with a linewidth of 7 MHz at 795 nm a rate of 70 pairs/(s mW MHz) is estimated, increasing the spectral brightness for indistinguishable photons by two orders of magnitude compared to similar previous sources.Comment: 7 pages, 3 figure

Entanglement-enhanced probing of a delicate material system

Quantum metrology uses entanglement and other quantum effects to improve the sensitivity of demanding measurements. Probing of delicate systems demands high sensitivity from limited probe energy and has motivated the field's key benchmark-the standard quantum limit. Here we report the first entanglement-enhanced measurement of a delicate material system. We non-destructively probe an atomic spin ensemble by means of near-resonant Faraday rotation, a measurement that is limited by probe-induced scattering in quantum-memory and spin-squeezing applications. We use narrowband, atom-resonant NOON states to beat the standard quantum limit of sensitivity by more than five standard deviations, both on a per-photon and per-damage basis. This demonstrates quantum enhancement with fully realistic loss and noise, including variable-loss effects. The experiment opens the way to ultra-gentle probing of single atoms, single molecules, quantum gases and living cells.Comment: 7 pages, 8 figures; Nature Photonics, advance online publication, 16 December 201

Polarization entanglement and quantum beats of photon pairs from four-wave mixing in a cold 87Rb ensemble

10.1088/1367-2630/17/9/093034New Journal of Physics1799303

Probing the quantum-classical boundary with compression software

10.1088/1367-2630/18/3/035011New Journal of Physics1833501