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  • 1.
    Bogdanski, Jan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ahrens, Johan
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Sagnac secret sharing over telecom fiber networks2009In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 17, no 2, p. 1055-1063Article in journal (Refereed)
    Abstract [en]

    We report the first Sagnac quantum secret sharing (in threeand four-party implementations) over 1550 nm single mode fiber (SMF) networks, using a single qubit protocol with phase encoding. Our secret sharing experiment has been based on a single qubit protocol, which has opened the door to practical secret sharing implementation over fiber telecom channels and in free-space. The previous quantum secret sharing proposals were based on multiparticle entangled states, difficult in the practical implementation and not scalable. Our experimental data in the three-party implementation show stable (in regards to birefringence drift) quantum secret sharing transmissions at the total Sagnac transmission loop distances of 55-75 km with the quantum bit error rates (QBER) of 2.3-2.4% for the mean photon number μ = 0.1 and 1.7-2.1% for μ = 0.3. In the four-party case we have achieved quantum secret sharing transmissions at the total Sagnac transmission loop distances of 45-55 km with the quantum bit error rates (QBER) of 3.0-3.7% for the mean photon number μ = 0.1 and 1.8-3.0% for μ = 0.3. The stability of quantum transmission has been achieved thanks to our new concept for compensation of SMF birefringence effects in Sagnac, based on a polarization control system and a polarization insensitive phase modulator. The measurement results have showed feasibility of quantum secret sharing over telecom fiber networks in Sagnac configuration, using standard fiber telecom components.

  • 2.
    Bogdanski, Jan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Ahrens, Johan
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Single mode fiber birefringence compensation in Sagnac and "plug & play" interferometric setups2009In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 17, no 6, p. 4485-4494Article in journal (Refereed)
    Abstract [en]

    Single mode fiber (SMF) birefringence effects have been a limiting factor for a variety of Sagnac applications over longer distance SMF links. In this report, we present a new concept of the SMF birefringence compensation in a Sagnac interferometric setup, based on a novel polarization control system. For the destructive interference, our control system guarantees a perfect compensation of both the SMF birefringence and imperfect propagation times matching of the setup’s components. For the stabilization of the constructive interference, we have applied a fiber stretcher and a simple proportional−integral−derivative (PID) controller. The enclosed experimental data of the setup’s visibility confirm validity of our polarization control system. We have also showed that the SMF birefringence model used in a “plug & play” interferometric setup [19], widely cited in the papers on quantum key distribution [11, 12, 13], cannot be applied in SMF Sagnac interferometric setup. However, the SMF birefringence model based on the Kapron equivalence well describes SMF Sagnac.

  • 3.
    Herbauts, Isabelle
    et al.
    Stockholm University, Faculty of Science, Department of Physics. University of Vienna, Austria.
    Blauensteiner, B.
    Poppe, A.
    Jennewein, T.
    Huebel, Hannes
    Stockholm University, Faculty of Science, Department of Physics. University of Vienna, Austria.
    Demonstration of active routing of entanglement in a multi-user network2013In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 21, no 23, p. 29013-29024Article in journal (Refereed)
    Abstract [en]

    We implement an entanglement distribution network based on wavelength-multiplexing and optical switching for quantum communication applications. Using a high-brightness source based on spontaneous parametric down-conversion in periodically-poled lithium niobate waveguides, we generate polarisation entangled photon pairs with a broad spectrum covering the telecom wavelengths around 1550 nm. The photon pairs have entanglement fidelities up to 99%, and are distributed via passive wavelength multiplexing in a static multi-user network. We furthermore demonstrate a possible network application in a scenario with a single centralised source dynamically allocating two-party entanglement to any pair of users by means of optical switches. The whole system, from the pump laser up to the receivers, is fibre and waveguide based, resulting in maximal stability, minimal losses and the advantage of readily integrable telecom components in the 1550 nm range.

  • 4.
    Pancaldi, Matteo
    et al.
    Stockholm University, Faculty of Science, Department of Physics. CIC nanoGUNE, Spain.
    Freeman, Ryan
    Hudl, Matthias
    Stockholm University, Faculty of Science, Department of Physics.
    Hoffmann, Matthias C.
    Urazhdin, Sergei
    Vavassori, Paolo
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics.
    Anti-reflection coating design for metallic terahertz meta-materials2018In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 26, no 3, p. 2917-2927Article in journal (Refereed)
    Abstract [en]

    We demonstrate a silicon-based, single-layer anti-reflection coating that suppresses the reflectivity of metals at near-infrared frequencies, enabling optical probing of nano-scale structures embedded in highly reflective surroundings. Our design does not affect the interaction of terahertz radiation with metallic structures that can be used to achieve terahertz near-field enhancement. We have verified the functionality of the design by calculating and measuring the reflectivity of both infrared and terahertz radiation from a silicon/gold double layer as a function of the silicon thickness. We have also fabricated the unit cell of a terahertz meta-material, a dipole antenna comprising two 20-nm thick extended gold plates separated by a 2 mu m gap, where the terahertz field is locally enhanced. We used the time-domain finite element method to demonstrate that such near-field enhancement is preserved in the presence of the anti-reflection coating. Finally, we performed magneto-optical Kerr effect measurements on a single 3-nm thick, 1-mu m wide magnetic wire placed in the gap of such a dipole antenna. The wire only occupies 2% of the area probed by the laser beam, but its magneto-optical response can be clearly detected. Our design paves the way for ultrafast time-resolved studies, using table-top femtosecond near-infrared lasers, of dynamics in nano-structures driven by strong terahertz radiation.

  • 5. Xiao, Ya
    et al.
    Kedem, Yaron
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Xu, Jin-Shi
    Li, Chuan-Feng
    Guo, Guang-Can
    Experimental nonlocal steering of Bohmian trajectories2017In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, no 13, p. 14463-14472Article in journal (Refereed)
    Abstract [en]

    Interpretations of quantum mechanics (QM), or proposals for underlying theories, that attempt to present a definite realist picture, such as Bohmian mechanics, require strong non-local effects. Naively, these effects would violate causality and contradict special relativity. However if the theory agrees with QM the violation cannot be observed directly. Here, we demonstrate experimentally such an effect: we steer the velocity and trajectory of a Bohmian particle using a remote measurement. We use a pair of photons and entangle the spatial transverse position of one with the polarization of the other. The first photon is sent to a double-slit-like apparatus, where its trajectory is measured using the technique of Weak Measurements. The other photon is projected to a linear polarization state. The choice of polarization state, and the result, steer the first photon in the most intuitive sense of the word. The effect is indeed shown to be dramatic, while being easy to visualize. We discuss its strength and what are the conditions for it to occur.

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