Multiphoton Quantum Key Distribution (QKD) has recently been proposed to exchange the secret keys using the rotational of polarization over a multi-stage protocol. It has the ability to outperform the weaknesses of a single photon QKD by improving the generation of key rate and distance range. This paper investigates the theoretical aspects of multiphoton QKD protocol’s performance over free space optic (FSO) networks. The most common setup for quantum communication is the single-beam approach. However, the single-beam setup has limitations in terms of high geometrical loss. In this paper, the symmetry multiple-beam for quantum communication which is called as Multiphoton Quantum Communication-Multiple Beam (MQC-MB) is proposed to transmit the multiphoton from the sender to the receiver in order to minimize the impact of geometrical loss that is faced by the single-beam setup. The analysis was carried out through mathematical analysis by establishing the FSO quantum model with the effects of atmospheric and geometrical loss as well as considering atmospheric turbulence modeled by log-normal distribution. The design criteria of FSO, such as the transmitter, receiver, beam divergence, and diameter of apertures, are analytically investigated. The numerical results demonstrate that the MQC-MB outperforms the single-beam in terms of reducing channel loss by about 8 dB and works well under strong turbulence channel. Furthermore, the MQC-MB reduces the quantum bit error rate (QBER) and improves the secret key rate (SKR) as compared to the single-beam system even though the distance between the sender and receiver increases.
Multiphoton Quantum Key Distribution (QKD) has recently been proposed to exchange the secret keys using the rotational of polarization over a multi-stage protocol. It has the ability to outperform the weaknesses of a single photon QKD by improving the generation of key rate and distance range. This paper investigates the theoretical aspects of multiphoton QKD protocol’s performance over free space optic (FSO) networks. The most common setup for quantum communication is the single-beam approach. However, the single-beam setup has limitations in terms of high geometrical loss. In this paper, the symmetry multiple-beam for quantum communication which is called as Multiphoton Quantum Communication-Multiple Beam (MQC-MB) is proposed to transmit the multiphoton from the sender to the receiver in order to minimize the impact of geometrical loss that is faced by the single-beam setup. The analysis was carried out through mathematical analysis by establishing the FSO quantum model with the effects of atmospheric and geometrical loss as well as considering atmospheric turbulence modeled by log-normal distribution. The design criteria of FSO, such as the transmitter, receiver, beam divergence, and diameter of apertures, are analytically investigated. The numerical results demonstrate that the MQC-MB outperforms the single-beam in terms of reducing channel loss by about 8 dB and works well under strong turbulence channel. Furthermore, the MQC-MB reduces the quantum bit error rate (QBER) and improves the secret key rate (SKR) as compared to the single-beam system even though the distance between the sender and receiver increases.
In order to mitigate the problem of low transmission rate and limited communication distance in Quantum Communication (QCs), multiphoton over multi-stages approach has been proven to be a possible alternative to the conventional single photon approach. Multiphoton has the ability to improve the range of distances and key generation rate over multi-stages photon transmission. However, the determination of optimal mean photon numbers and number of stages in multiphoton approach remains a key challenge to make the protocols well utilized during their operations. Following this concern, three problems and their corresponding proposed solutions in this thesis are presented below: Firstly, the existing multiphoton approaches involve multiple photons to travel throughout a number of stage. Furthermore, extra time is required to update the polarization angle of optical device for encoding purposes. These conditions would result in an increase in the total transmission time of the photons to be transmitted over the quantum channel. Accordingly, a Hybrid M-Ary in Braided Single-Stage (HMBSS) approach by utilizing data compression concept is proposed to address these issues. In HMBSS, the sender is able to compress the secret message using Huffman encoding over the braided single-stage operation. This compression mechanism has reduced the number of bits required to represent a string of symbols, thereby reducing the time to encode the photons. The simulation experiments shows that HMBSS achieved promising result by 75.9% and 91.7% total average transmission time decrease as compared to the well-known Multiphoton-BSS, Multiphoton-M-ary and Multiphoton-TSIV. Secondly, providing strong authentication is the main focus in this research which aims to make multiphoton QC secure against variety types of attacks. Current authentication procedure in multiphoton QC requires public agreement to pre-share the authentication key and secret angles before onset of the transmission, therefore increasing the communication cost. As a solution, a Secure Secret Authentication Key (SSAK) is proposed. In SSAK, the quantum handshake scheme is used to share initial secret polarization angle and authentication key which is utilized before quantum communication session. The results of simulation experiments reveal that SSAK significantly outperformed the Three-stage protocol in terms of average covered angle by Eve. The simulation experiments and security analysis of initial authentication and transmission of messages verified that an eavesdropper is unable to disclose any information about the transmitted message or the authentication key. Lastly, most of the proposed QKD protocol employs a single-beam set up to transmit the photons over the free space optic which results in low bit rate and limited distance coverage due to high impact of geometrical loss. To deal with this, a transmission technique of Multiphoton Quantum Communication using multiple beam concept (MQC-MB) is proposed. Comparison is conducted in terms of total loss and received power on different number of beam shows that 4-beam is acceptable to be adapted in MQC-MB. The statistical analysis shows that such approach has reduced the total attenuation by 6dB compared to single-beam setup.
Quantum key distribution (QKD) systems for use in future free-space optical communication links restrict communication to a low bit rate. In this paper, increasing the QKD bit rate using a communication system that includes a multiple-input-multiple-output quantum key distribution (MQKD) is proposed. Such a system can enable a number of receivers to communicate simultaneously with a number of transmitter elements, which consequently increases the overall QKD bit rate. However, due to scattering and turbulence in the atmospheric channel, this method may introduce interference effects when a photon transmitted from one transmitter element reaches the wrong detector element in the receiver. These interference effects reduce the system bit rate and increase the quantum bit error rate. A novel mathematical model for analyzing the effect of crosstalk and interference on the MQKD system performance is derived. The results of the analysis indicate that atmospheric effects impair performance. In order to mitigate the atmospheric effect, using several wavelengths simultaneously is suggested. Criteria to define the number of wavelengths that are required to achieve a given performance are also provided.
This paper proposes a novel two-way free-space quantum key distribution (QKD) protocol, which can be implemented using standard free-space optical (FSO) systems with subcarrier intensity-modulation (SIM) binary phase-shift-keying (BPSK) and direct-detection (DD) receiver. Different eavesdropping threats, including unauthorized receiver, beam-splitting, and intercept-resend attacks, are considered in the security analysis of the proposed protocol. Under the constraints on security requirements, we analytically investigate the design criteria for transmitter and receiver, in particular, the intensity modulation depth at the transmitter and the dual-threshold setting of the receiver in two-way free-space QKD systems. Quantum bit error rate (QBER), the ergodic secret-key rate, and the probability of detecting eavesdropper of the proposed system are analytically derived in closed-form expressions, considering the atmospheric loss, turbulence, and receiver noises.
BB84 is a single photon that has been widely used since 1984 as the unconditional security of key distribution. Recently, the idea of multiphoton QKD was proposed to exchange the key using the rotational of polarization by over multi-stages protocol. By utilizing the state of polarization, the system does not require one of the four states like it did in the BB84 scheme. The multiphoton has the ability to improve the key rate generation and range of distances as well. Indeed, multiphoton QKD needs to be setup properly to gain the optimal performance over the fiber optic. This paper presents the fundamental implementation of multi-stages and multiphoton approach over the fiber optic by applying secret unitary transformation between Alice and Bob. The step by step operation of unitary transformation is applied using half-wave plates represented as Mueller matrix. This paper also evaluates the effects of parameters in the QKD model. The result reveals that channel loss, dark count rate, detector efficiency, photons' repetition rate, and the average number of photons will affect the QBER and SKR.
When light is propagated through a channel that is not a waveguide, such as a fiber, it is known as a free-space optical channel. When used in communication systems, free-space optical channels provide the advantages of high communication rates, lower power, and minimal infrastructure installation. When the free-space optical channel has a random medium in it, such as the atmosphere, the distortions will be applied to the phase. The phase distortions can lead to intensity fluctuations and additional beam spreading. To compensate for the distortions adaptive optics can be used, allowing for the stability of the system to be improved. Additionally, post processing techniques on transmitted data can also be used to improve the stability of the channel. By significantly decreasing the number of photons transmitted, a classical communication channel can be considered as a quantum channel. Encoding information into the quantum states of the transmitted photons, quantum communication can be performed. When transmitting on the order of 1 photon per pulse, classical post processing techniques cannot be used to improve the stability of the channel. To stabilize the free-space optical channel, adaptive optics can be used at the transmitter or receiver. As no measurement of the quantum state is performed by the adaptive optics system, no information in the photon is lost. In this work, quantum key distribution will be implemented over the quantum channel. The focus of this work is on the design and study of how the channel degradations can be overcome, as well as the establishment of a free-space optical channel with adaptive optics in a quantum networking testbed. Quantum key distribution systems were simulated over a 30 km maritime channel as well as over a underwater channel. For the 30 km channel, the use of multiple parallel spatial modes results in the highest increases of the secure key rate at the cost of transmitting a lower average number of photons. When adaptive optics is applied, the secure key rates have a small increase, however as the strength of the turbulence increases from weak to moderate, the relative improvement is higher. For the underwater channel, operation at lower depths is optimal as fewer background photons reach the receiver and the channel attenuation is lower. In the experimental setup, it was found that the single photon detectors that are being used are the limiting the channel loss to be at most 6 dB. The free space optical link established between the Electrical and Computer Engineering building and College of Optical Sciences uses a corner cube retro-reflector to yield a distance of 1.45 km. The channel has been characterized at various times of day to determine the strength of the atmospheric turbulence, which ranges from a scintillation index of 0.07 to 2.21. A classical communication experiment was performed, resulting in error-free transmission while the input power to the EDFA before the photodetector was consistently above -25 dBm.
This paper proposes a novel design and analyzes security performance of quantum key distribution (QKD) protocol over free-space optics (FSO). Unlike conventional QKD protocols based on physical characteristics of quantum mechanics, the proposed QKD protocol can be implemented on standard FSO systems using subcarrier intensity modulation binary phase shift keying and direct detection with a dual-threshold receiver. Under security constraints, the design criteria for FSO transmitter and receiver, in particular, the modulation depth and the selection of dual-threshold detection, respectively, is analytically investigated. For the security analysis, quantum bit error rate, ergodic secret-key rate, and final key-creation rate are concisely derived in novel closed-form expressions in terms of finite power series, taking into account the channel loss, atmospheric turbulence-induced fading, and receiver noises. Furthermore, Monte-Carlo simulations are performed to verify analytical results and the feasibility of the proposed QKD protocol.