Wednesday, January 26, 2005

[1.26.2005] Creation of a polarization W state using optical fibre multiports

Journal of Modern Optics
Issue: Volume 52, Number 5 / March 20, 2005
Pages: 755 - 761
URL: JMO52.0755, 2005
DOI: 10.1080/09500340412331289565

Creation of a polarization W state using optical fibre multiports

Bao-Sen Shi and Akihisa Tomita
A1 Imai Quantum Computation and Information Project NEC Tsukuba Laboratories ERATO Japan Science and Technology Agency (JST) Fundamental Research Laboratories NEC 34 Miyukigaoka Tsukuba Ibaraki 305-8501 Japan

A scheme for experimental creation of a polarization W state is reported in this paper. This scheme is based on a common commercial multiport fibre coupler and single photon source and it is feasible using current technology. This scheme has a higher efficiency and can be easily extended to generate a multiphoton W state with a photon number larger than 4.

[1.26.2005] Quantum relays for long distance quantum cryptography

Journal of Modern Optics
Issue: Volume 52, Number 5 / March 20, 2005
Pages: 735 - 753

URL: JMO52.0735, 2005
Linking Options

DOI: 10.1080/09500340412331283633
Quantum relays for long distance quantum cryptography
Daniel Collins , Nicolas Gisin , Hugues De Riedmatten
A1 Group of Applied Physics University of Geneva 20 rue de l'Ecole-de-M├ędecine CH-1211 Geneva 4 Switzerland
Quantum cryptography is on the verge of commercial application. One of its greatest limitations is over long distance--secret key rates are low and the longest fibre over which any key has been exchanged is currently 100km. We investigate the quantum relay, which can increase the maximum distance at which quantum cryptography is possible. The relay splits the channel into sections and sends a different photon across each section, increasing the signal-to-noise ratio. The photons are linked as in teleportation, with entangled photon pairs and Bell measurements. We show that such a scheme could allow cryptography over hundreds of kilometres with today's detectors. It could not, however, improve the rate of key exchange over distances where the standard single section scheme already works. We also show that reverse key reconciliation, previously used in continuous variable quantum cryptography, gives a secure key over longer distances than forward key reconciliation.

[1.26.2005] Faked states attack on quantum cryptosystems

Journal of Modern Optics
Volume 52, Number 5 / March 20, 2005
691 - 705

URL: JMO52.0691,2005
DOI: 10.1080/09500340410001730986

Faked states attack on quantum cryptosystems
Vadim Makarov and Dag R. Hjelme
A1 Department of Electronics and Telecommunications Norwegian University of Science and Technology (NTNU) NO-7491 Trondheim Norway
A new type of attack on quantum cryptography systems is proposed. In this attack, Eve utilizes various optical imperfections in Bob's scheme and constructs light pulses so that Bob does not distinguish his detection results from normal, whereas they give Bob the basis and bit value chosen at Eve's discretion. Applying this attack to systems with passive basis choice on Bob's side is considered. Also, a general workflow of breaking into a running quantum cryptolink using this or Trojan horse attack is discussed.

Friday, January 07, 2005

[1/6/2005]Auto-compensating differential phase shift quantum key distribution

Authors: Xiaohong Han, Guang Wu, Chunyuan Zhou, Heping Zeng

We propose an auto-compensating differential phase shift scheme for quantum key distribution with a high key-creation efficiency, which skillfully makes use of automatic alignment of the photon polarization states in optical fiber with modified Michelson interferometers composed of unequal arms with Faraday mirrors at the ends. The Faraday-mirrors-based Michelson interferometers not only function as pulse splitters, but also enable inherent compensation of polarization mode dispersion in the optic-fiber paths at both Alice's and Bob's sites. The sequential pulses encoded by differential phase shifts pass through the quantum channel with the same polarization states, resulting in a stable key distribution immune to the polarization mode dispersion in the quantum channel. Such a system features perfect stability and higher key creation efficiency over traditional schemes.

Thursday, October 07, 2004

Secure Coherent-state Quantum Key Distribution Protocols with Efficient Reconciliation

Authors: G. Van Assche, S. Iblisdir, N. J. Cerf
Comments: REVTeX, 11 pages, 2 figures

We study the equivalence between a realistic quantum key distribution protocol using coherent states and homodyne detection and 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 coherent state, or equivalently for the realistic protocol, more than one secret key bit. The entanglement parameters are estimated using quantum tomography. We analyze the properties of the encoding scheme and investigate its application to the important case of the attenuation channel.

Friday, August 20, 2004

Practical quantum key distribution with polarization entangled photons

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.

Friday, August 06, 2004

Stability of Phase-modulated Quantum Key Distribution System

Title: Stability of Phase-modulated Quantum Key Distribution System
Authors: Zheng-Fu Han, Xiao-Fan Mo, You-Zhen Gui, Guang-Can Guo
Comments: 9 pages, 3 figues\Phase drift and random fluctuation of interference visibility in doubleunbalanced M-Z QKD system are observed and distinguished. It has been foundthat the interference visibilities are influenced deeply by the disturbance oftransmission fiber. Theory analysis shows that the fluctuation is derived fromthe envioronmental disturbance on polarization characteristic of fiber,especially including transmission fiber. Finally, stability conditions ofone-way anti-disturbed M-Z QKD system are given out, which provides atheoretical guide in pragmatic anti-disturbed QKD.\ ( , 372kb)

Tuesday, July 20, 2004

[20040720] CV attack 2

Title Security bounds for continuous variables quantum key distribution
Authors: Miguel Navascues, Antonio Acin
Comments: See also F. Grosshans, quant-ph/0407148

Security bounds for key distribution protocols using coherent and squeezed states and homodyne measurements are presented. These bounds refer to (i) general attacks and (ii) collective attacks where Eve interacts individually with the sent states, but delays her measurement until the end of the reconciliation process. For the case of a lossy line and coherent states, it is first proven that a secure key distribution is possible up to 1.9 dB of losses. For the second scenario, the security bounds are the same as for the completely incoherent attack.

[20040720] CV attack 1

Title: Collective attacks and unconditional security in continuous variable quantum key distribution
Authors: Frederic Grosshans

We present here an information theoretic study of Gaussian collective attacks on the continuous variable key distribution protocols based on Gaussian modulation of coherent states. These attacks, overlooked in previous security studies, give a finite advantage to the eavesdropper in the experimentally relevant lossy channel, but are not powerful enough to reduce the range of the reverse reconciliation protocols. Secret key rates are given for the ideal case where Bob performs optimal collective measurements, as well as for the realistic cases where he performs homodyne or heterodyne measurements. We also apply the generic security proof of Christiandl et. al. [quant-ph/0402131] to obtain unconditionally secure rates for these protocols.

Monday, July 19, 2004

Key distribution & Attack

Title: Cryptanalysis of a Practical Quantum Key Distribution With  Polarization-Entangled Photons
Authors: Thomas Beth and Joern Mueller-Quade and Rainer Steinwandt
Comments: 6 pages\\ 
 Recently, a quantum key exchange protocol has been described, which served asbasis for securing an actual bank transaction by means of quantum cryptography[quant-ph/0404115]. Here we show, that the authentication scheme applied isinsecure in the sense that an attacker can provoke a situation where initiatorand responder of a key exchange end up with different keys. Moreover, it mayhappen that an attacker can decrypt a part of the plaintext protected with thederived encryption key.\\
( ,  5kb)
------------------------------------------------------------------------------\Title: A Novel Protocol-Authentication Algorithm Ruling Out a Man-in-the-Middle  Attack in Quantum Cryptography
Authors: M. Peev, M. N\"olle, O. Maurhardt, T. Lor\"unser, M. Suda, A. Poppe,  R. Ursin, A. Fedrizzi and A. Zeilinger
Comments: 4 pages, submitted to the International Journal of Quantum  Information, Proceedings of the meeting "Foundations of Quantum Information",  Camerino, April 2004\\ 
In this work we review the security vulnerability of Quantum Cryptographywith respect to "man-in-the-middle attacks" and the standard authenticationmethods applied to counteract these attacks. We further propose a modifiedauthentication algorithm which features higher efficiency with respect toconsumption of mutual secret bits.\\
( ,  8kb)