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ABSTRACT: A theoretical investigation of quantum interference of photonic mul-tistates in simple devices like beam splitters, Mach–Zehnder interferometers and double-loop devices are presented. Variable transmission and reflection coefficients as well as variable phase shifts are included in order to calculate quantum states and mean photon numbers at the outputs. Various input states like Fock states and coherent states and a combination of both are considered as well as squeezed states. Two meth-ods are applied: The direct matrix method and the method of unitary representation. Remarkable results appear in a double-loop interferometer where for special phase shifts equal mean photon numbers in the three output ports are obtained provided certain input states are given. A computerized simulation of general networks using various input Fock states is presented. Multistate devices will be used in future linear quantum computation and quantum information processing schemes.
Quantum Information Processing 12/2012; 11. · 2.00 Impact Factor
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ABSTRACT: We derive analytical expressions for the single mode quantum field state at the individual output ports of a beam splitter when a single-photon Fock state and a coherent state are incident on the input ports. The output states turn out to be a statistical mixture between a displaced Fock state and a coherent state. Consequently we are able to find an analytical expression for the corresponding Wigner function. Because of the generality of our calculations the obtained results are valid for all passive and lossless optical four port devices. We show further how the results can be adapted to the case of the Mach–Zehnder interferometer. In addition we consider the case for which the single-photon Fock state is replaced with a general input state: a coherent input state displaces each general quantum state at the output port of a beam splitter with the displacement parameter being the amplitude of the coherent state.
Optics Communications 01/2011; 284:1907-1912. · 1.49 Impact Factor
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ABSTRACT: Quantum Key Distribution (QKD - also referred to as Quantum Cryptography) is a technique for secret key agreement. It has been shown that QKD rigged with Information-Theoretic Secure (ITS) authentication (using secret key) of the classical messages transmitted during the key distribution protocol is also ITS. Note, QKD without any authentication can trivially be broken by man-in-the-middle attacks. Here, we study an authentication method that was originally proposed because of its low key consumption; a two-step authentication that uses a publicly known hash function, followed by a secret strongly universal2 hash function, which is exchanged each round. This two-step authentication is not information-theoretically secure but it was argued that nevertheless it does not compromise the security of QKD. In the current contribution we study intrinsic weaknesses of this approach under the common assumption that the QKD adversary has access to unlimited resources including quantum memories. We consider one implementation of Quantum Cryptographic protocols that use such authentication and demonstrate an attack that fully extract the secret key. Even including the final key from the protocol in the authentication does not rule out the possibility of these attacks. To rectify the situation, we propose a countermeasure that, while not information-theoretically secure, restores the need for very large computing power for the attack to work. Finally, we specify conditions that must be satisfied by the two-step authentication in order to restore information-theoretic security.
SPIE 8189; 01/2011
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ABSTRACT: In this work, we respond to a comment by A. Abidin and J.-Å. Larsson on our previous paper, Int. J. Quant. Inf. 3 (2005) 225
International Journal of Quantum Information 10/2009; 07(07):1401. · 0.67 Impact Factor
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ABSTRACT: The rapid progress of QKD in theory and experiment has been reflected by a number of successful demonstrations during the last years. Many groups all over the world have put forward QKD-links, operating in the standard point-to-point mode. However, broad proliferation of QKD systems is slowed down by a number of road blocks: the point-to-point paradigm and correspondingly the quadratic scaling of the necessary initial secrets with the number of users, the question of integration in existing networks, high price and also by issues like missing standards. As a result, QKD appears still to be forced into a narrow niche market.
CLEO/Europe - EQEC 2009 - European Conference on Lasers and Electro-Optics and the European Quantum Electronics Conference; 06/2009
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ABSTRACT: The very recent demonstration of the SECOQC QKD-network convincingly extended single QKD-links to QKD-networks gaining new functionalities. The needed interfaces, protocols and node modules are explained.
CLEO/Europe - EQEC 2009 - European Conference on Lasers and Electro-Optics and the European Quantum Electronics Conference; 01/2009
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ABSTRACT: Integral identities for particular Bloch functions in finite periodic systems are derived. All following statements are proven for a finite domain consisting of an integer number of unit cells. It is shown that matrix elements of particular Bloch functions with respect to periodic differential operators vanish identically. The real valuedness, the time-independence and a summation property of the expectation values of periodic differential operators applied to superpositions of specific Bloch functions are derived. Comment: 10 pages
03/2007;
