# Quantum Key Distribution for 5G Networks: A Review, State of Art and Future Directions
## 4 Quantum Key Distribution Categories
### QKD Based on Multiphoton  
The enhanced approach where multiple photons are emitted from laser pulse or using a larger average number of photons is known as multiphoton approach [110]. The vulnerabilities in single-photon such as BB84 and protocol that derived from BB84 towards PNS attacks is significantly high since the difficulties for the photonic devices to periodically generating single photon at regular intervals due to a limited number of photons in the time slots [111]. A device that can produce single photons with periodicity on a consistent basis is technically difficult to be created. Multiphoton is claimed to be able to support long-distance photon and a higher rate of transmission compared to single-photon. In the multiphoton approach, information exchange is not limited to the single-photon source in a time slot, which means it allows transmitting the same quantum state more than once. The transmission of encoded bit between sender and receiver needs coherent non-decoyingquantum states with a mean number of photons greater than one [112; 113]. Even though the laser pulse generates multiple photons, any unitary transformation and their complex conjugate transformation will have similar photons as long as all the photons are in the same phase [114]. Multiple photons can be delivered simultaneously to represent one bit of information to increase the transmission success rate. The multiphoton approach is an enhanced version of a single photon with the benefits of high transmission rates and long photon travelling distance. In contrast, a single photon and entangled state are unable to do so [115]. In the multiphoton approach, information exchange is not limited to a single photon source in a time slot, allowing the same quantum state to be transmitted multiple times. Multiple photons could be transferred at the same time to represent one bit of information, increasing the transmission's success rate. To transfer the encoded bit from [PERSON] to [PERSON], coherent non-decoying quantum states with a mean number of photons greater than one have been used [112; 113]. Quantum communication using multiphoton approach are illustrated in Figure 4 where in the situation of [PERSON] wants to transmit a photon, bit 1 will be encoded with 90\({}^{\circ}\) or bit 0 will be encoded with 0\({}^{\circ}\), three photons were transferred, indicating of 90\({}^{\circ}\), 90\({}^{\circ}\), 90\({}^{\circ}\) or 0\({}^{\circ}\), 0\({}^{\circ}\) denoting as one bit of information [116]. In this approach, if an error happens in one of the three photons, the original photon can be easily recovered. As a result, the beam-splitting attack does not affect the multiphoton approach. The multiphoton approach utilizes the arbitrary state of polarization using a rotation operator, which can secure the information from the MITM attack. The polarization of rotation operator [117] adopted in Figure 4 can be described as:  
\[\mathrm{R}(\theta)=\left[ \begin{array}{cc}\cos\theta&-\sin\theta\\ \sin\theta&\cos\theta\end{array} \right] \tag{1}\]  
where \(\theta\) is the rotation angle. The secret unitary operation will be applied by [PERSON] and [PERSON] using \(\mathrm{U_{A}}\) and \(\mathrm{U_{B}}\) where \(\mathrm{U_{A}}\mathrm{U_{B}}=\mathrm{U_{B}}\mathrm{U_{A}}\). The values of the rotation operator are \(\mathrm{U_{A}}=\mathrm{R}(\theta)\) and \(\mathrm{U_{B}}=\mathrm{R}(\Phi)\), where \(\mathrm{R}(\theta)=\mathrm{R}(\Phi)\). Despite the advantages of multiphoton in terms of multiple photon emission, Eve can easily sypphon off multiphoton if only one stage is used. Therefore, it needs to be operated with the multi-stage transformation between sender and receiver. In theory, the Eve's capability to estimate the state of polarization might be improved as the numbers of photons in a beam increased. Therefore, the photon numbers and the stages must be carefully identified to make the protocols well utilized during their operations.