# MQC-MB: Multiphoton Quantum Communication Using Multiple-Beam Concept in Free Space Optical Channel
###### Abstract
The transmission of photons through free space may also be affected by atmospheric turbulence due to the intensity fluctuation of the laser beam, which will degrade the wireless signal. However, the effects of atmospheric turbulence give a negative impact on the photon loss at the source, not on the state of polarization. The state of polarization is impacted by technical issues, such as rotation noise of the half-wave plate at the stage of photon transmission. The concept of the multiphoton QKD [3; 7; 8] is to use rotational polarization to exchange the key. By applying the state of polarization, the system does not require the output of one of the four states as in the BB84 scheme. Indeed, it will create an arbitrary state of polarization, which can prevent the Man-in-the-Middle attack. The concept of the multiphoton protocol was proposed by [PERSON] [9] for quantum cryptography to allow exchange of the keys or direct transmission of the message using the secret rotational angle of the half-wave plate over a multi-stage protocol [7] in the fully quantum channel. Multiphoton is like passing more than once on the same information. Multiphoton is capable of resolving the limitations of the single-photon protocol caused by the emission of a single photon per pulse from the laser source, which restricts the number of photons travelling through the quantum channel. It is quite difficult to get one photon for each laser pulse using single-photon concepts. Due to a large number of empty pulses, the  
Figure 1: A simplified block diagram of QSDC over an FSO channel. transmission rate for a single photon is low. Besides, the single photon is vulnerable to the siphoning attack, which is also referred to as the PNS attack, which exploits several unintentionally emitted photons from the attenuated laser source. Apart from that, without disrupting the link, an eavesdropper only has to catch a single photon from multiple photons and try to guess the polarization states, while the remaining photons are transmitted to Bob. Despite the robust quantum network channel, the distance and key rate are the major limitations that need to be considered [10]. There are trades off between the distance and the key rate, which is the farther the photon needs to travel over a channel, the more photons will be lost due to device constraints and environmental issues, thus the key generation rate is low. Recently, an experimental setup was able to send the quantum signal over 100 km using fiber optic link. However, the distance covered for the free space optic is less than 100 km due to the total atmospheric attenuation. In FSO, the most common setup is using single-input single-output configuration (hereinafter, referred to as single-beam). Unfortunately, the FSO device itself primarily contributed to the geometrical loss. Thus, it is important to take into consideration the setup of the transmitter and receiver due to the geometrical loss occurring in all FSO links. What was done in most of the literature regarding free space quantum communication does not take into account the geometrical spreading loss, such as in [11; 12; 13]. To mitigate the aforementioned issues, this research proposes a multiple beam to replace the simplest form of communication; a single beam that consists of a transmitter and a receiver, which has been used widely by researchers in terrestrial free space. Terrestrial free space can be defined as the communication configuration with limited distance coverage due to the several losses caused by the complex environment [14]. The symmetry multiple beam offers advantages in terms of path redundancy, which is able to reduce the possibility of the path being blocked during the transmission of photons [15], and increase data throughput without additional bandwidth and transmission power. Besides, multiple transmitters and receiver designs can improve signal detection, reduce the probability of link downtime caused by the atmospheric factor, and overcome the problem of low per aperture power density [16].