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  • 1.
    Bourennane, Mohamed
    et al.
    KTH, Superseded Departments, Electronics.
    Ljunggren, Daniel
    KTH, Superseded Departments, Electronics.
    Karlsson, Anders
    KTH, Superseded Departments, Electronics.
    Jonsson, Per
    KTH, Superseded Departments, Electronics.
    Hening, A.
    KTH, Superseded Departments, Electronics.
    Peña Císcar, J.
    KTH, Superseded Departments, Electronics.
    Experimental long wavelength quantum cryptography: from single-photon transmission to key extraction protocols2000In: Journal of Modern Optics, ISSN 0950-0340, E-ISSN 1362-3044, Vol. 47, p. 563-579Article in journal (Refereed)
    Abstract [en]

    We present experiments on long wavelength (λ = 1.55 μm) 'plug and play' quantum cryptography systems. We discuss the performance of single-photon detectors at λ = 1.55 μm. Furthermore, we address the full implementation of the quantum cryptography protocol, discussing in detail the implementation of protocols for error correction and privacy amplification needed to get a secure key. We illustrate the theory with examples from a full software simulation to show the performance of the complete protocol in terms of final secure key creation rate

  • 2.
    Ljunggren, Daniel
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Entanglement in quantum communication: preparation and characterization of photonic qubits2006Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    At the heart of quantum physics lies the principle of superposition, and at the heart of information theory lies the bit. Perhaps the most useful property of quantum systems is that they can be loaded with information bits, so-called qubits, that are indefinitely both 0 and 1 until a measurement is made. Another consequence is that several qubits can become entangled, which is manifested by the non-classical correlations between such quantum systems when measured in all possible bases. Within the rapidly progressing fields of quantum information and quantum communication these quantum effects are utilized to perform tasks such as quantum computing and quantum cryptography.

    In this thesis we present experimental and theoretical work using single photon sources to prepare ``flying'' photonic qubits. We describe work using mainly quasi-phase-matched nonlinear crystals to generate beams of entangled photon pairs, that are either encoded in polarization at near-visible wavelengths, or in time at optical fiber telecommunication wavelengths (1550 nm). The optical fiber is the medium used for transporting the qubits over a long distance, and it is therefore essential to couple the photons well into the fibers. By focusing the beams optimally, we have investigated how this problem can meet the requirement of creating photons of a narrow frequency bandwidth and a high photon flux. Furthermore, we have generated truly single photons that are heralded by an electrical signal. As a result of modifying the statistics of such sources we have been able to show the effect of photon antibunching. In two separate works, we have implemented a quantum key distribution system based on faint laser pulses at the telecom wavelength of 1550 nm, as well as protocols based on entanglement for performing authentication of key distribution in quantum cryptography.

  • 3.
    Ljunggren, Daniel
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Bourennane, Mohamed
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Karlsson, Anders
    KTH, Superseded Departments, Electronics.
    Authority-based user authentication in quantum key distribution2000In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 62, no 2, p. 022305-Article in journal (Refereed)
    Abstract [en]

     We propose secure protocols for user authenticated quantum key distribution on jammable public channels between two parties, Alice and Bob. Via an arbitrator, Trent, these protocols provide data integrity and mutual identification of the messenger and recipient. The first three are based on single-photon generation and detection. The first and second require (initially) an unjammable channel between the arbitrator and each party. The third requires one broadcast from the arbitrator, disclosing what type of deterministic modification of the states sent through the quantum channel was done by him. The fourth and fifth protocols are based on two-particle entanglement with a preselection of nonorthogonal superpositions of Bell states. These two protocols also require one broadcast from the arbitrator disclosing the type of entangled state in each sending.

  • 4.
    Ljunggren, Daniel
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Tengner, Maria
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Optimal focusing for maximal collection of entangled narrow-band photon pairs into single-mode fibers2005In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 72, no 6, p. 062301-Article in journal (Refereed)
    Abstract [en]

    We present a theoretical and experimental investigation of the emission characteristics and the flux of photon pairs generated by spontaneous parametric downconversion in quasi-phase matched bulk crystals for the use in quantum communication sources. We show that, by careful design, one can attain well defined modes close to the fundamental mode of optical fibers and obtain high coupling efficiencies also for bulk crystals, these being more easily aligned than crystal waveguides. We distinguish between singles coupling, gamma(s) and gamma(i), conditional coincidence, mu(i vertical bar s), and pair coupling, gamma(c), and show how each of these parameters can be maximized by varying the focusing of the pump mode and the fiber-matched modes using standard optical elements. Specifically we analyze a periodically poled KTP-crystal pumped by a 532 nm laser creating photon pairs at 810 nm and 1550 nm. Numerical calculations lead to coupling efficiencies above 93% at optimal focusing, which is found by the geometrical relation L/z(R) to be approximate to 1 to 2 for the pump mode and approximate to 2 to 3 for the fiber-modes, where L is the crystal length and z(R) is the Rayleigh-range of the mode-profile. These results are independent on L. By showing that the single-mode bandwidth decreases proportional to 1/L, we can therefore design the source to produce and couple narrow bandwidth photon pairs well into the fibers. Smaller bandwidth means both less chromatic dispersion for long propagation distances in fibers, and that telecom Bragg gratings can be utilized to compensate for broadened photon packets-a vital problem for time-multiplexed qubits. Longer crystals also yield an increase in fiber photon flux proportional to root L, and so, assuming correct focusing, we can only see advantages using long crystals.

  • 5.
    Ljunggren, Daniel
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Tengner, Maria
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Marsden, Philip
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Pelton, Matthew
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Theory and experiment of entanglement in a quasi-phase-matched two-crystal source2006In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Phys. Rev. A, ISSN 1050-2947, Vol. 73, no 3, p. 032326-Article in journal (Refereed)
    Abstract [en]

    We report results regarding a source of polarization entangled photon pairs created by the process of spontaneous parametric downconversion in two orthogonally oriented, periodically poled, bulk KTiOPO4 crystals. The source emits light colinearly at the nondegenerate wavelengths of 810 and 1550 nm, and is optimized for a single-mode optical fiber collection and long-distance quantum communication. The configuration favors long crystals, which promote a high photon-pair production rate at a narrow bandwidth, together with a high pair probability in fibers. The quality of entanglement is limited by chromatic dispersion, which we analyze by determining the output state. We find that such a decoherence effect is strongly material dependent, providing for long crystals an upper bound on the visibility of the coincidence fringes of 41% for KTiOPO4, and zero for LiNbO3. The best obtained raw visibility, when canceling decoherence with an extra piece of crystal, was 91 +/- 0.2%, including background counts. We confirm by a violation of the CHSH-inequality (S=2.679 +/- 0.004 at 55 s(-1/2) standard deviations) and by complete quantum state tomography that the fibers carry high-quality entangled pairs at a maximum rate of 55x10(3) s(-1) THz(-1) mW(-1).

  • 6.
    Ljunggren, Daniel
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Tengner, Maria
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Pelton, Matthew
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Marsden, Phil
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    A source of entangled photon-pairs: Optimizing emission in two quasi-phasematched crystals2004In: Quantum Communication, Measurement And Computing / [ed] Barnett, SM; Andersson, E; Jeffers, J; Ohberg, P; Hirota, O, 2004, Vol. 734, p. 354-357Conference paper (Refereed)
    Abstract [en]

    We extend our earlier results [1] of a bright source of non-degenerate polarization entangled photon-pairs by making a thorough theoretical and experimental investigation of its single-mode emission and state-decoherence. Using two perpendicular bulk crystals of periodically poled KTP, the source directly creates the polarization-entangled state (\HH> + \VV>)/root2 with signal and idler wavelengths of 810 nm and 1550 nm, respectively. In order to optimize the coupling into single-mode fiber we have developed a numerical algorithm which determines the emission mode and the fiber coupling efficiency for a given crystal length and focussing of the pump. For a 4.5 mm long crystal the optimal focussing of the pump is to a beam waist radius of 14 mum. With these focussing conditions the computer calculations give the lowest M-2 value for the emission mode, M-2 approximate to 3, and the best possible fiber coupling efficiency, gamma approximate to 75%. Using the theoretical results to optimize our experimental setup we obtained photon count rates of 1.4 MHz singles and 12 kHz coincidences, and visibilities H, V: 99.6% and 45degrees: 94.2%.

  • 7. Nisbet-Jones, P. B. R.
    et al.
    Dilley, Jerome
    Ljunggren, Daniel
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP. Clarendon Laboratory, University of Oxford, United Kingdom .
    Kuhn, Axel
    Highly efficient source for indistinguishable single photons of controlled shape2011In: New Journal of Physics, ISSN 1367-2630, Vol. 13Article in journal (Refereed)
    Abstract [en]

    We demonstrate a straightforward implementation of a push-button like single-photon source, which is based on a strongly coupled atom-cavity system. The device operates intermittently for periods of up to 100μs, with single-photon repetition rates of 1.0MHz and an efficiency of 60%. Atoms are loaded into the cavity using an atomic fountain, with the upper turning point near the cavity's mode centre. This ensures long interaction times without any disturbances induced by trapping potentials. The latter is the key to reaching deterministic efficiencies as high as obtained in probabilistic photon-heralding schemes. The price to pay is the random loading of atoms into the cavity and the resulting intermittency. However, for all practical purposes, this has a negligible impact as an individual atom may emit up to 100 successive photons.

  • 8.
    Pelton, Matthew
    et al.
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Marsden, Philip
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Ljunggren, Daniel
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Tengner, Maria
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Karlsson, Anders
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Fragemann, Anna
    KTH, Superseded Departments, Physics.
    Canalias, Carlota
    KTH, Superseded Departments, Physics.
    Laurell, Fredrik
    KTH, Superseded Departments, Physics.
    Bright, single-spatial-mode source of frequency non-degenerate, polarization-entangled photon pairs using periodically poled KTP2004In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 12, no 15, p. 3573-3580Article in journal (Refereed)
    Abstract [en]

    We use two perpendicular crystals of periodically-poled KTP to directly generate polarization-entangled photon pairs, the majority of which are emitted into a single Gaussian spatial mode. The signal and idler photons have wavelengths of 810 nm and 1550 nm, respectively, and the photon-pair generation rate is 1.2 x 10(7) sec(-1) for a pump power of 62 mW. The apparatus is compact, flexible, and easily to use.

  • 9.
    Sauge, Sebastien
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Swillo, Marcin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Albert-Seifried, Sebastian
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Waldebäck, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Tengner, Maria
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Ljunggren, Daniel
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Karlsson, Anders
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Xavier, G. B.
    Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks2007In: CLEO/Europe IQEC 2007: 17-22 June 2007, Munich, Germany, 2007, p. 4387014-Conference paper (Refereed)
  • 10.
    Sauge, Sebastien
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Swillo, Marcin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Albert-Seifried, Sebastian
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Xavier, Guilherme
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Waldebäck, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Tengner, Maria
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Ljunggren, Daniel
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Karlsson, Anders
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks2007In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 15, no 11, p. 6926-6933Article in journal (Refereed)
    Abstract [en]

    We present a bright, narrowband, portable, quasi-phase- matched two-crystal source generating polarization- entangled photon pairs at 809 nm and 1555 nm at a maximum rate of 1.2 x 10(6) s(-1) THz(-1) mW(-1) after coupling to single- mode fiber. The quantum channel at 1555 nm and the synchronization signal gating the single photon detector are multiplexed in the same optical fiber of length 27 km by means of wavelength division multiplexers (WDM) having 100 GHz (0.8 nm) spacing between channels. This implementation makes quantum communication applications compatible with current high-speed optical networks.

  • 11.
    Sauge, Sebastien
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Swillo, Marcin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Albert-Seifried, Sebastian
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Xavier, Guilherme
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Waldebäck, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Tengner, Maria
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Ljunggren, Daniel
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Wang, Qin
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Karlsson, Anders
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Quantum communication in optical networks: an overview and selected recent results2007In: ICTON 2007: Proceedings of the 9th International Conference on Transparent Optical Networks, Vol 1 / [ed] Marciniak, M, 2007, p. 30-33Conference paper (Refereed)
    Abstract [en]

    We discuss recent work in quantum communication, and in some details present a bright, narrowband, portable, quasi-phase-matched two-crystal source generating polarization-entangled photon pairs at 809 nm and 1555 nm. We also show how the single-photon quantum channel at 1555 nm and a classical synchronization signal gating the single photon detector at the receiving side can be multiplexed in the same optical fiber of length 27 km by means of wavelength division multiplexers (WDM) having 100 GHz (0.8 nm) spacing between channels. This illustrates bow single-photon quantum communication applications is compatible with current high-speed optical networks.

  • 12.
    Tengner, Maria
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Ljunggren, Daniel
    KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
    Characterization of an asynchronous source of heralded single photons generated at a wavelength of 1550 nmArticle in journal (Refereed)
    Abstract [en]

    We make a thorough analysis of heralded single photon sources regarding how factors such as the detector gate-period, the photon rates, the fiber coupling efficiencies, and the system losses affect the performance of the source. In the course of this we give a detailed description of how to determine fiber coupling efficiencies from experimentally measurable quantities. We show that asynchronous sources perform, under most conditions, better than synchronous sources with respect to multiphoton events, but only for nearly perfect coupling efficiencies. We apply the theory to an asynchronous source of heralded single photons based on spontaneous parametric downconversion in a periodically poled, bulk, KTiOPO4 crystal. The source generates light with highly non-degenerate wavelengths of 810 nm and 1550 nm, where the 810 nm photons are used to announce the presence of the 1550 nm photons inside a single-mode optical fiber. For our setup we find the probability of having a 1550 nm photon present in the single-mode fiber, as announced by the 810 nm photon, to be 48%. The probability of multiphoton events is strongly suppressed compared to a Poissonian light source, giving highly sub-Poisson photon statistics.

  • 13.
    Tengner, Maria
    et al.
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Ljunggren, Daniel
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Sauge, Sebastien
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Waldeback, Johan
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Karlsson, Anders
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Single-photon correlations for secure communication2006In: Advanced Free-Space Optical Communication Techniques/Applications II and Photonic Components Architectures for Microwave Systems and Displays / [ed] Sjoqvist, LJ; Wilson, RA; Merlet, TJ, BELLINGHAM, WA: SPIE-INT SOC OPTICAL ENGINEERING , 2006, Vol. 6399, p. U124-U132Conference paper (Refereed)
    Abstract [en]

    We present two types of photon sources designed for secure quantum communication, e.g. for quantum cryptog raphy. Both types are based on the creation of photon pairs by spontaneous parametric downconversion in nonlinear crystals. The first is a heralded single photon source and the second is a source of polarization-entangled photon pairs. For the heralded single photon source the detection of one of the photons of a downconversion pair is used as a trigger to announce the presence of the other: the single photon. The source is characterized by a highly sub-Poisson photon number statistics making it very suitable for use in quantum cryptography protocols using single photonic qubits to create correlated information between a sender and a receiver. The entanglement source instead uses the inherent non-classical correlations between entangled qubits. We also present a hybrid-encoding where the sender uses polarization to encode information while the receiver uses time-bins. Both sources create photons with highly non-degenerate wavelengths of 810 nm and 1550 nm, taking advantage of the efficient detectors at near-infrared and the low transmission loss of optical fibers at telecommunication wavelengths.

  • 14. Vasilev, G. S.
    et al.
    Ljunggren, Daniel
    University of Oxford.
    Kuhn, A.
    Single photons made-to-measure2010In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 12, p. 063024-Article in journal (Refereed)
    Abstract [en]

    We investigate the efficiency of atom-cavity-based photongeneration schemes to deliver single photons of arbitrary temporal shape. Our model applies to Raman transitions in three-level atoms with one branch of the transition driven by a laser pulse and the other coupled to a cavity mode. For any possible shape of the single-photon wavepacket, we derive an unambiguous analytic expression for the shape of the required driving laser pulse. Furthermore, we discuss the constraints limiting the maximum probability for emitting any desired photon, and use these to estimate upper bounds for the efficiency of the process. The model is not only valid for vacuum-stimulated Raman adiabatic passages (V-STIRAP) in the strong-coupling and bad-cavity regime, but also generally allows the control of the coherence and population flow in any Raman process.

1 - 14 of 14
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