# Publications

Please find here publications by the project-partners done within SECOQC - or closely related to SECOQC.

Note: The list does not include publications of the time after SECOQC - please look them up at quant-ph at arxiv.org.

This is the main SECOQC publication:
Peev et. al. The SECOQC quantum key distribution network in Vienna

 High-fidelity transmission of polarization encoded qubits from an entangled source over 100 km of fiber Hannes Hübel, Michael R. Vanner, Thomas Lederer, Bibiane Blauensteiner, Thomas Lorünser, Andreas Poppe, and Anton Zeilinger Optics Express, Vol. 15, Issue 12, pp. 7853-7862 June 08, 2007 We demonstrate non-degenerate down-conversion at 810 and 1550 nm for long-distance fiber based quantum communication using polarization entangled photon pairs. Measurements of the two-photon visibility, without dark count subtraction, have shown that the quantum correlations (raw visibility 89%) allow secure quantum cryptography after 100 km of non-zero dispersion shifted fiber using commercially available single photon detectors. In addition, quantum state tomography has revealed little degradation of state negativity, decreasing from 0.99 at the source to 0.93 after 100 km, indicating minimal loss in fidelity during the transmission. The article @ Optics Express
 Oxide-apertured microcavity single-photon emitting diode D. J. P. Ellis, A. J. Bennett, A. J. Shields, P. Atkinson and D. A. Ritchie Appl. Phys. Lett. 90, 233514 (2007) June 04, 2007 The authors have developed a microcavity single-photon source based on a single quantum dot within a planar cavity in which wet oxidation of a high-aluminium content layer provides lateral confinement of both the photonic mode and the injection current. Lateral confinement of the optical mode in optically pumped structures produces a strong enhancement of the radiative decay rate. Using microcavity structures with doped contact layers, they demonstrate a single-photon emitting diode where current may be injected into a single dot. The article @ Appl Phys Lett
 Entanglement-based quantum communication over 144 km R. Ursin, F. Tiefenbacher, T. Schmitt-Manderbach, H. Weier, T. Scheidl, M. Lindenthal, B. Blauensteiner, T. Jennewein, J. Perdigues, P. Trojek, B. Ömer, M. Fürst, M. Meyenburg, J. Rarity, Z. Sodnik, C. Barbieri, H. Weinfurter & A. Zeilinger Nature Physics June 03, 2007 We demonstrate non-degenerate down-conversion at 810 and 1550 nm for long-distance fiber based quantum communication using polarization entangled photon pairs. Measurements of the two-photon visibility, without dark count subtraction, have shown that the quantum correlations (raw visibility 89%) allow secure quantum cryptography after 100 km of non-zero dispersion shifted fiber using commercially available single photon detectors. In addition, quantum state tomography has revealed little degradation of state negativity, decreasing from 0.99 at the source to 0.93 after 100 km, indicating minimal loss in fidelity during the transmission. The article @ nature physics
 Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks S. Sauge, M. Swillo, S. Albert-Seifried, G. B. Xavier, J. Waldebäck, M. Tengner, D. Ljunggren, and A. Karlsson Optics Express, Vol. 15, Issue 11, pp. 6926-6933 May 21, 2007 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 × 106 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. The article @ Optics Express
 Electrically driven telecommunication wavelength single-photon source M. B. Ward, T. Farrow, P. See, Z. L. Yuan, O. Z. Karimov, A. J. Bennett,A. J. Shields, P. Atkinson, K. Cooper, and D. A. Ritchie Appl. Phys. Lett. 90, 063512 (2007) February 5, 2007 An electrically driven ~1.3 µm single-photon source is demonstrated. The source contains InAs quantum dots within a planar cavity light-emitting diode. Electroluminescence (EL) spectra show clear emission lines and from time resolved EL we estimate a primary decay time of ~1 ns. Time-varying Stark shifts are studied and proposed for truncating the emission in jitter-sensitive applications (optimization for 2 ns detector gate width demonstrated) and for relaxing excitation pulse-length requirements. A correlation measurement demonstrates suppression of multiphoton emission to below 28% of the Poissonian level before correction for detector dark counts, suggesting g(2)(0)~0.19 for the source itself The article @ Appl Phys Lett
 SECOQC White Paper on Quantum Key Distribution and Cryptography Romain Alléaume, Jan Bouda, Cyril Branciard, Thierry Debuisschert, Mehrdad Dianati, Nicolas Gisin, Mark Godfrey, Philippe Grangier, Thomas Länger, Anthony Leverrier, Norbert Lütkenhaus, Philippe Painchault, Momtchil Peev, Andreas Poppe, Thomas Pornin, John Rarity, Renato Renner, Grégoire Ribordy, Michel Riguidel, Louis Salvail, Andrew Shields, Harald Weinfurter, Anton Zeilinger. quant-ph/0701168 Jan 22, 2007 The SECOQC White Paper on Quantum Key Distribution and Cryptography is the outcome on a thorough consultation and discussion among the participants of the European project SECOQC. This paper is a review article that attempts to position Quantum Key Distribution (QKD) in terms of cryptographic applications. A detailed comparison of QKD with the solutions currently in use to solve the key distribution problem, based on classical cryptography, is provided. We also detail how the work on QKD networks lead within SECOQC will allow the deployment of long-distance secure communication infrastructures based on quantum cryptography. The purpose of the White Paper is finally to promote closer collaboration between classical and quantum cryptographers. We believe that very fruitful research, involving both communities, could emerge in the future years and try to sketch what may be the next challenges in this direction. The article on quant-ph, pdf-version (pdf, ~500kB)
 Optomechanical Entanglement between a Movable Mirror and a Cavity Field D. Vitali, S. Gigan, A. Ferreira, H. R. Böhm, P. Tombesi, A. Guerreiro, V. Vedral, A. Zeilinger, and M. Aspelmeyer Phys. Rev. Lett. 98, 030405 (2007) published 19 January 2007 We show how stationary entanglement between an optical cavity field mode and a macroscopic vibrating mirror can be generated by means of radiation pressure. We also show how the generated optomechanical entanglement can be quantified, and we suggest an experimental readout scheme to fully characterize the entangled state. Surprisingly, such optomechanical entanglement is shown to persist for environment temperatures above 20 K using state-of-the-art experimental parameters. The article on PRL, pdf-version
 Experimental Demonstration of Free-Space Decoy-State Quantum Key Distribution over 144 km Tobias Schmitt-Manderbach, Henning Weier, Martin Fürst, Rupert Ursin, Felix Tiefenbacher, Thomas Scheidl, Josep Perdigues, Zoran Sodnik, Christian Kurtsiefer, John G. Rarity, Anton Zeilinger, and Harald Weinfurter Phys. Rev. Lett. 98, 010504 (2007) published 5 January 2007 We report on the experimental implementation of a Bennett-Brassard 1984 (BB84) protocol type quantum key distribution over a 144 km free-space link using weak coherent laser pulses. Optimization of the link transmission was achieved with bidirectional active telescope tracking, and the security was ensured by employing decoy-state analysis. This enabled us to distribute a secure key at a rate of 12.8 bit/s at an attenuation of about 35 dB. Utilizing a simple transmitter setup and an optical ground station capable of tracking a spacecraft in low earth orbit, this outdoor experiment demonstrates the feasibility of global key distribution via satellites. The article on PRL, Link to pdf-version
 Advanced quantum communication experiments with entangled photons M. Aspelmeyer, A. Zeilinger, H. R. Böhm, A. Fedrizzi, S. Gasparoni, M. Lindenthal, G. Molina-Terriza, A. Poppe, K. Resch, R. Ursin, P. Walther, T. D. Jennewein Book Contribution in: Quantum Communications and Cryptography, A.V. Sergienko, ed., CRC Taylor & Francis, Boca Raton, Florida, 2006, Ch. 3, pp. 45-81 (ISBN 0-8493-3684-8). Dec 2006 Link to pdf-version
 Simulating phase coding in quantum cryptography: influence of chromatic dispersion M.Suda, T.Herbst and A.Poppe Eur. Phys. J. D (2006) Dec 22, 2006 We demonstrate the impact of chromatic dispersion on the phase coding method in a double Mach-Zehnder implementation of an interferometric system for quantum cryptography. Formulas have been developed to explore detailed studies of the modifications on energy and position spectra which arise if chromatic dispersion is taken into account. Examples demonstrate the shifting of spectra and the appearance of oscillations depending on the wavelength and spectral broadness as well as on the phase shifters, absorbers and the dimensions of the interferometric set-up. The article on Eur. Phys J.D (online first)
 Passive Optical Network Approach to GigaHertz-Clocked Multiuser Quantum Key Distribution Veronica Fernandez, Robert J. Collins, Karen J. Gordon, Paul D. Townsend, Gerald S. Buller IEEE Journal of Quantum Electronics, Volume 43 (2), (2007) Dec 15, 2006 We present the application of quantum key distribution technologies to fiber-based broadband passive optical access networks. This application is based on our 850 nm wavelength gigahertz clock-rate single-receiver system, is compatible with existing telecommunications fiber and exploits a wavelength band not currently utilized in access networks. The developed quantum key distribution networks are capable of transmitting over distances consistent with the span of access links for metropolitan networks (10 km), at clock frequencies ranging up to 3 GHz. The article on quant-ph
 Low cost and compact quantum key distribution J L Duligall, M S Godfrey, K A Harrison, W J Munro and J G Rarity New J. Phys. 8 249 Octobre 24, 2006 We present the design of a novel free-space quantum cryptography system, complete with purpose-built software, that can operate in daylight conditions. The transmitter and receiver modules are built using inexpensive off-the-shelf components. Both modules are compact allowing the generation of renewed shared secrets on demand over a short range of a few metres. An analysis of the software is shown as well as results of error rates and therefore shared secret yields at varying background light levels. As the system is designed to eventually work in short-range consumer applications, we also present a use scenario where the consumer can regularly 'top up' a store of secrets for use in a variety of one-time-pad (OTP) and authentication protocols. The article @ New Journal of Physics
 Experimentally realizable quantum comparison of coherent states and its applications Erika Andersson, Marcos Curty, and Igor Jex Phys. Rev. A 74, 022304 (2006) (11 pages) Aug 3, 2006 When comparing quantum states to each other, it is possible to obtain an unambiguous answer, indicating that the states are definitely different, already after a single measurement. In this paper we investigate comparison of coherent states, which is the simplest example of quantum state comparison for continuous variables. The method we present has a high success probability, and is experimentally feasible to realize as the only required components are beam splitters and photon detectors. An easily realizable method for quantum state comparison could be important for real applications. As examples of such applications we present a "lock and key" scheme and a simple scheme for quantum public key distribution. The article on Physical Review A
 Finite key analysis for symmetric attacks in quantum key distribution Tim Meyer, Hermann Kampermann, Matthias Kleinmann, Dagmar Bruss quant-ph/0607141 July 20, 2006 We introduce a constructive method to calculate the achievable secret key rate for a generic class of quantum key distribution protocols, when only a finite number n of signals is given. Our approach is applicable to all scenarios in which the quantum state shared by Alice and Bob is known. In particular, we consider the six state protocol with symmetric eavesdropping attacks, and show that for a small number of signals, i.e. below the order of 10^4, the finite key rate differs significantly from the asymptotic value for n approaching infinity. However, for larger n, a good approximation of the asymptotic value is found. We also study secret key rates for protocols using higher-dimensional quantum systems. The article on quant-ph
 Quantum Communication and Quantum Computation with Entangled Photons Anton Zeilinger Foundations of Quantum Mechanics in the Light of New Technology: Proceedings, ISQM Tokyo '05, 2006, ISBN 981-256-858-1 published June 2006 Link to pdf file
 Asymptotic correctability of Bell-diagonal quantum states and maximum tolerable bit-error rates Kedar S. Ranade and Gernot Alber J. Phys. A: Math. Gen. 39 1701-1716 Feb 1, 2006 The general conditions are discussed which quantum state purification protocols have to fulfil in order to be capable of purifying Bell-diagonal qubit-pair states, provided they consist of steps that map Bell-diagonal states to Bell-diagonal states and they finally apply a suitably chosen Calderbank–Shor–Steane code to the outcome of such steps. As a main result a condition on asymptotic correctability is presented, which relates this problem to the magnitude of a characteristic exponent governing the relation between bit and phase errors under the purification steps. This condition allows a straightforward determination of maximum tolerable bit-error rates of quantum key distribution protocols whose security analysis can be reduced to the purification of Bell-diagonal states. The article on Journal of Physics A
 Quantum Cryptography Miloslav Dusek, Norbert Lutkenhaus, Martin Hendrych quant-ph/0601207, To appear in Progress in Optics, vol. 49, Edt. E. Wolf (Elsevier) Jan 31, 2006 Elementary review article on quantum cryptography. The article on quant-ph
 Quantum Information: Remember that Photon Philippe Grangier nature, Vol 438, page 749 Dec 8, 2005 Storing single photons in atomic memories, and releasing them at a later time, is a required step on the way to quantum repeaters and long-distance quantum cryptography networks. This step has now been taken. The article on nature (editors summary with link to article)
 Efficiency of coherent state quantum cryptography in the presence of loss: Influence of realistic error correction Matthias Heid, Norbert Lütkenhaus quant-ph/0512013 Dec 1, 2005 We investigate the performance of continuous variable (CV) quantum key distribution (QKD) scheme in a practical setting. More specifically, we take non-ideal error reconciliation procedure into account. The quantum channel connecting the two honest parties is assumed to be lossy but noiseless. Secret key rates are given for the case that the measurement outcomes are postselected or a reverse reconciliation scheme is applied. The reverse reconciliation scheme loses its initial advantage in the practical setting. If one combines postselection with reverse reconciliation however, much of this advantage can be recovered. The article on quant-ph
 Robustness of a quantum key distribution with two and three mutually unbiased bases Filippo Caruso, Helle Bechmann-Pasquinucci, Chiara Macchiavello Physical Review A 72, 032340 (2005) September 30,2005 We study the robustness of various protocols for quantum key distributions. We first consider the case of qutrits and study quantum protocols that employ two and three mutually unbiased bases. We then derive the optimal eavesdropping strategy for two mutually unbiased bases in dimension 4 and generalize the result to a quantum key distribution protocol that uses two mutually unbiased bases in arbitrary finite dimensions. The article on Phys Rev A
 Quantum Interferometry in Phase Space - Theory and Applications Martin Suda Book. Springer 2005. ISBN 3-540-26070-6 Sept 28, 2005 Quantum Interferometry in Phase Space is primarily concerned with quantum-mechanical distribution functions and their applications in quantum optics and neutron interferometry. In the first part of the book, the author describes the phase-space representation of quantum optical phenomena such as coherent and squeezed states. Applications to interferometry, e.g. in beam splitters and fiber networks, are also presented. In the second part of the book, the theoretical formalism is applied to neutron interferometry, including the dynamical theory of diffraction, choerence properties of superposed beams, and dephasing effects.
 Security bound of two-basis quantum-key-distribution protocols using qudits Georgios M. Nikolopoulos and Gernot Alber Phys. Rev. A 72, 032320 (2005) September 16, 2005 We investigate the security bounds of quantum-cryptographic protocols using d-level systems. In particular, we focus on schemes that use two mutually unbiased bases, thus extending the Bennett-Brassard 1984 quantum-key-distribution scheme to higher dimensions. Under the assumption of general coherent attacks, we derive an analytic expression for the ultimate upper security bound of such quantum-cryptography schemes. This bound is well below the predictions of optimal cloning machines. The possibility of extraction of a secret key beyond entanglement distillation is discussed. In the case of qutrits we argue that any eavesdropping strategy is equivalent to a symmetric one. For higher dimensions such an equivalence is generally no longer valid. The article on Phys Rev A
 Quantum erasing of biphoton visibility: the complementarity between entanglement and single-partite properties Matthias Jakob Proceedings of SPIE Volume: 5866; The Nature of Light: What Is a Photon?, ISBN: 0-8194-5871-6 August 2005 A quantum eraser is proposed that operates in a domain that does not have any classical counterpart. This quantum eraser utilizes the complementary aspect between entanglement and single-partite properties of composite quantum systems. Consequently, in contrast to the duality of visibility and which-path information which establish features of single quantum systems, here, properties of composite quantum systems are considered. In composite quantum systems entanglement might emerge which is of genuine quantum origin. This nonclassical correlation mutually exclude the single-partite properties of the subsystems of the composite quantum system. The single-partite properties can be describes by wave- and particle properties, i.e. the standard wave-particle duality. Remarkably, entanglement can be considered as a resource for observables that do not exist in classical physics. In a bipartite photon system, this observable is the two-particle visibility which describes the phase relations that are shared between both photons of the composite system. The complementary aspect between two-particle visibility and single-partite properties of the subsystems prevents us from the observation of single-partite properties in an entangled biphoton system. The quantum eraser erases the two-partite visibility and retrieves single-partite properties in form of single-particle visibility for both of the two photons. Thus, both observables contain phase information giving rise to interference effects. Here, complementarity is explicitly enforced by entanglement in a quantitative manner. The contribution online
 Cryptography In the Bounded Quantum-Storage Model Ivan Damgaard, Serge Fehr, Louis Salvail, Christian Schaffner Proceedings of FOCS 2005, 46th Annual IEEE Symposium on Foundations of Computer Science August 30, 2005 We initiate the study of two-party cryptographic primitives with unconditional security, assuming that the adversary's quantum memory is of bounded size. We show that oblivious transfer and bit commitment can be implemented in this model using protocols where honest parties need no q uantum memory, whereas an adversarial player needs quantum memory of size at least n/2 in order to break the protocol, where n is the number of qubits transmitted. This is in sharp contrast to the classical bounded-memory model, where we can only tolerate adversaries with memory of size quadratic in honest players' memory size. Our protocols are efficient, non-interactive and can be implemented using today's technology. On the technical side, a new entropic uncertainty relation involving min-entropy is established. The article on quant-ph
 Experimental demonstration of time coding quantum key distribution William Boucher, Thierry Debuisschert Proceedings of the QELS 2005 conference, Baltimore May 22-27 2005, paper JTuC112. quant-ph/0507059 July 6, 2005 Time coding quantum key distribution with coherent faint pulses is experimentally demonstrated. A measured 3.3 % quantum bit error rate and a relative contrast loss of 8.4 % allow a 0.49 bit/pulse advantage to Bob. The article on quant-ph
 Trojan Horse attacks on Quantum Key Distribution systems N. Gisin, S. Fasel, B. Kraus, H. Zbinden, G. Ribordy quant-ph/0507063 July 6, 2005 General Trojan horse attacks on quantum key distribution systems are analyzed. We illustrate the power of such attacks with today's technology and conclude that all system must implement active counter-measures. In particular all systems must include an auxiliary detector that monitors any incoming light. We show that such counter-measures can be efficient, provided enough additional privacy amplification is applied to the data. We present a practical way to reduce the maximal information gain that an adversary can gain using Trojan horse attacks. This does reduce the security analysis of the 2-way {\it Plug-&-Play} system to those of the standard 1-way systems. The article on quant-ph
 Reduced randomness in quantum cryptography with sequences of qubits encoded in the same basis L.-P. Lamoureux, H. Bechmann-Pasquinucci, N. J. Cerf, N. Gisin, C. Macchiavello quant-ph/0507058, submitted to PRA July 5, 2005 We consider the cloning of sequences of qubits prepared in the states used in the BB84 or 6-state quantum cryptography protocol, and show that the single-qubit fidelity is unaffected even if entire sequences of qubits are prepared in the same basis. This result is of great importance for practical quantum cryptosystems because it reduces the need for high-speed random number generation without impairing on the security against finite-size attacks. The article on quant-ph
 On the generalization of quantum state comparison M. Kleinmann, H. Kampermann, D. Bruss quant-ph/0503012 (2005), Phys. Rev. A 72, 032308 (2005) June 28, 2005 We investigate the unambiguous comparison of quantum states in a scenario that is more general than the one that was originally suggested by Barnett et al. First, we find the optimal solution for the comparison of two states taken from a set of two pure states with arbitrary a priori probabilities. We show that the optimal coherent measurement is always superior to the optimal incoherent measurement. Second, we develop a strategy for the comparison of two states from a set of N pure states, and find an optimal solution for some parameter range when N=3. In both cases we use the reduction method for the corresponding problem of mixed state discrimination, as introduced by Raynal et al., which reduces the problem to the discrimination of two pure states only for N=2. Finally, we provide a necessary and sufficient condition for unambiguous comparison of mixed states to be possible. The article on quant-ph
 High-performance guided-wave asynchronous heralded single-photon source O. Alibart, D. B. Ostrowsky, P. Baldi, and S. Tanzilli Opt. Lett. 30, 1539-1541 (2005) June 15, 2005 We report on a guided-wave asynchronous heralded single-photon source based on the creation of nondegenerate photon pairs by spontaneous parametric downconversion in a periodically poled lithium niobate waveguide. We show that, by use of the signal photon at 1310 nm as a trigger, a gated detection process permits announcement of the arrival of single photons at 1550 nm at the output of a single-mode optical fiber with a high probability of 0.37. At the same time the multiphoton emission probability is reduced by a factor of 10 compared with Poissonian light sources. Furthermore, the model we have developed to calculate those figures of merit is shown to be accurate. This study can therefore serve as a paradigm for the conception of new quantum communication and computation networks. Link to the article
 Fast and simple one-way Quantum Key Distribution Damien Stucki, Nicolas Brunner, Nicolas Gisin, Valerio Scarani, Hugo Zbinden quant-ph/0506097 and Appl. Phys. Lett. 87, 194108 (2005) June 13, 2005 We present and demonstrate a new protocol for practical quantum cryptography, tailored for an implementation with weak coherent pulses to obtain a high key generation rate. The key is obtained by a simple time-of-arrival measurement on the data line; the presence of an eavesdropper is checked by an interferometer on an additional monitoring line. The setup is experimentally simple; moreover, it is tolerant to reduced interference visibility and to photon number splitting attacks, thus featuring a high efficiency in terms of distilled secret bit per qubit. Link to the article
 On-demand single-photon source for 1.3 µm telecom fiber M.B. Ward, O.Z. Karimov, D.C. Unitt, Z.L. Yuan, P. See, D.G. Gevaux, and A.J. Shields; P. Atkinson and D. A. Ritchie Appl. Phys. Lett. 86, 201111 (2005) May 16, 2005 We demonstrate an on-demand single-photon source that is compatible with standard telecom optical fiber. Through careful control of the critical strains of InAs/GaAs self-assembled quantum dots, we produce a microcavity sample with a low density of large dots emitting into the fiber-optic transmission band at 1.3 µm. The second-order correlation function of the source reveals a strong suppression in the rate of multiphoton pulses at both 5 K and above 30 K. The source may be useful for fiber-optic-based single-photon applications, such as quantum metrology, quantum communications, and distributed quantum computing. The article in Appl.Phys.Lett
 Quantum key distribution with trusted quantum relay H. Bechmann-Pasquinucci, A. Pasquinucci quant-ph/0505089 May 12, 2005 A trusted quantum relay is introduced to enable quantum key distribution links to form the basic legs in a quantum key distribution network. The idea is based on the well-known intercept/resend eavesdropping. The same scheme can be used to make quantum key distribution between several parties. No entanglement is required. Link to the article
 Security of two quantum cryptography protocols using the same four qubit states Cyril Branciard, Nicolas Gisin, Barbara Kraus, Valerio Scarani quant-ph/0505035 and Phys. Rev. A 72, 032301 (2005) May 6, 2005 The first quantum cryptography protocol, proposed by Bennett and Brassard in 1984 (BB84), has been widely studied in the last years. This protocol uses four states (more precisely, two complementary bases) for the encoding of the classical bit. Recently, it has been noticed that by using the same four states, but a different encoding of information, one can define a new protocol which is more robust in practical implementations, specifically when attenuated laser pulses are used instead of single-photon sources [V. Scarani et al., Phys. Rev. Lett. {\bf 92}, 057901 (2004); referred to as SARG04]. We present a detailed study of SARG04 in two different regimes. In the first part, we consider an implementation with a single-photon source: we derive bounds on the error rate $Q$ for security against all possible attacks by the eavesdropper. The lower and the upper bound obtained for SARG04 ($Q\lesssim 10.95%$ and $Q\gtrsim 14.9%$ respectively) are close to those obtained for BB84 ($Q\lesssim 12.4%$ and $Q\gtrsim 14.6%$ respectively). In the second part, we consider the realistic source consisting of an attenuated laser and improve on previous analysis by allowing Alice to optimize the mean number of photons as a function of the distance. SARG04 is found to perform better than BB84, both in secret key rate and in maximal achievable distance, for a wide class of Eve's attacks. Link to the article
 Secure coherent-state quantum key distribution protocols with efficient reconciliation G. Van Assche, S. Iblisdir, and N. J. Cerf Phys. Rev. A 71, 052304 (2005) May 4, 2005 We study the equivalence of a realistic quantum key distribution protocol using coherent states and homodyne detection with a formal entanglement purification protocol. Maximally entangled qubit pairs that one can extract in the formal protocol correspond to secret key bits in the realistic protocol. More specifically, we define a qubit encoding scheme that allows the formal protocol to produce more than one entangled qubit pair per entangled oscillator pair or, equivalently for the realistic protocol, more than one secret key bit per coherent state. The entanglement parameters are estimated using quantum tomography. We analyze the properties of the encoding scheme and investigate the resulting secret key rate in the important case of the attenuation channel. The article in Phys.Rev.A
 Optimal copying of entangled two-qubit states J. Novotny, G. Alber, and I. Jex Phys. Rev. A 71, 042332 (2005) April 21, 2005 We investigate the problem of copying pure two-qubit states of a given degree of entanglement in an optimal way. Completely positive covariant quantum operations are constructed which maximize the fidelity of the output states with respect to two separable copies. These optimal copying processes hint at the intricate relationship between fundamental laws of quantum theory and entanglement. The article in Phys.Rev.A
 Economical phase-covariant cloning of qudits Francesco Buscemi, Giacomo Mauro D'Ariano, and Chiara Macchiavello Phys. Rev. A 71, 042327 (2005) April 19, 2005 We derive the optimal N->M phase-covariant quantum cloning for equatorial states in dimension d with M=kd+N, k integer. The cloning maps are optimal for both global and single-qudit fidelity. The map is achieved by an "economical" cloning machine, which works without ancilla. The article in Phys.Rev.A
 Quantum key distribution system clocked at 2 GHz Karen J. Gordon, Veronica Fernandez, and Gerald S. Buller; Ivan Rech and Sergio D. Cova; Paul D. Townsend Opt. Express 13, 3015-3020 (2005) April 18, 2005 An improved quantum key distribution test system operating at clock rates of up to 2GHz using a specially adapted commercially-available silicon single-photon counting module is presented. The use of an enhanced detector has improved the fiber-based quantum key distribution test system performance in terms of transmission distance and quantum bit error rate. The article on Optics Express
 Photon-number-splitting versus cloning attacks in practical implementations of the Bennett-Brassard 1984 protocol for quantum cryptography Armand Niederberger, Valerio Scarani, and Nicolas Gisin Phys. Rev. A 71, 042316 (2005), ePrint:quant-ph/0408122 April 11, 2005 In practical quantum cryptography, the source sometimes produces multi-photon pulses, thus enabling the eavesdropper Eve to perform the powerful photon-number-splitting (PNS) attack. Recently, it was shown by Curty and Lutkenhaus [Phys. Rev. A 69, 042321 (2004)] that the PNS attack is not always the optimal attack when two photons are present: if errors are present in the correlations Alice-Bob and if Eve cannot modify Bob's detection efficiency, Eve gains a larger amount of information using another attack based on a 2->3 cloning machine. In this work, we extend this analysis to all distances Alice-Bob. We identify a new incoherent 2->3 cloning attack which performs better than those described before. Using it, we confirm that, in the presence of errors, Eve's better strategy uses 2->3 cloning attacks instead of the PNS. However, this improvement is very small for the implementations of the Bennett-Brassard 1984 (BB84) protocol. Thus, the existence of these new attacks is conceptually interesting but basically does not change the value of the security parameters of BB84. The main results are valid both for Poissonian and sub-Poissonian sources. The article on quant-ph
 Relations between Entanglement Witnesses and Bell Inequalities P. Hyllus, O. Guehne, D. Bruss, M. Lewenstein quant-ph/0504079, Phys. Rev. A 72, 012321 (2005) April 11, 2005 Bell inequalities, considered within quantum mechanics, can be regarded as non-optimal witness operators. We discuss the relationship between such Bell witnesses and general entanglement witnesses in detail for the Bell inequality derived by Clauser, Horne, Shimony, and Holt (CHSH). We derive bounds on how much an optimal witness has to be shifted by adding the identity operator to make it positive on all states admitting a local hidden variable model. In the opposite direction, we obtain tight bounds for the maximal proportion of the identity operator that can be subtracted from such a CHSH witness, while preserving the witness properties. Finally, we investigate the structure of CHSH witnesses directly by relating their diagonalized form to optimal witnesses of two different classes. The article on quant-ph
 Entanglement and non-locality are different resources Nicolas Brunner, Nicolas Gisin and Valerio Scarani New J. Phys. 7 88 (2005), ePrint:quant-ph/0412109 April 5, 2005 Bell's theorem states that, to simulate the correlations created by measurement on pure entangled quantum states, shared randomness is not enough: some "non-local" resources are required. It has been demonstrated recently that all projective measurements on the maximally entangled state of two qubits can be simulated with a single use of a "non-local machine". We prove that a strictly larger amount of this non-local resource is required for the simulation of pure non-maximally entangled states of two qubits $\ket{\psi(\alpha)}= \cos\alpha\ket{00}+\sin\alpha\ket{11}$ with $0<\alpha\lesssim\frac{\pi}{7.8}$. The article on quant-ph
 Optimal Time-Reversal of Multi-phase Equatorial States Francesco Buscemi, Giacomo Mauro D'Ariano, Chiara Macchiavello quant-ph/0504016 April 4, 2005 Even though the time-reversal is unphysical (it corresponds to the complex conjugation of the density matrix), for some restricted set of states it can be achieved unitarily, typically when there is a common de-phasing in a n-level system. However, in the presence of multiple phases (i. e. a different de-phasing for each element of an orthogonal basis occurs) the time reversal is no longer physically possible. In this paper we derive the channel which optimally approaches in fidelity the time-reversal of multi-phase equatorial states in arbitrary (finite) dimension. We show that, in contrast to the customary case of the Universal-NOT on qubits (or the universal conjugation in arbitrary dimension), the optimal phase covariant time-reversal for equatorial states is a nonclassical channel, which cannot be achieved via a measurement/preparation procedure. Unitary realizations of the optimal time-reversal channel are given with minimal ancillary dimension, exploiting the simplex structure of the optimal maps. The article on quant-ph
 Probabilistic Model--Checking of Quantum Protocols Simon Gay, Rajagopal Nagarajan, Nikolaos Papanikolaou quant-ph/0504007, submitted for publication April 1, 2005 We establish fundamental and general techniques for formal verification of quantum protocols. Quantum protocols are novel communication schemes involving the use of quantum-mechanical phenomena for representation, storage and transmission of data. As opposed to quantum computers, quantum communication systems can and have been implemented using present-day technology; therefore, the ability to model and analyse such systems rigorously is of primary importance. While current analyses of quantum protocols use a traditional mathematical approach and require considerable understanding of the underlying physics, we argue that automated verification techniques provide an elegant alternative. We demonstrate these techniques through the use of PRISM, a probabilistic model-checking tool. Our approach is conceptually simpler than existing proofs, and allows us to disambiguate protocol definitions and assess their properties. It also facilitates detailed analyses of actual implemented systems. We illustrate our techniques by modelling a selection of quantum protocols (namely superdense coding, quantum teleportation, and quantum error correction) and verifying their basic correctness properties. Our results provide a foundation for further work on modelling and analysing larger systems such as those used for quantum cryptography, in which basic protocols are used as components. The article on quant-ph
 Eavesdropping without quantum memory H. Bechmann-Pasquinucci quant-ph/0504003 April 1, 2005 In quantum cryptography the optimal eavesdropping strategy requires that the eavesdropper uses quantum memories in order to optimize her information. What happens if the eavesdropper has no quantum memory? It is shown that the best strategy is actually to adopt the simple intercept/resend strategy. The article on quant-ph
 Photon-bunching measurement after 2x25km of standard optical fibers M. Halder, S. Tanzilli, H. de Riedmatten, A. Beveratos, H. Zbinden, N. Gisin quant-ph/0408092, Phys. Rev. A 71, 042335 (2005) March 18, 2005 To show the feasibility of a long distance partial Bell-State measurement, a Hong-Ou-Mandel experiment with coherent photons is reported. Pairs of degenerate photons at telecom wavelength are created by parametric down conversion in a periodically poled lithium niobate waveguide. The photon pairs are separated in a beam-splitter and transmitted via two fibers of 25km. The wave-packets are relatively delayed and recombined on a second beam-splitter, forming a large Mach-Zehnder interferometer. Coincidence counts between the photons at the two output modes are registered. The main challenge consists in the trade-off between low count rates due to narrow filtering and length fluctuations of the 25km long arms during the measurement. For balanced paths a Hong-Ou-Mandel dip with a visibility of 47.3% is observed, which is close to the maximal theoretical value of 50% developed here. This proves the practicability of a long distance Bell state measurement with two independent sources, as e.g. required in an entanglement swapping configuration in the scale of tens of km. The article on quant-ph
 Optimal state estimation and cloning for equatorial quantum systems with arbitrary dimension Ch. Macchiavello International Journal of Quantum Information, vol 3, 57 (2005) March 1, 2005 We review the problem of optimal estimation of multiple phases for systems with arbitrary finite dimension and derive the optimal estimation fidelity for equatorial states. We present optimal phase covariant cloning transformations for d-dimensional systems, relating these results to the optimal estimation procedure. The article in Int.J.Q.Info
 Intercept-resend attacks in the Bennett-Brassard 1984 quantum key distribution protocol with weak coherent pulses Marcos Curty, Norbert Lütkenhaus quant-ph/0411041, Phys. Rev. A 71, 062301 (2005) Feb 20, 2005 Unconditional security proofs of the Bennett-Brassard protocol of quantum key distribution have been obtained recently. These proofs cover also practical implementations that utilize weak coherent pulses in the four signal polarizations. Proven secure rates leave open the possibility that new proofs or new public discussion protocols obtain larger rates over increased distance. In this paper we investigate limits to error rate and signal losses that can be tolerated by future protocols and proofs. The article on quant-ph
 Efficient Single Photon Detection by Quantum Dot Resonant Tunneling Diodes J.C. Blakesley, P. See, A.J. Shields, B.E. Kardyna, P. Atkinson, I. Farrer, and D.A. Ritchie Phys. Rev. Lett. 94, 067401 (2005) February 16, 2005 We demonstrate that the resonant tunnel current through a double-barrier structure is sensitive to the capture of single photoexcited holes by an adjacent layer of quantum dots. This phenomenon could allow the detection of single photons with low dark count rates and high quantum efficiencies. The magnitude of the sensing current may be controlled via the thickness of the tunnel barriers. Larger currents give improved signal to noise and allow sub-µs photon time resolution. The article on PRL
 An information-theoretic security proof for QKD protocols R. Renner, N. Gisin, B. Kraus quant-ph/0502064 and Phys. Rev. A 72, 012332 (2005) February 11, 2005 We present a new technique for proving the security of quantum key distribution (QKD) protocols. It is based on direct information-theoretic arguments and thus also applies if no equivalent entanglement purification scheme can be found. Using this technique, we investigate a general class of QKD protocols with one-way classical post-processing. We show that, in order to analyze the full security of these protocols, it suffices to consider collective attacks. Indeed, we give new lower and upper bounds on the secret-key rate which only involve entropies of two-qubit density operators and which are thus easy to compute. As an illustration of our results, we analyze the BB84, the six-state, and the B92 protocol with one-way error correction and privacy amplification. Surprisingly, the performance of these protocols is increased if one of the parties adds noise to the measurement data before the error correction. In particular, this additional noise makes the protocols more robust against noise in the quantum channel. The article on quant-ph
 Detecting two-party quantum correlations in quantum-key-distribution protocols Marcos Curty, Otfried Gühne, Maciej Lewenstein, and Norbert Lütkenhaus Phys. Rev. A 71, 022306 (2005) February 11, 2005 A necessary precondition for secure quantum key distribution is that sender and receiver can prove the presence of entanglement in a quantum state that is effectively distributed between them. In order to deliver this entanglement proof one can use the class of entanglement witness (EW) operators that can be constructed from the available measurements results. This class of EWs can be used to provide a necessary and sufficient condition for the existence of quantum correlations even when a quantum state cannot be completely reconstructed. The set of optimal EWs for two well-known entanglement-based (EB) schemes, the six-state and the four-state EB protocols, has been obtained recently [M. Curty et al., Phys. Rev. Lett. 92, 217903 (2004).] Here we complete these results, now showing specifically the analysis for the case of prepare and measure (PM) schemes. For this, we investigate the signal states and detection methods of the four-state and the two-state PM schemes. For each of these protocols we obtain a reduced set of EWs. More importantly, each set of EWs can be used to derive a necessary and sufficient condition to prove that quantum correlations are present in these protocols. The article in Phys.Rev.A
 An Automated Analysis of the Security of Quantum Key Distribution Rajagopal Nagarajan, Nikolaos Papanikolaou, Garry Bowen, Simon Gay cs.CR/0502048, submitted for publication February 9, 2005 This paper discusses the use of computer-aided verification as a practical means for analysing quantum information systems; specifically, the BB84 protocol for quantum key distribution is examined using this method. This protocol has been shown to be unconditionally secure against all attacks in an information-theoretic setting, but the relevant security proof requires a thorough understanding of the formalism of quantum mechanics and is not easily adaptable to practical scenarios. Our approach is based on probabilistic model-checking; we have used the PRISM model-checker to show that, as the number of qubits transmitted in BB84 is increased, the equivocation of the eavesdropper with respect to the channel decreases exponentially. We have also shown that the probability of detecting the presence of an eavesdropper increases exponentially with the number of qubits. The results presented here are a testament to the effectiveness of the model-checking approach for systems where analytical solutions may not be possible or plausible. The article in arXiv.org/cs.CR
 Lower and upper bounds on the secret key rate for QKD protocols using one--way classical communication B. Kraus, N. Gisin, R. Renner quant-ph/0410215 February 6, 2005 We investigate a general class of quantum key distribution (QKD) protocols using one-way classical communication. We show that full security can be proven by considering only collective attacks. We derive computable lower and upper bounds on the secret key rate of those QKD protocol involving only entropies of two--qubit density operators. As an illustration of our results, we determine new bounds for the BB84, the six-state, and the B92 protocol. We show that in all these cases the first classical processing that the legitimate partners should apply consists in adding noise. This is precisely why any entanglement based proof would generally fail here. The article in quant-ph
 Four-photon correction in two-photon Bell experiments Valerio Scarani, Hugues de Riedmatten, Ivan Marcikic, Hugo Zbinden, Nicolas Gisin Eur. Phys. J. D 32, 129 (2005), ePrint: quant-ph/0407189 January 26, 2005 Correlated photons produced by spontaneous parametric down-conversion are an essential tool for quantum communication, especially suited for long-distance connections. To have a reasonable count rate after all the losses in the propagation and the filters needed to improve the coherence, it is convenient to increase the intensity of the laser that pumps the non-linear crystal. By doing so, however, the importance of the four-photon component of the down-converted field increases, thus degrading the quality of two-photon interferences. In this paper, we present an easy derivation of this nuisance valid for any form of entanglement generated by down-conversion, followed by a full study of the problem for time-bin entanglement. We find that the visibility of two-photon interferences decreases as V=1-2\rho, where \rho is, in usual situations, the probability per pulse of creating a detectable photon pair. In particular, the decrease of V is independent of the coherence of the four-photon term. Thanks to the fact that \rho can be measured independently of V, the experimental verification of our prediction is provided for two different configuration of filters. The article on quant-ph
 Continuous operation of a one-way quantum key distribution system over installed telecom fibre Zhiliang Yuan and A. J. Shields Opt. Express 13, 660-665 (2005) 25 January 2005 We demonstrate a robust, compact and automated quantum key distribution system, based upon a one-way Mach-Zender interferometer, which is actively compensated for temporal drifts in the photon phase and polarization. The system gives a superior performance to passive compensation schemes with an average quantum bit error rate of 0.87% and a duty cycle of 99.6% for a continuous quantum key distribution session of 19 hours over a 20.3km installed telecom fibre. The results suggest that actively compensated QKD systems are suitable for practical applications. The article on Optics Express
 Impossibility of perfect quantum sealing of classical information H. Bechmann-Pasquinucci, G. M. D'Ariano, C. Macchiavello quant-ph/0501073 and Int. J. Quant. Inf. 3 435-440 (2005) January 14, 2005 Sealing information means making it publicly available, but with the possibility of knowing if it has been read. Commenting on [1], we will show that perfect quantum sealing is not possible for perfectly retrievable information, due to the possibility of performing a perfect measurement without disturbance, even on unknown states. The measurement is a collective one, and this makes the protocol of quantum sealing very interesting as the only example of the power of collective measurements in breaking security. The article on quant-ph
 Robustness of the BB84 quantum key distribution protocol against general coherent attacks Georgios M. Nikolopoulos, Gernot Alber quant-ph/0403148 January 11, 2005 It is demonstrated that for the entanglement-based version of the Bennett-Brassard (BB84) quantum key distribution protocol, Alice and Bob share provable entanglement if and only if the estimated qubit error rate is below 25% or above 75%. In view of the intimate relation between entanglement and security, this result sheds also new light on the unconditional security of the BB84 protocol in its original prepare-and-measure form. In particular, it indicates that for small qubit error rates 25% is the ultimate upper security bound for any prepare-and-measure BB84-type QKD protocol. On the contrary, for qubit error rates between 25% and 75% we demonstrate that the correlations shared between Alice and Bob can always be explained by separable states and thus, no secret key can be distilled in this regime. The article on quant-ph
 Distributing entanglement and single photons through an intra-city, free-space quantum channel K. J. Resch, M. Lindenthal, B. Blauensteiner, H.R. Böhm, A. Fedrizzi, C. Kurtsiefer, A. Poppe, T. Schmitt-Manderbach, M. Taraba, R. Ursin, P. Walther, H. Weier, H. Weinfurter, A. Zeilinger Opt. Express 13, 202-209 (2005) January 10, 2005 We have distributed entangled photons directly through the atmosphere to a receiver station 7.8 km away over the city of Vienna, Austria at night. Detection of one photon from our entangled pairs constitutes a triggered single photon source from the sender. With no direct time-stable connection, the two stations found coincidence counts in the detection events by calculating the cross-correlation of locally-recorded time stamps shared over a public internet channel. For this experiment, our quantum channel was maintained for a total of 40 minutes during which time a coincidence lock found approximately 60000 coincident detection events. The polarization correlations in those events yielded a Bell parameter, S=2.27±0.019, which violates the CHSH-Bell inequality by 14 standard deviations. This result is promising for entanglement-based freespace quantum communication in high-density urban areas. It is also encouraging for optical quantum communication between ground stations and satellites since the length of our free-space link exceeds the atmospheric equivalent. The article on Optics Express
 On feedback and the classical capacity of a noisy quantum channel Bowen, G. Nagarajan, R. IEEE Transactions on Information Theory, vol 51, issue 1, 320- 324 (2005) January 10, 2005 In Shannon information theory, the capacity of a memoryless communication channel cannot be increased by the use of feedback from receiver to sender. In this correspondence, the use of classical feedback is shown to provide no increase in the unassisted classical capacity of a memoryless quantum channel when feedback is used across nonentangled input states, or when the channel is an entanglement-breaking channel. This gives a generalization of the Shannon theory for certain classes of feedback protocols when transmitting through noisy quantum communication channels. The article on IEEE Transactions on Inf.Theory
 Surface acoustic wave-induced electroluminescence intensity oscillation in planar light-emitting devices Marco Cecchini, Vincenzo Piazza, Fabio Beltram, D. G. Gevaux, M. B. Ward, A. J. Shields, H. E. Beere, D. A. Ritchie cond-mat/0501136 (2005), published in Appl.Phys.Lett vol. 86, 241107 (2005) Jan 7, 2005 Electroluminescence emission from surface acoustic wave-driven light-emitting diodes (SAWLEDs) is studied by means of time-resolved techniques. We show that the intensity of the SAW-induced electroluminescence is modulated at the SAW frequency (~1 GHz), demonstrating electron injection into the p-type region synchronous with the SAW wavefronts. The article on cond-mat
 Unconditional quantum cloning of coherent states with linear optics Ulrik L. Andersen, Vincent Josse and Gerd Leuchs quant-ph/0501005, Phys. Rev. Lett. Volume 94, Number 24, p. 240503 January 3, 2005 A scheme for optimal Gaussian cloning of optical coherent states is proposed and experimentally demonstrated. Its optical realization is based entirely on simple linear optical elements and homo- dyne detection. The optimality of the presented scheme is only limited by detection inefficiencies. Experimentally we achieved a cloning fidelity of about 65%, which almost touches the optimal value of 2/3. The article on quant-ph
 Complete hierarchies of efficient approximations to problems in entanglement theory J. Eisert, P. Hyllus, O. Guehne, M. Curty quant-ph/0407135 Dec 21, 2004 We investigate several problems in entanglement theory from the perspective of convex optimization. This list of problems comprises (A) the decision whether a state is multi-party entangled, (B) the minimization of expectation values of entanglement witnesses with respect to pure product states, (C) the closely related evaluation of the geometric measure of entanglement to quantify pure multi-party entanglement, (D) the test whether states are multi-party entangled on the basis of witnesses based on second moments and on the basis of linear entropic criteria, and (E) the evaluation of instances of maximal output purities of quantum channels. We show that these problems can be formulated as certain optimization problems: as polynomially constrained problems employing polynomials of degree three or less. We then apply very recently established known methods from the theory of semi-definite relaxations to the formulated optimization problems. By this construction we arrive at a hierarchy of efficiently solvable approximations to the solution, approximating the exact solution as closely as desired, in a way that is asymptotically complete. For example, this results in a hierarchy of novel, efficiently decidable sufficient criteria for multi-particle entanglement, such that every entangled state will necessarily be detected in some step of the hierarchy. Finally, we present numerical examples to demonstrate the practical accessibility of this approach. The article on quant-ph
 Distributed Quantum Dense Coding D. Bruß, G. M. D'Ariano, M. Lewenstein, C. Macchiavello, A. Sen(De), and U. Sen Phys. Rev. Lett. 93, 210501 (2004) November 19, 2004 We introduce the notion of distributed quantum dense coding, i.e., the generalization of quantum dense coding to more than one sender and more than one receiver. We show that global operations (as compared to local operations) of the senders do not increase the information transfer capacity, in the case of a single receiver. For the case of two receivers, using local operations and classical communication, a nontrivial upper bound for the capacity is derived. We propose a general classification scheme of quantum states according to their usefulness for dense coding. In the bipartite case (for any dimensions), bound entanglement is not useful for this task. The article in Phys.Rev.Lett
 High-quality asynchronous heralded single-photon source at telecom wavelength Sylvain Fasel, Olivier Alibart, Sébastien Tanzilli, Pascal Baldi, Alexios Beveratos, Nicolas Gisin and Hugo Zbinden New J. Phys. 6 (2004) 163 Nov 12, 2004 We report on the experimental realization and characterization of an asynchronous heralded single-photon source based on spontaneous parametric down-conversion. Photons at 1550 nm are heralded as being inside a single-mode fibre with more than 60% probability, and the multi-photon emission probability is reduced by a factor of up to more than 500 compared to Poissonian light sources. These figures of merit, together with the choice of telecom wavelength for the heralded photons, are compatible with practical applications needing very efficient and robust single-photon sources. The article on New Journal of Physics
 Detecting quantum correlations for quantum key distribution Marcos Curty, Otfried Gühne, Maciej Lewensteinc and Norbert Lütkenhaus Proceedings accepted for the conference "Quantum Potics and Applications in Computing and Communications II" at the Phototnics Asia 2004 November 8, 2004 Practical quantum key distribution can be understood as a two-step procedure: in a first step two parties exchange quantum mechanical signals and perform measurements on them, in a second step auxiliary classical communication protocols are performed over an authenticated public channel to transform the data of the first step into an information-theoretic secure key. In this article we address the question of necessary conditions on the correlated (classical) data of the first step such that there can be a successful second step at all. As it turns out, a necessary condition is that these data, together with the knowledge about the physical set-up of sender and receiver, allow to establish a proof of effective entanglement between the two parties. We then demonstrate methods to systematically search for such a proof in basic settings, involving the 2-, 4-, and 6-state protocols. Link to pdf-file
 Intercept-resend attacks in the Bennett-Brassard 1984 quantum key distribution protocol with weak coherent pulses M. Curty, and N. Lütkenhaus quant-ph/0411041 November 05, 2004 Unconditional security proofs of the Bennett-Brassard protocol of quantum key distribution have been obtained recently. These proofs cover also practical implementations that utilize weak coherent pulses in the four signal polarizations. Proven secure rates leave open the possibility that new proofs or new public discussion protocols obtain larger rates over increased distance. In this paper we investigate limits to error rate and signal losses that can be tolerated by future protocols and proofs. The article on quant-ph
 Towards practical and fast Quantum Cryptography N. Gisin, G. Ribordy, H. Zbinden, D. Stucki, N. Brunner, V. Scarani quant-ph/0411022 November 03, 2004 We present a new protocol for practical quantum cryptography, tailored for an implementation with weak coherent pulses. The key is obtained by a very simple time-of-arrival measurement on the data line; an interferometer is built on an additional monitoring line, allowing to monitor the presence of a spy (who would break coherence by her intervention). Against zero-error attacks (the analog of photon-number-splitting attacks), this protocol performs as well as standard protocols with strong reference pulses: the key rate decreases only as the transmission t of the quantum channel. We present also two attacks that introduce errors on the monitoring line: the intercept-resend, and a coherent attack on two subsequent pulses. Finally, we sketch several possible variations of this protocol. The article on quant-ph
 Distribution of Time-Bin Entangled Qubits over 50 km of Optical Fiber I. Marcikic, H. de Riedmatten, W. Tittel, H. Zbinden, M. Legré, and N. Gisin Phys. Rev. Lett. 93, 180502 (2004) Oct 28, 2004 We report experimental distribution of time-bin entangled qubits over 50 km of optical fibers. Using actively stabilized preparation and measurement devices we demonstrate violation of the Clauser-Horne-Shimony-Holt Bell inequality by more than 15 standard deviations without removing the detector noise. In addition we report a proof-of principle experiment of quantum key distribution over 50 km of optical fibers using entangled photon. The article in Phys.Rev.Lett
 Quantum cloning without external control G. De Chiara, R. Fazio, C. Macchiavello, S. Montangero, G. M. Palma quant-ph/0410211 Oct 26, 2004 In this paper we present an approach to quantum cloning with unmodulated spin networks. The cloner is realized by a proper design of the network and a choice of the coupling between the qubits. We show that in the case of phase covariant cloner the XY coupling gives the best results. In the 1->2 cloning we find that the value for the fidelity of the optimal cloner is achieved, and values comparable to the optimal ones in the general N->M case can be attained. If a suitable set of network symmetries are satisfied, the output fidelity of the clones does not depend on the specific choice of the graph. We show that spin network cloning is robust against the presence of static imperfections. Moreover, in the presence of noise, it outperforms the conventional approach. In this case the fidelity exceeds the corresponding value obtained by quantum gates even for a very small amount of noise. Furthermore we show how to use this method to clone qutrits and qudits. By means of the Heisenberg coupling it is also possible to implement the universal cloner although in this case the fidelity is 10% off that of the optimal cloner. The article on quant-ph
 Security of Quantum Key Distribution with Coherent States and Homodyne Detection S. Iblisdir, G. Van Assche, and N. J. Cerf Phys. Rev. Lett. 93, 170502 (2004) Oct 19, 2004 We assess the security of a quantum key distribution protocol relying on the transmission of Gaussian-modulated coherent states and homodyne detection. This protocol is shown to be equivalent to an entanglement purification protocol using CSS codes followed by key extraction, and is thus secure against any eavesdropping strategy. The article on Phys.Rev.Lett
 Complete hierarchies of efficient approximations to problems in entanglement theory J. Eisert, P. Hyllus, O. Guehne, M. Curty quant-ph/0407135; accepted for publication in Physical Review A October 11, 2004 We investigate several problems in entanglement theory from the perspective of convex optimization. This list of problems comprises (A) the decision whether a state is multi-party entangled, (B) the minimization of expectation values of entanglement witnesses with respect to pure product states, (C) the closely related evaluation of the geometric measure of entanglement to quantify pure multi-party entanglement, (D) the test whether states are multi-party entangled on the basis of witnesses based on second moments and on the basis of linear entropic criteria, and (E) the evaluation of instances of maximal output purities of quantum channels. We show that these problems can be formulated as certain optimization problems: as polynomially constrained problems employing polynomials of degree three or less. We then apply very recently established known methods from the theory of semi-definite relaxations to the formulated optimization problems. By this construction we arrive at a hierarchy of efficiently solvable approximations to the solution, approximating the exact solution as closely as desired, in a way that is asymptotically complete. For example, this results in a hierarchy of novel, efficiently decidable sufficient criteria for multi-particle entanglement, such that every entangled state will necessarily be detected in some step of the hierarchy. Finally, we present numerical examples to demonstrate the practical accessibility of this approach. The article on quant-ph
 Time coding protocols for quantum key distribution Thierry Debuisschert and William Boucher Phys. Rev. A 70, 042306 (2004) October 8, 2004 We propose quantum key distribution protocols based on coherent single-photon optical pulses with duration T and with minimum time-frequency uncertainty. The pulses are sent with possible delays (e.g., 0, T/2) that are used to code the information (e.g., bit 0, bit 1) and that are shorter than the pulse width. Therefore, the time detection of the photons may result in a ambiguity of the delay evaluation for a potential eavesdropper. The duration of the received pulses is controlled thanks to a contrast measurement using an interferometer. A quantum formalism is given, allowing us to model the transmission of the key and the consequences of a possible eavesdropping. Two protocols are proposed and discussed. The first one involves two states and is limited to channels with losses lower than 50%. The second one involves four states, which prevents the eavesdropper from exploiting the losses of the line. The security of each protocol is evaluated as a function of channel losses, quantum bit error rate, and contrast loss in the case of intercept-resend attacks. It is applied to situations where photocounters dark counts are the main limitation of the system. The resulting maximum propagation distance allowing secure communication is evaluated. The article on Phys.Rev.A
 Quantum entanglement can be simulated without communication N. J. Cerf, N. Gisin, S. Massar, S. Popescu quant-ph/0410027 October 4, 2004 It has recently been shown that all causal correlations between two parties which output each one bit, a and b, when receiving each one bit, x and y, can be expressed as convex combinations of local correlations (i.e., correlations that can be simulated with local random variables) and non-local correlations of the form a+b=xy mod 2. We show that a single instance of the latter elementary non-local correlation suffices to simulate exactly all possible projective measurements that can be performed on the singlet state of two qubits, with no communication needed at all. This elementary non-local correlation thus defines some unit of non-locality, which we call a nl-bit. The article on quant-ph
 Byzantine agreement with two quantum-key-distribution setups S. Iblisdir and N. Gisin Phys. Rev. A 70, 034306 (2004), ePrint: quant-ph/0405167 Sept. 29, 2004 It is pointed out that two separated quantum channels and three classical authenticated channels are sufficient resources to achieve detectable broadcast. The article on quant-ph
 Embedded Quantum Cryptography on RTS Boards Roland Lieger, Thomas Lorünser, Gerhard Humer, Florian Schupfer Proceedings of 12th European Signal Processing Conference - EUSIPCO 2004 Vienna, 6.-10. Sept. 2004, ISBN 3-200-00148-8 Sept., 2004 Quantum cryptography is the only system for key generation that can provably not be tampered by an eavesdropper without being noticed. While its theoretical basis is already reasonably well understood, commercial application is hampered by the lack of ready-to-use embedded encryption systems. In this paper we will describe our hardware solution, developed for setting up an application oriented quantum cryptography embedded-system. Link to EUSIPCO 2004-Conference
 Long distance entanglement swapping with photons from separated sources H. de Riedmatten, I. Marcikic, J.A.W. van Houwelingen, W. Tittel, H. Zbinden, N. Gisin quant-ph/0409093 Sept 15, 2004 We report the first experimental realization of entanglement swapping over large distances in optical fibers. Two photons separated by more than two km of optical fibers are entangled, although they never directly interacted. We use two pairs of time-bin entangled qubits created in spatially separated sources and carried by photons at telecommunication wavelengths. A partial Bell state measurement is performed with one photon from each pair which projects the two remaining photons, formerly independent onto an entangled state. A visibility high enough to violate a Bell inequality is reported, after both photons have each travelled through 1.1 km of optical fiber. The article on quant-ph
 Detecting two-party quantum correlations in quantum key distribution protocols M. Curty, O. Gühne, M. Lewenstein and Norbert Lütkenhaus quant-ph 0409047 September 08, 2004 A necessary precondition for secure quantum key distribution (QKD) is that sender and receiver can prove the presence of entanglement in a quantum state that is effectively distributed between them. In order to deliver this entanglement proof one can use the class of entanglement witness (EW) operators that can be constructed from the available measurements results. This class of EWs can be used to provide a necessary and sufficient condition for the existence of quantum correlations even when a quantum state cannot be completely reconstructed. The set of optimal EWs for two well-known entanglement based (EB) schemes, the 6-state and the 4-state EB protocols, has been obtained recently [M. Curty et al., Phys. Rev. Lett. 92, 217903 (2004)]. Here we complete these results, now showing specifically the analysis for the case of prepare&measure (P&M) schemes. For this, we investigate the signal states and detection methods of the 4-state and the 2-state P&M schemes. For each of these protocols we obtain a reduced set of EWs. More importantly, each set of EWs can be used to derive a necessary and sufficient condition to prove that quantum correlations are present in these protocols. The article on quant-ph
 Photon-induced conductance steps and in situ modulation of disorder in mesoscopic electron systems N. S. Beattie, B. E. Kardyna, A. J. Shields, I. Farrer, D. A. Ritchie, and M. Pepper Phys. Rev. B 70, 081304(R) (2004) August 23, 2004 We study the conductance of a disordered two-dimensional electron system (2DES) at a mesoscopic length scale in which the localization length can be controlled and varied during an experiment. The localization is induced by a quantum dot layer adjacent to the 2DES, whose charge occupancy can be controlled optically. Under illumination the 2DES conductance increases in sharp steps due to the discharging of individual dots by single photo-excited holes. As the 2DES localization length increases, electron transport evolves from hopping through a small network to direct tunneling across the sample. The article in Phys.Rev.Lett
 Practical Quantum Key Distribution with Polarization-Entangled Photons A. Poppe, A. Fedrizzi, T. Loruenser, O. Maurhardt, R. Ursin, H. R. Boehm, M. Peev, M. Suda, C. Kurtsiefer, H. Weinfurter, T. Jennewein, A. Zeilinger Opt. Express 12, 3865-3871 (2004) August 09, 2004 We present an entangled-state quantum cryptography system that operated for the first time in a real world application scenario. The full key generation protocol was performed in real time between two distributed embedded hardware devices, which were connected by 1.45 km of optical fiber, installed for this experiment in the Vienna sewage system. The generated quantum key was immediately handed over and used by a secure communication application. The article on Optics Express
 Quantum correlations for quantum key distribution protocols M. Curty, O. Gühne, M. Lewenstein and N. Lütkenhaus Proceedinges of the 7th International Conference on Quantum Communication, Measurement and Computing QCMC 2004 in Glasgow, UK, Pages 307-311, American Institute of Physics, New York, 2004 July 25-29, 2004 A necessary precondition for secure quantum key distribution (QKD) is that sender and receiver can prove the presence of entanglement in a quantum state that is effectively distributed between them. In order to deliver this entanglement proof one can use the class of entanglement witness operators that can be constructed from the available measurements results. This criterion provides a necessary and sufficient condition for the existence of quantum correlations even when a quantum state cannot be completely reconstructed. We apply such analysis to three well-known qubit-based QKD protocols, namely the 6-state, the 4-state and the 2-state protocols. Link to pdf-file
 A source of entangled photon-pairs: optimizing emission in two quasi-phasematched crystals D. Ljunggren, M. Teneger, M. Pelton and P. Marsden Proceedings of the 7th International Conference on Quantum Communication, Measurement and Computing, Glasgow, UK, July 24-29 (2004), ISBN 0-7354-0216-7 (published by the American Institute of Physics) July 24-29, 2004 We extend our earlier results 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 1/sqrt(2)(HH + VV) 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 microns. With these focussing conditions the computer calculations give the lowest M2 value for the emission mode, M^2 = 3, and the best possible fiber coupling efficiency = 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 45 : 94 2%.
 Lateral n-p junction for acoustoelectric nanocircuits T. Hosey, V. Talyanskii, S.Vijendran, G.A.C Jones, M.B.Ward, D.C Unit, C.E Norman and A.J. Shields Applied Physics Letters, 85, 491-493 (2004) July 19, 2004 We report the experimental realization of a device comprising a lateral n–p junction grown by focused-ion molecular-beam epitaxy and a transducer to generate a surface acoustic wave. Acoustic charge transport across the junction and the accompanying photon emission are demonstrated. This type of light-emitting diode is suitable for integration into acoustoelectric nanocircuits in which quasione-dimensional semiconductor channels serve as "wires" through which packets of charge are transported by surface acoustic waves. The diode provides a means by which to extend the functionality of acoustoelectric nanocircuits into the optical domain. ©2004 American Institute of Physics The article on Applied Physics Letters
 Cryptanalysis of a Practical Quantum Key Distribution With Polarization-Entangled Photons Thomas Beth, Joern Mueller-Quade, Rainer Steinwandt quant-ph/0407130 July 16, 2004 Recently, a quantum key exchange protocol has been described, which served as basis for securing an actual bank transaction by means of quantum cryptography [quant-ph/0404115]. Here we show, that the authentication scheme applied is insecure in the sense that an attacker can provoke a situation where initiator and responder of a key exchange end up with different keys. Moreover, it may happen that an attacker can decrypt a part of the plaintext protected with the derived encryption key. The article on quant-ph
 A Novel Protocol-Authentication Algorithm Ruling Out a Man-in-the-Middle Attack in Quantum Cryptography M. Peev, M. Nölle, O. Maurhardt, T. Lorünser, M. Suda, A. Poppe, R. Ursin, A. Fedrizzi, A. Zeilinger quant-ph0407131 (2004), published in Int.J.Quantum Inform. 3 (2005), 225-232 July 16, 2004 In this work we review the security vulnerability of Quantum Cryptography with respect to "man-in-the-middle attacks" and the standard authentication methods applied to counteract these attacks. We further propose a modified authentication algorithm which features higher efficiency with respect to consumption of mutual secret bits. The article on quant-ph
 Optimal State Estimation and cloning for equatorial quantum systems with arbitrary dimension Chiara Macchiavello International Journal of Quantum Information, Vol. 3, No. 1 (2005) 57-63 July 6, 2004 We review the problem of optimal estimation of multiple phases for systems with arbitrary finite dimension and derive the optimal estimation fidelity for equatorial states. We present optimal phase covariant cloning transformations for d-dimensional systems, relating these results to the optimal estimation procedure. Link to the article
 Continuous-variable quantum key distribution using polarization encoding and post selection S. Lorenz, N. Korolkova and G. Leuchs Appl.Phys.B 79, 273-277 (2004) June 22, 2004 We present an experimental demonstration of a quantum key distribution protocol using coherent polarization states. Post selection is used to ensure a low error rate and security against beam-splitting attacks even in the presence of high losses. Signal encoding and readout in polarization bases avoids the difficult task of sending a local oscillator with the quantum channel. This makes our setup robust and easy to implement. A shared key was established for losses up to 64%. The article in Appl.Phys.B
 Side-Information Coding with Turbo Codes and its Application to Quantum Key Distribution Kim-Chi Nguyen, Gilles Van Assche, Nicolas J. Cerf Proc. of International Symposium on Information Theory and its Applications (ISITA); ePrint at arXiv/cs.IT June 1, 2004 Turbo coding is a powerful class of forward error correcting codes, which can achieve performances close to the Shannon limit. The turbo principle can be applied to the problem of side-information source coding, and we investigate here its application to the reconciliation problem occurring in a continuous-variable quantum key distribution protocol. The article on arXiv/cs.IT
 New intensity and visibility aspects of a double loop neutron interferometer M. Suda, H. Rauch, M. Peev quant-ph0405091 (2004) May 17, 2004 Various phase shifters and absorbers can be put into the arms of a double loop neutron interferometer. The mean intensity levels of the forward and diffracted beams behind an empty four plate interferometer of this type have been calculated. It is shown that the intensities in the forward and diffracted direction can be made equal using certain absorbers. In this case the interferometer can be regarded as a 50/50 beam splitter. Furthermore the visibilities of single and double loop interferometers are compared to each other by varying the transmission in the first loop using different absorbers. It can be shown that the visibility becomes exactly 1 using a phase shifter in the second loop. In this case the phase shifter in the second loop must be strongly correlated to the transmission coefficient of the absorber in the first loop. Using such a device homodyne-like measurements of very weak signals should become possible. The article on quant-ph
 Quantum key distribution over 30km of standard fiber using energy-time entangled photon pairs: a comparison of two chromatic dispersion reduction methods Sylvain Fasel, Nicolas Gisin, Grégoire Ribordy, Hugo Zbinden Eur. Phys. J. D 30, 143-148 (2004), ePrint: quant-ph/0403144 March 19, 2004 We present a full implementation of a quantum key distribution system using energy-time entangled photon pairs and functioning with a 30km standard telecom fiber quantum channel. Two bases of two orthogonal states are implemented and the setup is quite robust to environmental constraints such as temperature variation. Two different ways to manage chromatic dispersion in the quantum channel are discussed. The article on quant-ph
 Quantum Key Distribution: How Do We Know It's Secure? Norbert Lütkenhaus Optics & Photonics News, Vol 15 No. 3 March 2004 Quantum key distribution - the creation of secret keys from quantum mechanical correlations - is an example of how physical methods can be used to solve problems in classical information theory. The author describes the basic principles that can be used to confirm the security of the quantum key distribution systems now being marketed to banks, governments and network service providers. The article in Optics & Photonics News