# MQC-MB: Multiphoton Quantum Communication Using Multiple-Beam Concept in Free Space Optical Channel
###### Abstract  
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. m 2020 2  
hotly debated by researchers, especially in mobile devices technology. The configuration of non-directed line-of-sight would face loss and noise issues. Although the usage of beam steering to control device alignment might solve such issues, it would increase the cost of the handheld devices and setting complexity [4]. Quantum cryptography is used primarily for key distribution, called quantum key distribution (QKD), and for directly sharing the message without using the step of key distribution and entanglement, called quantum secure direct communication (QSDC). The structure of quantum cryptography over the free space channel is illustrated in Figure 1, where the design of FSO consists of the transmitter, atmospheric channel, and receiver. Photon is the particle that is responsible for carrying the basic unit of quantum information, which is also known as qubit. Practically, minimum numbers of quantum communication system technology are supported for the current mobile devices as they need a specific detector to detect the photons. In addition, the research of the quantum communication system in the mobile telecommunication environment has not been profoundly studied even though the technology of mobile networks evolves rapidly. FSO solves the issue of interception attacks due to it being hard to tap the narrow beam of divergence angles compared to easily tapping the FO wired [5]. However, quantum communication in the free space environment suffers from environmental effects, such as bad weather, and it also generates total lost channel due to the geometrical and atmospheric losses [6]. For practical implementation of quantum communication over the FSO link, channel loss, as well as the geometrical and atmospheric attenuation, needs to be carefully evaluated. Firstly, geometrical attenuation can be defined as the loss that occurred due to the spreading of the transmitted beam between the transmitter and receiver. The loss will occur when the light beam deviates from the transmitter to the receiver, resulting in most light beams do not hit the telescope's receiver. Secondly, atmospheric attenuation happens when the signal is disturbed by air molecules or other particles, such as smoke, fog, etc., while traversing over the atmosphere. The performance of the free space link degrades when the distance between the sender and receiver increases.