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  • 1. Jogenfors, Jonathan
    et al.
    Elhassan, Ashraf Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Ahrens, Johan
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Larsson, Jan-Åke
    Hacking the Bell test using classical light in energy-time entanglement-based quantum key distribution2015In: Science Advances, E-ISSN 2375-2548, Vol. 1, no 11, article id e1500793Article in journal (Refereed)
    Abstract [en]

    Photonic systems based on energy-time entanglement have been proposed to test local realism using the Bell inequality. A violation of this inequality normally also certifies security of device-independent quantum key distribution (QKD) so that an attacker cannot eavesdrop or control the system. We show how this security test can be circumvented in energy-time entangled systems when using standard avalanche photodetectors, allowing an attacker to compromise the system without leaving a trace. We reach Bell values up to 3.63 at 97.6% faked detector efficiency using tailored pulses of classical light, which exceeds even the quantum prediction. This is the first demonstration of a violation-faking source that gives both tunable violation and high faked detector efficiency. The implications are severe: the standard Clauser-Horne-Shimony-Holt inequality cannot be used to show device-independent security for energy-time entanglement setups based on Franson's configuration. However, device-independent security can be reestablished, and we conclude by listing a number of improved tests and experimental setups that would protect against all current and future attacks of this type.

  • 2.
    Mohamed El Hassan, Ashraf
    Stockholm University, Faculty of Science, Department of Physics.
    Multiparty Quantum Communication and fs-laser Written Integrated Optics Circuits2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Quantum information science, the rapidly developing interdisciplinary field,  gives power to the information and communications technologies (ICT) by  providing secure communication, precision measurements, ultra-powerful simulation and ultimately computation. It is well known that photons are an ideal candidate for encoding the quantum bit, or "qubit", in quantum information and specially for quantum communication. This thesis consists of two main parts. In the first part, realization of quantum security tasks using optical fibers has been implemented. Bell tests are a cornerstone of quantum key distribution and are necessary for device-independent security. Device-independent Bell inequality test must be performed with care to avoid loopholes. Time-energy entanglement has a distinct advantage over polarization as it is easier transmitted over longer distances, therefore, it may be preferable as a quantum resource to perform reliable key distribution. Novel multi-party communication protocols: secret sharing, detectable Byzantine agreement, clock synchronization, and reduction of communication complexity, all these quantum protocols has been realized without compromising on detection efficiency or generating extremely complex many-particle entangled states. These protocols are realized in an optical fiber setup with sequential phase modulation on single photons. In recent years there has been great interest in fabricating ICT optical setups in low scale in glass chips, which would replace the bulk setups on tables used today. In the second part of the thesis, realization of photonic waveguides in glass has been implemented. Using femtosecond laser inscription of waveguides in glass, photonic quantum technologies and integrated optical circuits are becoming more and more important in miniaturization of optical circuits written in different glass samples for the quantum optics and quantum information processing. These platforms offer stability over the time-scales required for multi-photon coincidence based measurements. The study and optimization the different building blocks for integrated photonic quantum circuits, for instance the directional coupler and Mach-Zehnder interferometer is very important. The principal goal is to develop a method for design, fabrication and characterization of integrated optics circuits for further applications in quantum information. Incorporation of photon sources, detectors, and circuits integrating waveguides technology can be used to produce integrated photonics devices.

  • 3.
    Mohamed El Hassan, Ashraf
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Andler, Guillermo
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Tunable integrated devices for quantum optics experiments, based on fs-laser writing of optical waveguides in glassManuscript (preprint) (Other academic)
  • 4.
    Mohamed El Hassan, Ashraf
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Kunst, Flore
    Stockholm University, Faculty of Science, Department of Physics.
    Moritz, Alexander
    Stockholm University, Faculty of Science, Department of Physics.
    Andler, Guillermo
    Stockholm University, Faculty of Science, Department of Physics.
    Bergholtz, Emil
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Corner state of light in photonic waveguides2018In: Article in journal (Refereed)
  • 5.
    Smania, Massimiliano
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Elhassan, Ashraf M.
    Stockholm University, Faculty of Science, Department of Physics.
    Tavakoli, Armin
    Stockholm University, Faculty of Science, Department of Physics.
    Bourennane, Mohamed
    Stockholm University, Faculty of Science, Department of Physics.
    Experimental quantum multiparty communication protocols2016In: npj Quantum Information, ISSN 2056-6387, Vol. 2, article id 16010Article in journal (Refereed)
    Abstract [en]

    Quantum information science breaks limitations of conventional information transfer, cryptography and computation by using quantum superpositions or entanglement as resources for information processing. Here we report on the experimental realisation of three-party quantum communication protocols using single three-level quantum system (qutrit) communication: secret-sharing, detectable Byzantine agreement and communication complexity reduction for a three-valued function. We have implemented these three schemes using the same optical fibre interferometric setup. Our realisation is easily scalable without compromising on detection efficiency or generating extremely complex many-particle entangled states.

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