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Contribuições à análise de segurança de protocolos de criptografia quântica

Francisco Franklin Sousa Rios

Published Jun 29, 2018• 0 citations
OpenAlex

Abstract

This thesis can be divided in three parts: I) Security analysis of quantum bit commitment (QBC). II) Quantum-chaotic key distribution (QCKD) using multiphoton pulses. III) Homodyne detection of weak coherent states. In respect to the security analysis of quantum bit commitment, it was shown that if one takes into account the spectral width of the light pulses and the frequency dependence of optical quantum gates, the QBC, contrary to what was previously considered, is not unconditionally insecure. In what concerns the QCKD, the usage of coherent states with continuous phase forces the eavesdropper to use an attack based on quantum homodyne tomography. In this case, it was shown that the transmitter can use coherent states with mean photon number much larger than the value traditionally used in quantum key distribution QKD, and the QCKD will still be secure. At last, in the homodyne detection part, an experiment was realized showing the detection, by an optical receiver based on PIN photodiode, of an analog modulating signal carried by a strongly attenuated coherent state.

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This thesis can be divided in three parts: I) Security analysis of quantum bit commitment (QBC). II) Quantum-chaotic key distribution (QCKD) using multiphoton pulses. III) Homodyne detection of weak coherent states. In respect to the security analysis of quantum bit commitment, it was shown that if one takes into account the spectral width of the light pulses and the frequency dependence of optical quantum gates, the QBC, contrary to what was previously considered, is not unconditionally insecure. In what concerns the QCKD, the usage of coherent states with continuous phase forces the eavesdropper to use an attack based on quantum homodyne tomography. In this case, it was shown that the transmitter can use coherent states with mean photon number much larger than the value traditionally used in quantum key distribution QKD, and the QCKD will still be secure. At last, in the homodyne detection part, an experiment was realized showing the detection, by an optical receiver based on PIN photodiode, of an analog modulating signal carried by a strongly attenuated coherent state.
In this dissertation it is realized a study about quantum key distribution (QKD) in optical networks. Initially, a review of the theory of quantum key distribution and some of its implementations with strongly attenuated coherent states, as well a review of an optical system for physical encryptation using mesoscopic coherent states are realized. Following, it is analyzed the use of an error correction scheme in the physical encryptation system, and two new schemes for quantum key distribution are proposed. The first is a possible implementation of a hybrid system, that is, using weak and mesoscopic coherent states, for QKD and quantum authentication of classical messages. The second is an implementation of a QKD system based on relative phase modulation of sidebands frequency, using two RF carriers and an amplitude modulator in Alice and a phase modulator in Bob. After, an experimental realization of a simple QKD setup using polarization states of strongly attenuated coherent states for B92 protocol is presented. At last, it is realized an experimental characterization of an optical receiver for optical communication applications.
Quantum Key Distribution (QKD) provides a method for securely sharing a secret key between two distant parties, Alice and Bob, by leveraging the principles of quantum mechanics. This thesis focuses on Continuous-Variable (CV) QKD, a variant of QKD where information is encoded in the quadrature components of coherent light states, allowing for efficient and cost-effective detection using homodyne or heterodyne receivers. The thesis is divided into three main parts: an introduction to quantum cryptography concepts, the presentation of a CV-QKD protocol, and a detailed security proof of the protocol against different types of attacks. The security proof considers both finite and infinite symbol transmission and tackles various eavesdropping attacks, including individual, collective, and coherent attacks. The use of Gaussian states and operations provides a promising approach to enhance QKD security, particularly in the context of practical quantum cryptography. The final sections provide a mathematical framework for the optimization of secure key rates and strategies for addressing the photon-number cutoff assumption in practical implementations.
Quantum key distribution is a technique in which secret key bits are encoded into quantum states which are transmitted over a quantum channel, e.g. an optical link, so that the security is guaranteed by the laws of quantum physics. Most experimental realizations to date have relied on discrete protocols, involving ideally single-photons states (or, in practice, strongly attenuated light pulses) as well as single-photon detectors. In this chapter, we present an overview of the recent continuous-variable quantum cryptosystems, which instead rely on continuously-modulated Gaussian states (e.g. coherent states) and homodyne detection. The series of security proofs of these protocols against increasingly powerful attacks will be reviewed. A particular emphasis will be put on the op-timality of Gaussian attacks in this context, which holds provided that the second-order moments of the relevant variables are monitored.
The development of new quantum communication technologies for optical networks will enable inviolable protocols to exchange secure information. The first and more ro-bust among them is the quantum key distribution (QKD). Although in theory the security is proven unconditionally safe, the implementation in optical networks can still produce flaws due the non-ideal behavior of the optical and optoelectronic devices. In this direc-tion, this thesis presents two contributions to improve security of QKD systems. The first is the redesign of two QKD protocols to use two quantum states of light, the coherent and thermal state, with a polarization randomness. The protocols are the differential quadrature phase shift QKD and QKD using homodyne detection. The second contribution is the experimental characterization of the light emitted by avalanche photodiodes during a detection and its implication in the security of the BB84 QKD protocol.
In this dissertation it is realized a study about quantum key distribution (QKD) in optical networks. Initially, a review of the theory of quantum key distribution and some of its implementations with strongly attenuated coherent states, as well a review of an optical system for physical encryptation using mesoscopic coherent states are realized. Following, it is analyzed the use of an error correction scheme in the physical encryptation system, and two new schemes for quantum key distribution are proposed. The first is a possible implementation of a hybrid system, that is, using weak and mesoscopic coherent states, for QKD and quantum authentication of classical messages. The second is an implementation of a QKD system based on relative phase modulation of sidebands frequency, using two RF carriers and an amplitude modulator in Alice and a phase modulator in Bob. After, an experimental realization of a simple QKD setup using polarization states of strongly attenuated coherent states for B92 protocol is presented. At last, it is realized an experimental characterization of an optical receiver for optical communication applications.