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    AuthorTitleYearJournal/ProceedingsReftypeDOI/URL
    Abuter, R., di Lieto, N. & Schmid, C. VLTI fringe tracking real time computer architecture
    2012 Proc. SPIE
    Vol. 8445, pp. 844535-844535-8 
    other DOI URL 
    Abstract: Fringe tracking is a CPU intensive real time application. Rates of up to 1 KHz are needed to be able to freeze the atmosphere; in addition, each available baseline must be tracked independently. To be able to overcome the CPU performance limits, VLTI 1st generation trackers PRIMA fringe sensor unit (FSU) and FINITO were implemented in a distributed architecture separating the sensor from the controller. A side effect of this design was that the control loop delay increases as the system runs asynchronously causing an increase of the phase RMS value. This is in conflict with the objective of stabilizing the fringes with the smallest possible phase RMS. VLTI 2nd generation trackers (GRAVITY 6 and ESO 2GFT) currently being designed, will operate with up to six baselines and need to achieve phase RMS values smaller than 200 [nm]. This paper will present the computer architecture of the first generation trackers. Using simulations carried out in the PRIMA testbed, the induction of phase RMS by additional pure delays of the control loop will be precisely quantify. Afterwards, using the current architecture as reference, expected values of pure delays will be estimated for a six baselines tracker. Finally, an in order to overcome this problem, a new design that exploits the inherent parallelism of the multiples baselines and integrates the sensor and the controller in a single computer will be briefly proposed.
    BibTeX:
    @other{doi:10.1117/12.926005,
      author = {Abuter, Roberto and di Lieto, Nicola and Schmid, Christian},
      title = {VLTI fringe tracking real time computer architecture
    }, journal = {Proc. SPIE}, year = {2012}, volume = {8445}, pages = {844535-844535-8}, url = { http://dx.doi.org/10.1117/12.926005}, doi = {http://dx.doi.org/10.1117/12.926005} }
    Bourennane, M., Schmid, C., Trojek, P., Kurtsiefer, C., Brukner, C., Zukowski, M. & Weinfurter, H. NATO Science for Peace and Security 2007
    Vol. 11: Quantum Communication and Security, pp. 22-30 
    inbook URL 
    Abstract: We present simple and practical quantum solution for secure multiparty communication protocols, the secret sharing and the communication complexity and their proof-of-principle experimental realizations. In the secret sharing protocol, a secret is split among several parties in a way that its reconstruction requires the collaboration of the participating parties. In the communication complexity problem, the goal is to maximize the success probability of the partners for solving for giving communication resources some N partner communication complexity tasks. Our quantum solution is based on sequential transformations on a single qubit. In contrast with recently proposed schemes involving multiparticle GHZ states.
    BibTeX:
    @inbook{Bou07,
      author = {Mohamed Bourennane and Christian Schmid and Pavel Trojek and Christain Kurtsiefer and Caslav Brukner and Marek Zukowski and Harald Weinfurter},
      title = {NATO Science for Peace and Security},
      publisher = {IOS Press},
      year = {2007},
      volume = {11: Quantum Communication and Security},
      pages = {22-30},
      url = {http://ebooks.iospress.nl/publication/23516}
    }
    
    Flitney, A.P., Schlosshauer, M., Schmid, C., Laskowski, W. & Hollenberg, L.C. Equivalence between Bell inequalities and quantum minority games 2009 Physics Letters A
    Vol. 373(5), pp. 521 - 524 
    article DOI URL 
    Abstract: We show that, for a continuous set of entangled four-partite states, the task of maximizing the payoff in the symmetric-strategy four-player quantum Minority game is equivalent to maximizing the violation of a four-particle Bell inequality. We conclude the existence of direct correspondences between (i) the payoff rule and Bell inequalities, and (ii) the strategy and the choice of measured observables in evaluating these Bell inequalities. We also show that such a correspondence is unique to minority-like games.
    BibTeX:
    @article{Fli09,
      author = {Adrian P. Flitney and Maximilian Schlosshauer and Christian Schmid and WiesŇāaw Laskowski and Lloyd C.L. Hollenberg},
      title = {Equivalence between Bell inequalities and quantum minority games },
      journal = {Physics Letters A },
      year = {2009},
      volume = {373},
      number = {5},
      pages = {521 - 524},
      url = {http://www.sciencedirect.com/science/article/pii/S0375960108017246},
      doi = {http://dx.doi.org/10.1016/j.physleta.2008.12.003}
    }
    
    Gomes, N., Schmid, C., Sahlmann, J., Menardi, S., Abuter, R., Merand, A. & Delplancke, F. Estimating visibility amplitudes with the PRIMA fringe trackers
    2012 Proc. SPIE
    Vol. 8445, pp. 844522-844522-9 
    other DOI URL 
    Abstract: The quality of the tracking performed by the fringe sensor units (FSUs) of PRIMA, the ESO's dual feed facility for the VLTI, is affected by the angular separation between the two objects being observed simultaneously, the detector integration time (DIT) and the atmospheric observational conditions. We describe the algorithm we developed to compute visibilities from the FSU data and discuss their validity for the study on the angular anisoplanatism measured through the dependence of the visibility amplitudes on the angular separation.
    BibTeX:
    @other{doi:10.1117/12.925445,
      author = {Gomes, Nuno and Schmid, Christian and Sahlmann, Johannes and Menardi, Serge and Abuter, Roberto and Merand, Antoine and Delplancke, Francoise},
      title = {Estimating visibility amplitudes with the PRIMA fringe trackers
    }, journal = {Proc. SPIE}, year = {2012}, volume = {8445}, pages = {844522-844522-9}, url = { http://dx.doi.org/10.1117/12.925445}, doi = {http://dx.doi.org/10.1117/12.925445} }
    Gruca, J., Laskowski, W., ifmmode Zelse Zukowski, M., Kiesel, N., Wieczorek, W., Schmid, C. & Weinfurter, H. Nonclassicality thresholds for multiqubit states: Numerical analysis 2010 Phys. Rev. A
    Vol. 82, pp. 012118 
    article DOI URL 
    Abstract: States that strongly violate Bellís inequalities are required in many quantum-informational protocols as, for example, in cryptography, secret sharing, and the reduction of communication complexity. We investigate families of such states with a numerical method which allows us to reveal nonclassicality even without direct knowledge of Bellís inequalities for the given problem. An extensive set of numerical results is presented and discussed.
    BibTeX:
    @article{Gru10,
      author = {Gruca, Jacek and Laskowski, Wiesław and ifmmode Zelse Zukowski, Marek and Kiesel, Nikolai and Wieczorek, Witlef and Schmid, Christian and Weinfurter, Harald},
      title = {Nonclassicality thresholds for multiqubit states: Numerical analysis},
      journal = {Phys. Rev. A},
      publisher = {American Physical Society},
      year = {2010},
      volume = {82},
      pages = {012118},
      url = {http://link.aps.org/doi/10.1103/PhysRevA.82.012118},
      doi = {http://dx.doi.org/10.1103/PhysRevA.82.012118}
    }
    
    Haguenauer, P., Abuter, R., Andolfato, L., Alonso, J., Blanchard, G., Berger, J.-P., Bourget, P., Brillant, S., Derie, F., Delplancke, F., Di Lieto, N., Dupuy, C., Gilli, B., Gitton, P., Gonzalez, J.C., Guisard, S., Guniat, S., Hudepohl, G., Kaufer, A., Leveque, S., Menardi, S., Merand, A., Morel, S., Percheron, I., Phan Duc, T., Poupar, S., Ramirez, A., Reineiro, C., Rengaswamy, S., Rivinius, T., Schoeller, M., Schmid, C., Segovia, A., Schuhler, N., Valdes, G., de Wit, W.J. & Wittkowski, M. The Very Large Telescope Interferometer v2012+
    2012 Proc. SPIE
    Vol. 8445, pp. 84450D-84450D-10 
    other DOI URL 
    Abstract: The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8-m Unit Telescopes (UT) and the four 1.8-m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in northern Chile. The two VLTI instruments, MIDI and AMBER deliver regular scientific results. In parallel to the operation, the instruments developments are pursued, and new modes are studied and commissioned to offer a wider range of scientific possibilities to the community and increase sensitivity. New configurations of the ATs have been offered and are frequently discussed with the science users of the VLTI and implemented to optimize the scientific return. The PRIMA instrument, bringing astrometry capability to the VLTI and phase referencing to the instruments is being commissioned. The visitor instrument PIONIER is now fully operational and bringing imaging capability to the VLTI. The current status of the VLTI is described with successes and scientific results, and prospects on future evolution are presented.
    BibTeX:
    @other{doi:10.1117/12.925323,
      author = {Haguenauer, Pierre and Abuter, Roberto and Andolfato, Luigi and Alonso, Jaime and Blanchard, Guillaume and Berger, Jean-Philippe and Bourget, Pierre and Brillant, Stephane and Derie, Frederic and Delplancke, Francoise and Di Lieto, Nicola and Dupuy, Christophe and Gilli, Bruno and Gitton, Philippe and Gonzalez, Juan Carlos and Guisard, Stephane and Guniat, Serge and Hudepohl, Gerhard and Kaufer, Andreas and Leveque, Samuel and Menardi, Serge and Merand, Antoine and Morel, Sebastien and Percheron, Isabelle and Phan Duc, Than and Poupar, Sebastien and Ramirez, Andres and Reineiro, Claudio and Rengaswamy, Sridharan and Rivinius, Thomas and Schoeller, Markus and Schmid, Christian and Segovia, Alex and Schuhler, Nicolas and Valdes, Guillermo and de Wit, Willem Jan and Wittkowski, Markus},
      title = {The Very Large Telescope Interferometer v2012+
    }, journal = {Proc. SPIE}, year = {2012}, volume = {8445}, pages = {84450D-84450D-10}, url = { http://dx.doi.org/10.1117/12.925323}, doi = {http://dx.doi.org/10.1117/12.925323} }
    Haguenauer, P., Alonso, J., Bourget, P., Brillant, S., Gitton, P., Guisard, S., Poupar, S., Schuhler, N., Abuter, R., Andolfato, L., Blanchard, G., Berger, J.-P., Cortes, A., Derie, F., Delplancke, F., Di Lieto, N., Dupuy, C., Gilli, B., Glindemann, A., Guniat, S., Huedepohl, G., Kaufer, A., Le Bouquin, J.-B., Leveque, S., Menardi, S., Merand, A., Morel, S., Percheron, I., Phan Duc, T., Pino, A., Ramirez, A., Rengaswamy, S., Richichi, A., Rivinius, T., Sahlmann, J., Schoeller, M., Schmid, C., Stefl, S., Valdes, G., van Belle, G., Wehner, S. & Wittkowski, M. The very large telescope Interferometer: 2010 edition 2010 Proc. SPIE
    Vol. 7734, pp. 773404-773404-11 
    other DOI URL 
    Abstract: The ESO Very Large Telescope Interferometer (VLTI) offers access to the four 8-m Unit Telescopes (UT) and the four 1.8-m Auxiliary Telescopes (AT) of the Paranal Observatory located in the Atacama Desert in northern Chile. The two VLTI instruments, MIDI and AMBER deliver regular scientific results. In parallel to the operation, the instruments developments are pursued, and new modes are studied and commissioned to offer a wider range of scientific possibilities to the community. New configurations of the ATs array are discussed with the science users of the VLTI and implemented to optimize the scientific return. The monitoring and improvement of the different systems of the VLTI is a continuous work. The PRIMA instrument, bringing astrometry capability to the VLTI and phase referencing to the instruments has been successfully installed and the commissioning is ongoing. The possibility for visiting instruments has been opened to the VLTI facility.
    BibTeX:
    @other{doi:10.1117/12.857070,
      author = {Haguenauer, Pierre and Alonso, Jaime and Bourget, Pierre and Brillant, Stephane and Gitton, Philippe and Guisard, Stephane and Poupar, Sebastien and Schuhler, Nicolas and Abuter, Roberto and Andolfato, Luigi and Blanchard, Guillaume and Berger, Jean-Philippe and Cortes, Angela and Derie, Frederic and Delplancke, Francoise and Di Lieto, Nicola and Dupuy, Christophe and Gilli, Bruno and Glindemann, Andreas and Guniat, Serge and Huedepohl, Gerhard and Kaufer, Andreas and Le Bouquin, Jean-Baptiste and Leveque, Samuel and Menardi, Serge and Merand, Antoine and Morel, Sebastien and Percheron, Isabelle and Phan Duc, Than and Pino, Andres and Ramirez, Andres and Rengaswamy, Sridharan and Richichi, Andrea and Rivinius, Thomas and Sahlmann, Johannes and Schoeller, Markus and Schmid, Christian and Stefl, Stan and Valdes, Guillermo and van Belle, Gerard and Wehner, Stefan and Wittkowski, Markus},
      title = {The very large telescope Interferometer: 2010 edition},
      journal = {Proc. SPIE},
      year = {2010},
      volume = {7734},
      pages = {773404-773404-11},
      url = { http://dx.doi.org/10.1117/12.857070},
      doi = {http://dx.doi.org/10.1117/12.857070}
    }
    
    Kiesel, N., Schmid, C., Tóth, G., Solano, E. & Weinfurter, H. Experimental Observation of Four-Photon Entangled Dicke State with High Fidelity 2007 Phys. Rev. Lett.
    Vol. 98, pp. 063604 
    article DOI URL 
    Abstract: We present the experimental observation of the symmetric four-photon entangled Dicke state with two excitations |D4(2)>. A simple experimental setup allowed quantum state tomography yielding a fidelity as high as 0.844Ī0.008. We study the entanglement persistency of the state using novel witness operators and focus on the demonstration of a remarkable property: depending on the orientation of a measurement on one photon, the remaining three photons are projected into both inequivalent classes of genuine tripartite entanglement, the Greenberger-Horne-Zeilinger and W class. Furthermore, we discuss possible applications of |D4(2)> in quantum communication.
    BibTeX:
    @article{Kie07,
      author = {Kiesel, N. and Schmid, C. and Tóth, G. and Solano, E. and Weinfurter, H.},
      title = {Experimental Observation of Four-Photon Entangled Dicke State with High Fidelity},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2007},
      volume = {98},
      pages = {063604},
      url = {http://link.aps.org/doi/10.1103/PhysRevLett.98.063604},
      doi = {http://dx.doi.org/10.1103/PhysRevLett.98.063604}
    }
    
    Kiesel, N., Schmid, C., Weber, U., Tóth, G., Gühne, O., Ursin, R. & Weinfurter, H. Experimental Analysis of a Four-Qubit Photon Cluster State 2005 Phys. Rev. Lett.
    Vol. 95, pp. 210502 
    article DOI URL 
    Abstract: Linear-optics quantum logic operations enabled the observation of a four-photon cluster state. We prove genuine four-partite entanglement and study its persistency, demonstrating remarkable differences from the usual Greenberger-Horne-Zeilinger (GHZ) state. Efficient analysis tools are introduced in the experiment, which will be of great importance in further studies on multiparticle entangled states.
    BibTeX:
    @article{Kie05a,
      author = {Kiesel, Nikolai and Schmid, Christian and Weber, Ulrich and Tóth, Géza and Gühne, Otfried and Ursin, Rupert and Weinfurter, Harald},
      title = {Experimental Analysis of a Four-Qubit Photon Cluster State},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2005},
      volume = {95},
      pages = {210502},
      url = {http://link.aps.org/doi/10.1103/PhysRevLett.95.210502},
      doi = {http://dx.doi.org/10.1103/PhysRevLett.95.210502}
    }
    
    Kiesel, N., Schmid, C., Weber, U., Ursin, R. & Weinfurter, H. Linear Optics Controlled-Phase Gate Made Simple 2005 Phys. Rev. Lett.
    Vol. 95, pp. 210505 
    article DOI URL 
    Abstract: Linear optics quantum logic gates are the best tool to generate multiphoton entanglement. Simplifying a recent approach [ Phys. Rev. A 65 062324 (2002); 66 024308 (2002); Nature (London) 426 264 (2003); Phys. Rev. Lett. 93 080502 (2004)], we were able to implement the conditional phase gate with only one second-order interference at a polarization dependent beam splitter, thereby significantly increasing its stability. The improved quality of the gate is evaluated by analyzing its entangling capability and by performing full process tomography. The achieved results ensure that this device is well suited for implementation in various multiphoton quantum information protocols.
    BibTeX:
    @article{Kie05,
      author = {Kiesel, Nikolai and Schmid, Christian and Weber, Ulrich and Ursin, Rupert and Weinfurter, Harald},
      title = {Linear Optics Controlled-Phase Gate Made Simple},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2005},
      volume = {95},
      pages = {210505},
      url = {http://link.aps.org/doi/10.1103/PhysRevLett.95.210505},
      doi = {http://dx.doi.org/10.1103/PhysRevLett.95.210505}
    }
    
    Mueller, A., Pott, J.-U., Morel, S., Abuter, R., van Belle, G., van Boekel, R., Burtscher, L., Delplancke, F., Henning, T., Jaffe, W., Leinert, C., Lopez, B., Matter, A., Meisenheimer, K., Schmid, C., Tristram, K. & Verhoeff, A.P. First results using PRIMA FSU as a fringe tracker for MIDI 2010 Proc. SPIE
    Vol. 7734, pp. 773420-773420-15 
    other DOI URL 
    Abstract: We report first results obtained from observations using a PRIMA FSU (Fringe Sensor Unit) as a fringe tracker for MIDI on the VLTI when operating with the 1.8-m ATs. Interferometric observations require the correction of the disturbance in the optical path induced by atmospheric turbulence ("piston"). The PRIMA FSU is able to compensate for such disturbances in real-time which makes it a suitable facility to stabilize the fringe signal for other VLTI instruments, like AMBER, MIDI or later MATISSE. Currently, the atmospheric coherence time in the N band (8 to 13 mum) observed by MIDI, as well as the thermal background in this band, require a minimum target flux of 20 Jy and a correlated flux of 10 Jy (in PRISM/HIGH SENSE mode and using the ATs under standard conditions) to allow self-fringe-tracking and data reduction. However, we show that if the fringes are stabilized by the FSU, coherent integration allows a reliable data reduction even for the observation of faint targets (Fcorr <10 Jy) with MIDI at standard detector exposure times. We were able to measure the correlated flux of a 0.5 Jy source, which pushes the current limits of MIDI down to regions where numerous new targets become accessible on ATs. For faint object observations we will discuss the usage of VISIR photometry for calibration purposes. The observational tests done so far and the obtained results represent a first step towards Phase Referenced Imaging with the VLTI in the mid-infrared.
    BibTeX:
    @other{doi:10.1117/12.857754,
      author = {Mueller, A. and Pott, J.-U. and Morel, S. and Abuter, R. and van Belle, G. and van Boekel, R. and Burtscher, L. and Delplancke, F. and Henning, Th. and Jaffe, W. and Leinert, Ch. and Lopez, B. and Matter, A. and Meisenheimer, K. and Schmid, C. and Tristram, K. and Verhoeff, A. P.},
      title = {First results using PRIMA FSU as a fringe tracker for MIDI},
      journal = {Proc. SPIE},
      year = {2010},
      volume = {7734},
      pages = {773420-773420-15},
      url = { http://dx.doi.org/10.1117/12.857754},
      doi = {http://dx.doi.org/10.1117/12.857754}
    }
    
    Pachos, J.K., Wieczorek, W., Schmid, C., Kiesel, N., Pohlner, R. & Weinfurter, H. Revealing anyonic features in a toric code quantum simulation 2009 New Journal of Physics
    Vol. 11(8), pp. 083010 
    article URL 
    Abstract: Anyons are quasiparticles in two-dimensional systems that show statistical properties very distinct from those of bosons or fermions. While their isolated observation has not yet been achieved, here we perform a quantum simulation of anyons on the toric code model. By encoding the model in the multi-partite entangled state of polarized photons, we are able to demonstrate various manipulations of anyonic states and, in particular, their characteristic fractional statistics.
    BibTeX:
    @article{Pac09,
      author = {J K Pachos and W Wieczorek and C Schmid and N Kiesel and R Pohlner and H Weinfurter},
      title = {Revealing anyonic features in a toric code quantum simulation},
      journal = {New Journal of Physics},
      year = {2009},
      volume = {11},
      number = {8},
      pages = {083010},
      url = {http://stacks.iop.org/1367-2630/11/i=8/a=083010}
    }
    
    Sahlmann, J., Abuter, R., Menardi, S., Schmid, C., Di Lieto, N., Delplancke, F., Frahm, R., Gomes, N., Haguenauer, P., Leveque, S., Morel, S., Mueller, A., Phan Duc, T., Schuhler, N. & van Belle, G. First results from fringe tracking with the PRIMA fringe sensor unit 2010 Proc. SPIE
    Vol. 7734, pp. 773422-773422-12 
    other DOI URL 
    Abstract: The fringe sensor unit (FSU) is the central element of the phase referenced imaging and micro-arcsecond astrometry (PRIMA) dual-feed facility for the Very Large Telescope interferometer (VLTI). It has been installed at the Paranal observatory in August 2008 and is undergoing commissioning and preparation for science operation. Commissioning observations began shortly after installation and first results include the demonstration of spatially encoded fringe sensing and the increase in VLTI limiting magnitude for fringe tracking. However, difficulties have been encountered because the FSU does not incorporate real-time photometric correction and its fringe encoding depends on polarisation. These factors affect the control signals, especially their linearity, and can disturb the tracking control loop. To account for this, additional calibration and characterisation efforts are required. We outline the instrument concept and give an overview of the commissioning results obtained so far. We describe the effects of photometric variations and beam-train polarisation on the instrument operation and propose possible solutions. Finally, we update on the current status in view of the start of astrometric science operation with PRIMA.
    BibTeX:
    @other{doi:10.1117/12.856896,
      author = {Sahlmann, J. and Abuter, R. and Menardi, S. and Schmid, C. and Di Lieto, N. and Delplancke, F. and Frahm, R. and Gomes, N. and Haguenauer, P. and Leveque, S. and Morel, S. and Mueller, A. and Phan Duc, T. and Schuhler, N. and van Belle, G.},
      title = {First results from fringe tracking with the PRIMA fringe sensor unit},
      journal = {Proc. SPIE},
      year = {2010},
      volume = {7734},
      pages = {773422-773422-12},
      url = { http://dx.doi.org/10.1117/12.856896},
      doi = {http://dx.doi.org/10.1117/12.856896}
    }
    
    Sahlmann, J., Segransan, D., Merand, A., Zimmerman, N., Abuter, R., Chazelas, B., Delplancke, F., Henning, T., Kaminski, A., Koehler, R., Launhardt, R., Mohler, M., Pepe, F., Queloz, D., Quirrenbach, A., Reffert, S., Schmid, C., Schuhler, N. & Schulze-Hartung, T. Narrow-angle astrometry with PRIMA
    2012 Proc. SPIE
    Vol. 8445, pp. 84450S-84450S-11 
    other DOI URL 
    Abstract: The Extrasolar Planet Search with PRIMA project (ESPRI) aims at characterising and detecting extrasolar planets by measuring the host star's reflex motion using the narrow-angle astrometry capability of the PRIMA facility at the Very Large Telescope Interferometer. A first functional demonstration of the astrometric mode was achieved in early 2011. This marked the start of the astrometric commissioning phase with the purpose of characterising the instrument's performance, which ultimately has to be sufficient for exoplanet detection. We show results obtained from the observation of bright visual binary stars, which serve as test objects to determine the instrument's astrometric precision, its accuracy, and the plate scale. Finally, we report on the current status of the ESPRI project, in view of starting its scientific programme.
    BibTeX:
    @other{doi:10.1117/12.926214,
      author = {Sahlmann, J. and Segransan, D. and Merand, A. and Zimmerman, N. and Abuter, R. and Chazelas, B. and Delplancke, F. and Henning, T. and Kaminski, A. and Koehler, R. and Launhardt, R. and Mohler, M. and Pepe, F. and Queloz, D. and Quirrenbach, A. and Reffert, S. and Schmid, C. and Schuhler, N. and Schulze-Hartung, T.},
      title = {Narrow-angle astrometry with PRIMA
    }, journal = {Proc. SPIE}, year = {2012}, volume = {8445}, pages = {84450S-84450S-11}, url = { http://dx.doi.org/10.1117/12.926214}, doi = {http://dx.doi.org/10.1117/12.926214} }
    Sahlmann, J., Henning, T., Queloz, D., Quirrenbach, A., Elias, N.M., Launhardt, R., Pepe, F., Reffert, S., Ségransan, D., Setiawan, J., Abuter, R., Andolfato, L., Bizenberger, P., Baumeister, H., Chazelas, B., Delplancke, F., Dérie, F., Di Lieto, N., Duc, T.P., Fleury, M., Graser, U., Kaminski, A., Köhler, R., Lévêque, S., Maire, C., Mégevand, D., Mérand, A., Michellod, Y., Moresmau, J.-M., Mohler, M., Müller, A., Müllhaupt, P., Naranjo, V., Sache, L., Salvade, Y., Schmid, C., Schuhler, N., Schulze-Hartung, T., Sosnowska, D., Tubbs, B., van Belle, G.T., Wagner, K., Weber, L., Zago, L. & Zimmerman, N. The ESPRI project: astrometric exoplanet search with PRIMA 2013 A&A
    Vol. 551, pp. A52 
    article DOI URL 
    Abstract: Context. The ESPRI project relies on the astrometric capabilities offered by the PRIMA facility of the Very Large Telescope Interferometer for discovering and studying planetary systems. Our survey consists of obtaining high-precision astrometry for a large sample of stars over several years to detect their barycentric motions due to orbiting planets. We present the operationís principle, the instrumentís implementation, and the results of a first series of test observations.

    Aims. We give a comprehensive overview of the instrument infrastructure and present the observation strategy for dual-field relative astrometry in the infrared K-band. We describe the differential delay lines, a key component of the PRIMA facility that was delivered by the ESPRI consortium, and discuss their performance within the facility. This paper serves as reference for future ESPRI publications and for the users of the PRIMA facility.

    Methods. Observations of bright visual binaries were used to test the observation procedures and to establish the instrumentís astrometric precision and accuracy. The data reduction strategy for the astrometry and the necessary corrections to the raw data are presented. Adaptive optics observations with NACO were used as an independent verification of PRIMA astrometric observations.

    Results. The PRIMA facility was used to carry out tests of astrometric observations. The astrometric performance in terms of precision is limited by the atmospheric turbulence at a level close to the theoretical expectations and a precision of 30 muas was achieved. In contrast, the astrometric accuracy is insufficient for the goals of the ESPRI project and is currently limited by systematic errors that originate in the part of the interferometer beamtrain that is not monitored by the internal metrology system.

    Conclusions. Our observations led to defining corrective actions required to make the facility ready for carrying out the ESPRI search for extrasolar planets.

    BibTeX:
    @article{Sa13,
      author = {Sahlmann, J. and Henning, T. and Queloz, D. and Quirrenbach, A. and Elias, N. M. and Launhardt, R. and Pepe, F. and Reffert, S. and Ségransan, D. and Setiawan, J. and Abuter, R. and Andolfato, L. and Bizenberger, P. and Baumeister, H. and Chazelas, B. and Delplancke, F. and Dérie, F. and Di Lieto, N. and Duc, T. P. and Fleury, M. and Graser, U. and Kaminski, A. and Köhler, R. and Lévêque, S. and Maire, C. and Mégevand, D. and Mérand, A. and Michellod, Y. and Moresmau, J.-M. and Mohler, M. and Müller, A. and Müllhaupt, P. and Naranjo, V. and Sache, L. and Salvade, Y. and Schmid, C. and Schuhler, N. and Schulze-Hartung, T. and Sosnowska, D. and Tubbs, B. and van Belle, G. T. and Wagner, K. and Weber, L. and Zago, L. and Zimmerman, N.},
      title = {The ESPRI project: astrometric exoplanet search with PRIMA},
      journal = {A&A},
      year = {2013},
      volume = {551},
      pages = {A52},
      url = {http://dx.doi.org/10.1051/0004-6361/201220569},
      doi = {http://dx.doi.org/10.1051/0004-6361/201220569}
    }
    
    Schmid, C., Abuter, R., Menardi, S., Andolfato, L., Delplancke, F., Derie, F., Di Lieto, N., Frahm, R., Gitton, P., Gomes, N., Haguenauer, P., Leveque, S., Morel, S., Mueller, A., Phan Duc, T., Pozna, E., Sahlmann, J., Schuhler, N. & van Belle, G. Status of PRIMA for the VLTI or the quest for user-friendly fringe tracking 2010 Proc. SPIE
    Vol. 7734, pp. 77340F-77340F-13 
    other DOI URL 
    Abstract: The Phase Referenced Imaging and Micro Arcsecond Astrometry (PRIMA) facility for the Very Large Telescope Interferometer (VLTI), is being installed and tested in the observatory of Paranal. Most of the tests have been concentrated on the characterization of the Fringe Sensor Unit (FSU) and on the automation of the fringe tracking in preparation of dual-field observations. The status of the facility, an analysis of the FSU performance and the first attempts towards dual-field observations will be presented in this paper. In the FSU, the phase information is spatially encoded into four independent combined beams (ABCD) and the group delay comes from their spectral dispersion over 5 spectral channels covering the K-band. During fringe tracking the state machine of the optical path difference controller is driven by the Signal to Noise Ratio (SNR) derived from the 4 ABCD measurements. We will describe the strategy used to define SNR thresholds depending on the star magnitude for automatically detecting and locking the fringes. Further, the SNR as well as the phase delay measurements are affected by differential effects occurring between the four beams. We will shortly discuss the contributions of these effects on the measured phase and SNR noises. We will also assess the sensitivity of the group delay linearity to various instrumental parameters and discuss the corresponding calibration procedures. Finally we will describe how these calibrations and detection thresholds are being automated to make PRIMA as much as possible a user-friendly and efficient facility.
    BibTeX:
    @other{doi:10.1117/12.857419,
      author = {Schmid, C. and Abuter, R. and Menardi, S. and Andolfato, L. and Delplancke, F. and Derie, F. and Di Lieto, N. and Frahm, R. and Gitton, Ph. and Gomes, N. and Haguenauer, P. and Leveque, S. and Morel, S. and Mueller, A. and Phan Duc, T. and Pozna, E. and Sahlmann, J. and Schuhler, N. and van Belle, G.},
      title = {Status of PRIMA for the VLTI or the quest for user-friendly fringe tracking},
      journal = {Proc. SPIE},
      year = {2010},
      volume = {7734},
      pages = {77340F-77340F-13},
      url = { http://dx.doi.org/10.1117/12.857419},
      doi = {http://dx.doi.org/10.1117/12.857419}
    }
    
    Schmid, C., Abuter, R., Merand, A., Sahlmann, J., Alonso, J., Andolfato, L., van Belle, G., Delplancke, F., Derie, F., Di Lieto, N., Frahm, R., Gitton, P., Gomes, N., Haguenauer, P., Justen, B., Leveque, S., Menardi, S., Morel, S., Mueller, A., Phan Duc, T., Pozna, E., Ramirez, A., Schuhler, N. & Segransan, D. Status of PRIMA for the VLTI: heading to astrometry
    2012 Proc. SPIE
    Vol. 8445, pp. 84450F-84450F-12 
    other DOI URL 
    Abstract: The Phase Referenced Imaging and Micro Arcsecond Astrometry (PRIMA) facility for the Very Large Telescope Interferometer (VLTI), is being installed and tested in the observatory of Paranal. Since January 2011 the integration and individual testing of the different subsystem has come to a necessary minimum. At the same time the astrometric commissioning phase has begun. In this contribution we give an update on the status of the facility and present some highlights and difficulties on our way from first dual-feed fringe detection to first astrometric measurements. We focus on technical and operational aspects. In particular, within the context of the latter we are going to present a modified mode of operation that scans across the fringes. We will show that this mode, originally only intended for calibration purposes, facilitates the detection of dual-fringes.
    BibTeX:
    @other{doi:10.1117/12.926426,
      author = {Schmid, C. and Abuter, R. and Merand, A. and Sahlmann, J. and Alonso, J. and Andolfato, L. and van Belle, G. and Delplancke, F. and Derie, F. and Di Lieto, N. and Frahm, R. and Gitton, Ph. and Gomes, N. and Haguenauer, P. and Justen, B. and Leveque, S. and Menardi, S. and Morel, S. and Mueller, A. and Phan Duc, T. and Pozna, E. and Ramirez, A. and Schuhler, N. and Segransan, D.},
      title = {Status of PRIMA for the VLTI: heading to astrometry
    }, journal = {Proc. SPIE}, year = {2012}, volume = {8445}, pages = {84450F-84450F-12}, url = { http://dx.doi.org/10.1117/12.926426}, doi = {http://dx.doi.org/10.1117/12.926426} }
    Schmid, C., Flitney, A.P., Wieczorek, W., Kiesel, N., Weinfurter, H. & Hollenberg, L.C.L. Experimental implementation of a four-player quantum game 2010 New Journal of Physics
    Vol. 12(6), pp. 063031 
    article URL 
    Abstract: Game theory is central to the understanding of competitive interactions arising in many fields, from the social and physical sciences to economics. Recently, as the definition of information is generalized to include entangled quantum systems, quantum game theory has emerged as a framework for understanding the competitive flow of quantum information. Up till now, only two- and three-player quantum games have been demonstrated with restricted strategy sets. Here, we report the first experiment that implements a four-player quantum minority game over tunable four-partite entangled states encoded in the polarization of single photons. Experimental application of appropriate player strategies gives equilibrium payoff values well above those achievable in the classical game. These results are in excellent quantitative agreement with our theoretical analysis of the symmetric Pareto optimal strategies. Our results demonstrate for the first time how nontrivial equilibria can arise in a competitive situation involving quantum agents.
    BibTeX:
    @article{Sch10,
      author = {C Schmid and A P Flitney and W Wieczorek and N Kiesel and H Weinfurter and L C L Hollenberg},
      title = {Experimental implementation of a four-player quantum game},
      journal = {New Journal of Physics},
      year = {2010},
      volume = {12},
      number = {6},
      pages = {063031},
      url = {http://stacks.iop.org/1367-2630/12/i=6/a=063031}
    }
    
    Schmid, C., Kiesel, N., Laskowski, W., Solano, E., Toth, G., Zukowski, M. & Weinfurter, H. NATO Science for Peace and Security 2007
    Vol. 11: Quantum Communication and Security, pp. 113-122 
    inbook URL 
    Abstract: We present an experimental and theoretical characterization of the symmetric four-qubit entangled Dicke state with two excitations D(2)4. We investigate the state's violation of local realism and study its characteristic properties with respect to quantum information applications. For the experimental observation of the state we used photons obtained from parametric down conversion. This allowed, in a simple experimental set-up, quantum state tomography yielding a fidelity as high as 0.844 Ī 0.008.
    BibTeX:
    @inbook{Sch07c,
      author = {Christian Schmid and Nikolai Kiesel and Wieslaw Laskowski and Enrique Solano and Geza Toth and Marek Zukowski and Harald Weinfurter},
      title = {NATO Science for Peace and Security},
      publisher = {IOS Press},
      year = {2007},
      volume = {11: Quantum Communication and Security},
      pages = {113-122},
      url = {http://ebooks.iospress.nl/publication/23528}
    }
    
    Schmid, C., Kiesel, N., Laskowski, W., Wieczorek, W., ifmmode Zelse Zukowski, M. & Weinfurter, H. Discriminating Multipartite Entangled States 2008 Phys. Rev. Lett.
    Vol. 100, pp. 200407 
    article DOI URL 
    Abstract: The variety of multipartite entangled states enables numerous applications in novel quantum information tasks. In order to compare the suitability of different states from a theoretical point of view, classifications have been introduced. Accordingly, here we derive criteria and demonstrate how to experimentally discriminate an observed state against the ones of certain other classes of multipartite entangled states. Our method, originating in Bell inequalities, adds an important tool for the characterization of multiparty entanglement.
    BibTeX:
    @article{Sch08,
      author = {Schmid, Christian and Kiesel, Nikolai and Laskowski, Wiesław and Wieczorek, Witlef and ifmmode Zelse Zukowski, Marek and Weinfurter, Harald},
      title = {Discriminating Multipartite Entangled States},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2008},
      volume = {100},
      pages = {200407},
      url = {http://link.aps.org/doi/10.1103/PhysRevLett.100.200407},
      doi = {http://dx.doi.org/10.1103/PhysRevLett.100.200407}
    }
    
    Schmid, C., Kiesel, N., Weber, U., Ursin, R. & Weinfurter, H. Experimental Analysis of a Simple Linear Optics Phase Gate 2007 International Journal of Quantum Information
    Vol. 05(01n02), pp. 235-240 
    article DOI URL 
    Abstract: Linear optics two-qubit logic gates are essential tools in photonic quantum information. We describe a recently introduced simple conditional phase gate for photons, which relies on only one second order interference at a polarization dependent beam splitter, thereby making additional stability precautions dispensable. The improved quality of the gate is evaluated by performing full process tomography. The obtained process tomography data is fitted by a model based on experimental parameters of the setup which allows predictions on the performance of the gate in multi-photon experiments.
    BibTeX:
    @article{Sch07a,
      author = {Schmid, C. and Kiesel, N. and Weber, U. and Ursin, R. and Weinfurter, H.},
      title = {Experimental Analysis of a Simple Linear Optics Phase Gate},
      journal = {International Journal of Quantum Information},
      year = {2007},
      volume = {05},
      number = {01n02},
      pages = {235-240},
      url = {http://www.worldscientific.com/doi/abs/10.1142/S0219749907002682},
      doi = {http://dx.doi.org/10.1142/S0219749907002682}
    }
    
    Schmid, C., Kiesel, N., Weber, U.K., Ursin, R., Zeilinger, A. & Weinfurter, H. Quantum teleportation and entanglement swapping with linear optics logic gates 2009 New Journal of Physics
    Vol. 11(3), pp. 033008 
    article URL 
    Abstract: We report on the usage of a linear optics phase gate for distinguishing all four Bell states simultaneously in a quantum teleportation and entanglement swapping protocol. This is demonstrated by full-state tomography of the one- and two-qubit output states of the two protocols, yielding average state fidelities of about 0.83 and 0.77, respectively. In addition, the performance of the teleportation channel is characterized by the quantum process tomography. The non-classical properties of the entanglement swapping output states are further confirmed by the violation of a CHSH-type Bell inequality of 2.14 on average.
    BibTeX:
    @article{Sch09,
      author = {Christian Schmid and Nikolai Kiesel and Ulrich K Weber and Rupert Ursin and Anton Zeilinger and Harald Weinfurter},
      title = {Quantum teleportation and entanglement swapping with linear optics logic gates},
      journal = {New Journal of Physics},
      year = {2009},
      volume = {11},
      number = {3},
      pages = {033008},
      url = {http://stacks.iop.org/1367-2630/11/i=3/a=033008}
    }
    
    Schmid, C., Kiesel, N., Wieczorek, W., Pohlner, R. & Weinfurter, H. Multiphoton entanglement engineering via projective measurements 2007 Proc. SPIE
    Vol. 6780, pp. 67800E-67800E-10 
    other DOI URL 
    Abstract: We present strategies to obtain different classes of three and four photon entangled symmetric states from a single experimental setup. The basic idea originates from the property of the symmetric Dicke state with two excitations to connect the two inequivalent types of genuine tripartite entanglement. We experimentally confirm the distinct types of entanglement of the observed states. We further propose an extension of the applied scheme that allows one to obtain different classes of four-photon entanglement by adding a fifth photon. The requirement of a single fifth photon is currently a technical challenge, and thus we consider the approach of using a strongly attenuated weak coherent beam instead.
    BibTeX:
    @other{Sch07d,
      author = {Schmid, Christian and Kiesel, Nikolai and Wieczorek, Witlef and Pohlner, Reinhold and Weinfurter, Harald},
      title = {Multiphoton entanglement engineering via projective measurements},
      journal = {Proc. SPIE},
      year = {2007},
      volume = {6780},
      pages = {67800E-67800E-10},
      url = { http://dx.doi.org/10.1117/12.734697},
      doi = {http://dx.doi.org/10.1117/12.734697}
    }
    
    Schmid, C., Kiesel, N., Wieczorek, W. & Weinfurter, H. The entanglement of the four-photon cluster state 2007 New Journal of Physics
    Vol. 9(7), pp. 236 
    article URL 
    Abstract: Measurement based quantum computation draws its power from the remarkable entanglement properties of the cluster states. This contribution analyses the entanglement and its persistency of an experimentally realized four-photon cluster state. We describe how spontaneous parametric down conversion together with linear optics quantum logic can be used for the experimental study.
    BibTeX:
    @article{Sch07b,
      author = {C Schmid and N Kiesel and W Wieczorek and H Weinfurter},
      title = {The entanglement of the four-photon cluster state},
      journal = {New Journal of Physics},
      year = {2007},
      volume = {9},
      number = {7},
      pages = {236},
      url = {http://stacks.iop.org/1367-2630/9/i=7/a=236}
    }
    
    Schmid, C., Kiesel, N., Wieczorek, W., Weinfurter, H., Mintert, F. & Buchleitner, A. Experimental Direct Observation of Mixed State Entanglement 2008 Phys. Rev. Lett.
    Vol. 101, pp. 260505 
    article DOI URL 
    Abstract: We report on the direct estimation of concurrence for mixed quantum states. The used method relies on joint measurements on two copies of an entangled state. In the experimental demonstration two polarization-entangled photon pairs emitted from spontaneous parametric down-conversion are analyzed together using a linear optics controlled phase gate. We demonstrate that the measured data, without need for further numerical processing, directly yield reliable estimates, despite experimental imperfections.
    BibTeX:
    @article{Sch08a,
      author = {Schmid, Christian and Kiesel, Nikolai and Wieczorek, Witlef and Weinfurter, Harald and Mintert, Florian and Buchleitner, Andreas},
      title = {Experimental Direct Observation of Mixed State Entanglement},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2008},
      volume = {101},
      pages = {260505},
      url = {http://link.aps.org/doi/10.1103/PhysRevLett.101.260505},
      doi = {http://dx.doi.org/10.1103/PhysRevLett.101.260505}
    }
    
    Schmid, C., Trojek, P., Bourennane, M., Kurtsiefer, C., ifmmode Zelse Zukowski, M. & Weinfurter, H. Schmid et~al. Reply: 2007 Phys. Rev. Lett.
    Vol. 98, pp. 028902 
    article DOI URL 
    BibTeX:
    @article{Sch07,
      author = {Schmid, Christian and Trojek, Pavel and Bourennane, Mohamed and Kurtsiefer, Christian and ifmmode Zelse Zukowski, Marek and Weinfurter, Harald},
      title = {Schmid et~al. Reply:},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2007},
      volume = {98},
      pages = {028902},
      url = {http://link.aps.org/doi/10.1103/PhysRevLett.98.028902},
      doi = {http://dx.doi.org/10.1103/PhysRevLett.98.028902}
    }
    
    Schmid, C., Trojek, P., Bourennane, M., Kurtsiefer, C., ifmmode Zelse Zukowski, M. & Weinfurter, H. Experimental Single Qubit Quantum Secret Sharing 2005 Phys. Rev. Lett.
    Vol. 95, pp. 230505 
    article DOI URL 
    Abstract: We present a simple and practical protocol for the solution of a secure multiparty communication task, the secret sharing, and its proof-of-principle experimental realization. In this protocol, a secret is split among several parties in a way that its reconstruction requires the collaboration of the participating parties. In our scheme the parties solve the problem by sequential transformations on a single qubit. In contrast with recently proposed schemes involving multiparticle Greenberger-Horne-Zeilinger states, the approach demonstrated here is much easier to realize and scalable in practical applications.
    BibTeX:
    @article{Sch05,
      author = {Schmid, Christian and Trojek, Pavel and Bourennane, Mohamed and Kurtsiefer, Christian and ifmmode Zelse Zukowski, Marek and Weinfurter, Harald},
      title = {Experimental Single Qubit Quantum Secret Sharing},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2005},
      volume = {95},
      pages = {230505},
      url = {http://link.aps.org/doi/10.1103/PhysRevLett.95.230505},
      doi = {http://dx.doi.org/10.1103/PhysRevLett.95.230505}
    }
    
    Schmid, C., Trojek, P., Gaertner, S., Bourennane, M., Kurtsiefer, C., Zukowski, M. & Weinfurter, H. Elements of Quantum Information 2007 , pp. 303  inbook URL 
    BibTeX:
    @inbook{Sch07e,
      author = {Schmid, C. and Trojek, P. and Gaertner, S. and Bourennane, M. and Kurtsiefer, C. and Zukowski, M. and Weinfurter, H.},
      title = {Elements of Quantum Information},
      publisher = {Wiley-Vch},
      year = {2007},
      pages = {303},
      edition = {1},
      url = {http://www.wiley-vch.de/publish/dt/books/bySubjectPH00/ISBN3-527-40725-1/?sID=i1th2cocen0rvm29g20afv4q53}
    }
    
    Schmid, C., Trojek, P., Gaertner, S., Bourennane, M., Kurtsiefer, C., Zukowski, M. & Weinfurter, H. Experimental quantum secret sharing 2006 Fortschritte der Physik
    Vol. 54(8-10), pp. 831-839 
    article DOI URL 
    Abstract: We consider two simple and practical protocols for multiparty communication and show their experimental realization. These protocols deal with the task of secret sharing in which a secret message is split among several parties in a way that its reconstruction requires their mutual collaboration. In the presented schemes the parties solve the problem by two different approaches: The first uses as a resource the multiqubit entangled state |psi4->. As no interferometric setups are required here, contrary to known schemes, involving Greenberger-Horne-Zeilinger states, its implementation is simpler and more stable. In the second scheme only sequential transformations on a single qubit are used. This further tremendously simplifies the method, makes it scalable with regard to the number of participating partners and above all, technologically comparable to quantum key distribution.
    BibTeX:
    @article{Sch06,
      author = {Schmid, Ch. and Trojek, P. and Gaertner, S. and Bourennane, M. and Kurtsiefer, Ch. and Zukowski, M. and Weinfurter, H.},
      title = {Experimental quantum secret sharing},
      journal = {Fortschritte der Physik},
      publisher = {WILEY-VCH Verlag},
      year = {2006},
      volume = {54},
      number = {8-10},
      pages = {831--839},
      url = {http://dx.doi.org/10.1002/prop.200610320},
      doi = {http://dx.doi.org/10.1002/prop.200610320}
    }
    
    Spaleniak, I., Giessler, F., Geiss, R., Minardi, S., Pertsch, T., Neuhaeuser, R., Becker, M., Rothhardt, M., Delplancke, F., Richichi, A., Menardi, S. & Schmid, C. MAMMUT: mirror vibration metrology for VLTI 2010 Proc. SPIE
    Vol. 7734, pp. 77343Y-77343Y-7 
    other DOI URL 
    Abstract: MAMMUT (Mirror vibrAtion Metrolology systeM for the Unit Telescope) is an ESO funded feasibility project for the development of a fiber interferometer prototype designed for optical path laser-metrology along the optical train of the Unit Telescopes (UT) of the Very Large Telescope Interferometer (VLTI). Fast mechanical vibrations originating in the VLTI cause fast variations of the optical path difference between two arms of the stellar interferometer, thus reducing the contrast of measured interference fringes. MAMMUT aims at monitoring in real time the optical path variations inside the Coud√ɬ© train of the UT, for active control purposes. MAMMUT features a 250-meter-long optical fiber which can be used to deliver and inject a laser beam at 1353 nm into the UT. The injected beam can be dropped from the telescope in the Coud√ɬ© room and interfered with a phase reference, provided by the second 250-meter-long arm of the fiber interferometer. The optical path variations are measured by means of an active homodyne scheme. Coherence between the beam at the injection point and the phase reference is provided by active fiber stabilization, made possible by the implementation of an internal metrology channel in MAMMUT. Here we present the initial laboratory performance results of the MAMMUT prototype, which will be able to sense optical path variations of +/- 5 őľm with sub-10 nm precision within a bandwidth of at least 100 Hz.
    BibTeX:
    @other{doi:10.1117/12.856898,
      author = {Spaleniak, Izabela and Giessler, Frank and Geiss, Reinhard and Minardi, Stefano and Pertsch, Thomas and Neuhaeuser, Ralph and Becker, Martin and Rothhardt, Manfred and Delplancke, Francoise and Richichi, Andrea and Menardi, Serge and Schmid, Christian},
      title = {MAMMUT: mirror vibration metrology for VLTI},
      journal = {Proc. SPIE},
      year = {2010},
      volume = {7734},
      pages = {77343Y-77343Y-7},
      url = { http://dx.doi.org/10.1117/12.856898},
      doi = {http://dx.doi.org/10.1117/12.856898}
    }
    
    Trojek, P., Schmid, C., Bourennane, M., Brukner, eckC. C., ifmmode Zelse Zukowski, M. & Weinfurter, H. Experimental quantum communication complexity 2005 Phys. Rev. A
    Vol. 72, pp. 050305 
    article DOI URL 
    Abstract: We prove that by communicating N-1 times a single qubit, instead of N-1 classical bits, the success probability for solving some N partner communication complexity tasks is strongly enhanced. The superiority, as measured by the quantum-to-classical fidelity ratio, of the quantum scheme grows exponentially with N. We report an experimental implementation of these tasks for N=5. Even without correcting for any inefficiencies of the state-of-the-art setup, our multiparty quantum protocol still outperforms the best classical protocols.
    BibTeX:
    @article{Tro05,
      author = {Trojek, Pavel and Schmid, Christian and Bourennane, Mohamed and Brukner, ifmmode eckCelse Časlav and ifmmode Zelse Zukowski, Marek and Weinfurter, Harald},
      title = {Experimental quantum communication complexity},
      journal = {Phys. Rev. A},
      publisher = {American Physical Society},
      year = {2005},
      volume = {72},
      pages = {050305},
      url = {http://link.aps.org/doi/10.1103/PhysRevA.72.050305},
      doi = {http://dx.doi.org/10.1103/PhysRevA.72.050305}
    }
    
    Trojek, P., Schmid, C., Bourennane, M., Weinfurter, H. & Kurtsiefer, C. Compact source of polarization-entangled photon pairs 2004 Opt. Express
    Vol. 12(2), pp. 276-281 
    article DOI URL 
    Abstract: We present a compact source of polarization-entangled photon pairs at a wavelength of 805 nm using a violet single-mode laser diode as the pump source of type-II spontaneous parametric down-conversion. The source exhibits entanglement and pair-rate comparable to conventional systems utilizing large frame ion lasers thus significantly increases the practicality of novel quantum information or quantum metrology applications.
    BibTeX:
    @article{Tro04,
      author = {Pavel Trojek and Ch. Schmid and M. Bourennane and H. Weinfurter and Ch. Kurtsiefer},
      title = {Compact source of polarization-entangled photon pairs},
      journal = {Opt. Express},
      publisher = {OSA},
      year = {2004},
      volume = {12},
      number = {2},
      pages = {276--281},
      url = {http://www.opticsexpress.org/abstract.cfm?URI=oe-12-2-276},
      doi = {http://dx.doi.org/10.1364/OPEX.12.000276}
    }
    
    Wieczorek, W., Kiesel, N., Schmid, C., Laskowski, W., Zukowski, M. & Weinfurter, H. Multiphoton Interference as a Tool to Observe Families of Multiphoton Entangled States 2009 Selected Topics in Quantum Electronics, IEEE Journal of
    Vol. 15(6), pp. 1704-1712 
    article DOI  
    Abstract: Spontaneous parametric downconversion in combination with linear optics was successfully used to observe a variety of multiphoton entangled states. Yet, experiments performed so far lacked flexibility, as each of the various setups was useful for only a particular multiphoton entangled state. In this paper, we describe how, by using multiphoton interference, one can observe entire families of multiphoton entangled states in the very same linear optical setup. Our method thus goes beyond the commonly used two-photon interference and turns out to be a very useful tool for state observation. We will discuss the interference of four and six photons at different types of beam splitters and show which families of entangled states are observable. The benefits of this approach are demonstrated in a four-photon interference experiment by observing a variety of highly entangled multiphoton states.
    BibTeX:
    @article{Wie09a,
      author = {Wieczorek, W. and Kiesel, N. and Schmid, C. and Laskowski, W. and Zukowski, M. and Weinfurter, H.},
      title = {Multiphoton Interference as a Tool to Observe Families of Multiphoton Entangled States},
      journal = {Selected Topics in Quantum Electronics, IEEE Journal of},
      year = {2009},
      volume = {15},
      number = {6},
      pages = {1704-1712},
      doi = {http://dx.doi.org/10.1109/JSTQE.2009.2025697}
    }
    
    Wieczorek, W., Kiesel, N., Schmid, C. & Weinfurter, H. Quantum Communication and Quantum Measurement 2007 , pp. 487  inbook  
    Abstract: The efficient characterization of multipartite entangled states is possible with
    analysis tools not requiring a full tomographic state estimation. We describe several
    methods and apply them to two different, genuinely four-partite entangled states,
    that were recently observed in the experiment.
    BibTeX:
    @inbook{Wie07,
      author = {Wieczorek, W. and Kiesel, N. and Schmid, C. and Weinfurter, H.},
      title = {Quantum Communication and Quantum Measurement},
      publisher = {NICT Press},
      year = {2007},
      pages = {487}
    }
    
    Wieczorek, W., Kiesel, N., Schmid, C. & Weinfurter, H. Multiqubit entanglement engineering via projective measurements 2009 Phys. Rev. A
    Vol. 79, pp. 022311 
    article DOI URL 
    Abstract: So far, various multiphoton entangled states have been observed experimentally by using different experimental setups. We present a scheme to realize many stochastic local operations and classical communication inequivalent states of three and four qubits via projective measurements on suitable entangled states. We demonstrate how these states can be observed experimentally in a single setup and study the feasibility of implementation with present-day technology.
    BibTeX:
    @article{Wie09,
      author = {Wieczorek, Witlef and Kiesel, Nikolai and Schmid, Christian and Weinfurter, Harald},
      title = {Multiqubit entanglement engineering via projective measurements},
      journal = {Phys. Rev. A},
      publisher = {American Physical Society},
      year = {2009},
      volume = {79},
      pages = {022311},
      url = {http://link.aps.org/doi/10.1103/PhysRevA.79.022311},
      doi = {http://dx.doi.org/10.1103/PhysRevA.79.022311}
    }
    
    Wieczorek, W., Krischek, R., Kiesel, N., Schmid, C. & Weinfurter, H. Entanglement enhanced quantum sensing 2010 Proc. SPIE
    Vol. 7608, pp. 76080P-76080P-5 
    other DOI URL 
    Abstract: Entanglement between quantum objects can be used to enhance the sensitivity of measurements. We demonstrate this effect by using entangled multi-photon states to go beyond the shot noise limit when observing polarization rotations.
    BibTeX:
    @other{doi:10.1117/12.837531,
      author = {Wieczorek, W. and Krischek, R. and Kiesel, N. and Schmid, Ch. and Weinfurter, H.},
      title = {Entanglement enhanced quantum sensing},
      journal = {Proc. SPIE},
      year = {2010},
      volume = {7608},
      pages = {76080P-76080P-5},
      url = { http://dx.doi.org/10.1117/12.837531},
      doi = {http://dx.doi.org/10.1117/12.837531}
    }
    
    Wieczorek, W., Schmid, C., Kiesel, N., Pohlner, R., Gühne, O. & Weinfurter, H. Experimental Observation of an Entire Family of Four-Photon Entangled States 2008 Phys. Rev. Lett.
    Vol. 101, pp. 010503 
    article DOI URL 
    Abstract: A single linear-optical setup is used to observe an entire family of four-photon entangled states. This approach breaks with the inflexibility of present linear-optical setups usually designed for the observation of a particular multipartite entangled state only. The family includes several prominent entangled states that are known to be highly relevant for quantum information applications.
    BibTeX:
    @article{Wie08,
      author = {Wieczorek, Witlef and Schmid, Christian and Kiesel, Nikolai and Pohlner, Reinhold and Gühne, Otfried and Weinfurter, Harald},
      title = {Experimental Observation of an Entire Family of Four-Photon Entangled States},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2008},
      volume = {101},
      pages = {010503},
      url = {http://link.aps.org/doi/10.1103/PhysRevLett.101.010503},
      doi = {http://dx.doi.org/10.1103/PhysRevLett.101.010503}
    }
    
    Wieczorek, W.and Kiesel, N.S.C., Schwemmer, C., Toth, G., Laskowski, W. & Weinfurter, H. Experimental characterization of multipartite entanglement   unpublished  
    Abstract: Quantum information processing (QIP) is heavily based
    on the entanglement between multiple particles. Therefore, a variety of physical systems
    have been established to prepare, manipulate and utilize multi-particle entangled
    states. A significant part of the experimental implementation is the characterization of
    the prepared states. On the one hand, this characterization is required for evaluating
    the performance of the experimental set-up. This includes for example such important
    tasks as measuring the quality of the prepared states by using the fidelity or detecting
    entanglement by using witness operators. On the other hand, a more detailed characterization
    enables to judge the applicability of the prepared states for QIP and for studying
    fundamental, physical questions. This comprises, for example, the more involved
    phenomenological description of the stateís properties by evaluating its entanglement
    structure. The measurement of relevant data for calculating operator expectation values
    is required to characterize states. However, retrieval of complete state information
    becomes increasingly unpractical with a growing number of particles, which prevents
    the full characterization of the state via complete state tomography. Mostly, depending
    on the specific purpose, only certain properties of entangled states are of interest,
    as for example the quality of the preparation, the detection and quantification of the
    stateís entanglement or its entanglement structure. In this review, a concise methodology
    suitable for state characterization of multipartite entangled states is surveyed,
    especially suitable for the experimental setting. Methods are discussed that require only
    few measurement settings and are, thus, favorable for an experimental characterization
    along with methods based on full quantum state tomography. The presented tools are
    exemplarily applied to two- and four-qubit states to illustrate the main methods summarized
    in this review. This review is intended as a guide for state characterization in
    any physical system realizing entanglement between multiple, individually addressable
    qubits.
    BibTeX:
    @unpublished{Wie,
      author = {Wieczorek, W.and Kiesel, N.and Schmid, C. and Schwemmer, C. and Toth, G. and Laskowski, W. and Weinfurter, H.},
      title = {Experimental characterization of multipartite entanglement},
      note = {to be published}
    }
    

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