Quantum Communication Technology

Quantum communication is built on a set of disruptive concepts and technologies. It is driven by fascinating physics and by promising applications. It requires a new mix of competencies, from telecom engineering to theoretical physics, from theoretical computer science to mechanical and electronic engineering. First applications have already found their way to niche markets and university labs are working on futuristic quantum networks, but most of the surprises are still ahead of us. Quantum communication, and more generally quantum information science and technologies, are here to stay and will have a profound impact on the XXI century

Entanglement Manipulation and Concentration

We introduce a simple, experimentally realisable, entanglement manipulation protocol for exploring mixed state entanglement. We show that for both non-maximally entangled pure, and mixed polarisation-entangled two qubit states, an increase in the degree of entanglement and purity, which we define as concentration, is achievable.Comment: Accepted as Rapid Communication PR

Coherent frequency-down-conversion interface for quantum repeaters

We report a coherence-preserving photon frequency down-conversion experiment based on difference-frequency generation in a periodically poled Lithium niobate waveguide, at the single-photon level. The coherence of the process has been demonstrated by measuring the phase coherence of pseudo single-photon time-bin qubits after frequency conversion with an interference visibility of > 96 %. This interface could be of interest for quantum repeater based hybrid networks.Comment: 6 pages, 3 figure

Tunable Up-Conversion Photon Detector

We introduce a simple approach for a tunable up-conversion detector. This scheme is relevant for both single photon detection or anywhere where low light levels at telecom wavelengths need to be detected with a high degree of temporal resolution or where high count rates are desired. A system combining a periodically poled Lithium niobate waveguide for the nonlinear wavelength conversion and a low jitter Silicon avalanche photodiode are used in conjunction with a tunable pump source. We report more than a ten-fold increase in the detectable bandwidth using this tuning scheme.Comment: 3 pages, 3 figures, Accepted for publication in AP

Practical fast gate rate InGaAs/InP single-photon avalanche photodiodes

We present a practical and easy-to-implement method for high-speed near infrared single-photon detection based on InGaAs/InP single-photon avalanche photodiodes (SPADs), combining aspects of both sine gating and self-differencing techniques. At a gating frequency of 921 MHz and temperature of -30 $^{\circ}$C we achieve: a detection efficiency of 9.3 %, a dark count probability of 2.8$\times10^{-6}$ ns$^{-1}$, while the afterpulse probability is 1.6$\times10^{-4}$ ns$^{-1}$, with a 10 ns "count-off time" setting. In principle, the maximum count rate of the SPAD can approach 100 MHz, which can significantly improve the performance for diverse applications.Comment: 3 pages and a few lines, 5 figures, 1 table. Accepted by Applied Physics Letter

A Bell-type test of energy-time entangled qutrits

We have performed a Bell-type test for energy-time entangled qutrits. A method of inferring the Bell violation in terms of an associated interference visibility is derived. Using this scheme we obtained a Bell value of $2.784 \pm 0.023$, representing a violation of $34 \sigma$ above the limit for local variables. The scheme has been developed for use at telecom wavelengths and using proven long distance quantum communication architecture to optimize the utility of this high dimensional entanglement resource.Comment: replaced lost acknowledement

Youth participation and agency in the United Nations Framework Convention on Climate Change

This article examines the participation and agency of young non-state actors (NSAs) in the United Nations Framework Convention on Climate Change (UNFCCC). It utilizes the constituency of Youth NGOs: YOUNGO, as a case study to examine the relationship between selection of participatory strategies, power sources (following Nasiritousi et al. in Int Environ Agreem Politics Law Econ 16(1):109–126, 2016), recognition and agency using ego and alter perceptions. It finds that young people’s selection of participatory strategies and power sources is shaped by the level of agency which they perceive to be available to them. When self-perception of agency is high, young participants offer constructive policy amendments which can lead to recognition and agency, though only within certain policy areas and the silos in which they are negotiated. When self-perception of agency is low, youth interpret this as lack of recognition, leading to efforts to assert their relevance and/or to challenge procedural legitimacy: neither of which are well received by decision-makers. In reality, several of the challenges faced by young participants are not structurally unique to their constituency; however, their lack of financial resources does hinder their ability to fully utilize modes of participation which previous studies have found to be beneficial to other NSAs, such as side-events. Financial constraints also restrict the ability of youth participants, many of whom are volunteers, to develop professional relationships with key actors over time, meaning that the level of agency developed by more established, better-resourced NSAs remains largely out of reach. These findings have implications for the understanding of NSA agency, which has previously been treated as homogeneous and raises further questions regarding procedural legitimacy of the UNFCCC and its role in mobilizing and empowering the next generation

Linear optics schemes for entanglement distribution with realistic single-photon sources

We study the operation of linear optics schemes for entanglement distribution based on nonlocal photon subtraction when input states, produced by imperfect single-photon sources, exhibit both vacuum and multiphoton contributions. Two models for realistic photon statistics with radically different properties of the multiphoton "tail" are considered. The first model assumes occasional emission of double photons and linear attenuation, while the second one is motivated by heralded sources utilizing spontaneous parametric down-conversion. We find conditions for the photon statistics that guarantee generation of entanglement in the relevant qubit subspaces and compare it with classicality criteria. We also quantify the amount of entanglement that can be produced with imperfect single-photon sources, optimized over setup parameters, using as a measure entanglement of formation. Finally, we discuss verification of the generated entanglement by testing Bell's inequalities. The analysis is carried out for two schemes. The first one is the well-established one-photon scheme, which produces a photon in a delocalized superposition state between two nodes, each of them fed with one single photon at the input. As the second scheme, we introduce and analyze a linear-optics analog of the robust scheme based on interfering two Stokes photons emitted by atomic ensembles, which does not require phase stability between the nodes.Comment: 12 pages, 7 figures, title change, minor corrections in the tex