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.
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.
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.
In this dissertation, it is realized a study of light polarization and its applications in quantum communications. Initially, the mathematical tools needed for single-photon and coherent states polarization analyses are presented: coherence matrix, Stokes parameters and polarization degree. Following, using numerical simulations, it is presented the variation of the degree of polarization of a single-photon pulse during depolarizing channel propagation. At last, the result of an experiment using coherent states, aiming to measure the degree of polarization after propagation in 200 m optical fiber is presented. The experiment is useful for determination of the parameter of the qubit depolarizing channel model. Since light polarization is easily changed when environment fluctuations are present during fiber propagation, optical schemes able to correct unpredictable polarization changes are studied. Such schemes are employed for error correction in quantum communication systems. Based on already proposed error correction schemes, two new schemes are proposed, one for systems that employs polarization of singlephoton light, and other for systems that employs polarization of two-mode coherent states, being the first active, that is, requiring a control and synchronism protocol, while the second is passive. At last, it is realized a security analysis of a quantum key distribution system, employing the proposed error correction scheme, when the same is under Fuch-Peres-Brandt attack. It is shown that using the proposed error correction scheme the presence of an eavesdropper monitoring the quantum channel is more visible.
Quantum key distribution is an application of quantum physics that allows two distant partners to securely distribute shared random secrets which they can use for secure message encryption, authentication or other communication tasks. In this thesis, I study and implement several concepts, protocols and their components, in particular single photon detectors. I evaluate its compatibility with fiber infrastructures with the aim to improve applicability, performance and profitability of quantum key distribution in different scenarios. I present superconducting single photon detectors implemented for long distance quantum communication, as well as semiconductor detectors employing rapid sine gating schemes. Wavelength-multiplexing of quantum and classical communication channels is demonstrated to stabilize a polarization-encoded quantum channel against birefringence fluctuations, and to realize quantum and encrypted communication over one fiber. An implementation of the coherent one-way protocol is presented which features a hardware distillation engine, wavelength-multiplexing, and an analysis of finite key effects and authentication costs.
Quantum key distribution (QKD) is a technology that allows two users to communicate with theoretically perfect security using standard optical fibres. This is possible by transmitting the key on single photons, meaning a measurement by an eavesdropper disturbs the system in a way observable to the legitimate parties. The technology has advanced since the first protocol proposed in 1984, to the stage where there are now many protocols that can be experimentally implemented. These protocols have allowed secure keys to be generated over distances greater than 400 km and with secure key rates over 10 Mbit/s. In a metropolitan QKD network, it would be desirable for as many users to be connected as possible. Unfortunately, each protocol comes with different requirements on the transmitter and receiver. Even within a single protocol, different clock rates can require individualised transmitter and receiver hardware. This prohibits users from communicating with all receivers, unless they have complex transmitters with hardware for many protocols. This thesis develops a transmitter for practical QKD that is able to adapt to a number of different protocols with no changes to the hardware. The transmitter works using optical injection locking, where a pulse preparation laser adopts the phase of a phase preparation laser. Controlling the phase and intensity of the pulses in this way removes the side channel ordinarily present with direct modulation, in that the phase, intensity and frequency simultaneously change in response to an applied current. The cavity-enhanced electro-optic effect allows for the first demonstration of sub-volt half-wave phase modulation at high clock rates. The transmitter successfully demonstrates phase encoding, intensity encoding and on-demand phase randomisation. This allows for the experimental realisation and direct comparison of different QKD protocols, including one that has never before been implemented due to experimental complexity. A stable intensity modulator is also developed, based on a Sagnac interferometer. This removes a side channel in QKD systems and integrates well with the directly-modulated quantum transmitter. This development also means that the transmitter can implement all current two-party QKD protocols based on weak coherent pulses. The transmitter has the potential to become the standard transmitter for future quantum communication networks due to its stability, versatility and power efficiency. The design could also be demonstrated on a photonic chip, making it compact enough to fit in small transmitter units.
Quantum key distribution (QKD) has been developed within the last decade that is provably secure against arbitrary computing power, and even against quantum computer attacks. Now there is a strong need of research to exploit this technology in the existing communication networks. In this paper we have presented various experimental results pertaining to QKD like Raw key rate and Quantum bit error rate (QBER). We found these results over 25 km single mode optical fiber. The experimental setup implemented the enhanced version of BB84 QKD protocol. Based upon the results obtained, we have presented a network design which can be implemented for the realization of large scale QKD networks. Furthermore, several new ideas are presented and discussed to integrate the QKD technique in the classical communication networks.
Quantum cryptography, or quantum key distribution (QKD), provides a means of unconditionally secure communication. The security is in principle based on the fundamental laws of physics. Security proofs show that if quantum cryptography is appropriately implemented, even the most powerful eavesdropper cannot decrypt the message from a cipher.
The implementations of quantum crypto-systems in real life may not fully comply with the assumptions made in the security proofs. Such discrepancy between the experiment and the theory can be fatal to the security of a QKD system. In this thesis we address a number of these discrepancies.
A perfect single-photon source is often assumed in many security proofs. However, a weak coherent source is widely used in a real-life QKD implementation. Decoy state protocols have been proposed as a novel approach to dramatically improve the performance of a weak coherent source based QKD implementation without jeopardizing its security. Here, we present the first experimental demonstrations of decoy state protocols. Our experimental scheme was later adopted by most decoy state QKD implementations.
In the security proof of decoy state protocols as well as many other QKD protocols, it is widely assumed that a sender generates a phase-randomized coherent state. This assumption has been enforced in few implementations. We close this gap in two steps: First, we implement and verify the phase randomization experimentally; second, we prove the security of a QKD implementation without the coherent state assumption.
In many security proofs of QKD, it is assumed that all the detectors on the receiver's side have identical detection efficiencies. We show experimentally that this assumption may be violated in a commercial QKD implementation due to an eavesdropper's malicious manipulation. Moreover, we show that the eavesdropper can learn part of the final key shared by the legitimate users as a consequence of this violation of the assumptions.