# Quantum Key Distribution for 5G Networks: A Review, State of Art and Future Directions
## 4 Quantum Key Distribution Categories
### QKD Based on Entanglement  
Entanglement is an area of quantum physics that can be described as the correlation among particles that can't be separated despite their physical separation. When the state of one qubit is affected by the measurement of the other qubit, two qubits are said to be entangled. The disruption to the entangled qubits may cause them to lose their correlation. There are various states of entanglement, such as Greenberger-Home-Zeilinger (GHZ), Einstein-Podolsky-Rosen (EPR) and Bell state. The measurement is critical in quantum entanglement to determine whether the state is entangled or separable. There are two kinds of entanglement states: qubits spinning in the same direction and qubits spinning in different directions [93]. In quantum cryptography, especially in QKD, entanglement has widely been applied. The entangled state is applied to identify the existence of an eavesdropper and establish a secure key. The first QKD protocol using entangled state was proposed by [PERSON] in 1991 [94]. In entangled state QKD, the photon pairs entangled in the polarization degree of freedom are shared between [PERSON] and [PERSON]. One of the photons is transmitted to [PERSON] whereas the other photon is transmitted to [PERSON]. When they received the photons, the measurement will be conducted. After then, the entangled state is used to create a secure key and discover the existence of an eavesdropper. The states that have different results during  
Figure 3: Classification of QKD Protocols. measurement will be eliminated. Next, Error Correction (EC) and Privacy Amplification (PA) will be conducted in the same procedure as BB84. In [95], EPR-QC is introduced where an entangled state in an EPR channel is used to perform an authentication procedure while the quantum channel transfers the shared key in QKD protocols. The process of authentication and key sharing is performed simultaneously to provide a sustainable flow of data. Semi-QKD was proposed in [96] where [PERSON], as the third party prepares Green-Horne-Zeilinger (GHZ) states, divides them into a sequence and inserts decoy states into the sequence. Then, the quantum [PERSON] requests the sequence of GHZ states and shares it with classical [PERSON]. The security of Semi-QKD is preserved because [PERSON] cannot obtain the value of two particles by only knowing the value of one particle. However, the practical implementation of QKD should not rely on a third party. Therefore, Multi-party Bell state QKD [97] is introduced based on bi-directional communication. As a result, information leakage has been eliminated accordingly. However, despite the benefits of entangled state QKD, the difficulty of efficiently creating, transmitting, and storing the entangled state make it impossible to implement in today's communication system. Furthermore, a research study conducted by [98] discovered that quantum cryptography based on entanglement is technically more cost than a single qubit. Due to the noise and error act independently in each qubit, the expensive quantum resource might be reduced using the single-qubit approach. In addition, entanglement suffers from photon source issues. [PERSON] will try to split the copy of entangled states in case the photon laser source accidentally emits multiple simultaneous pairs of photons [99]. Consequently, [PERSON] might be capable to measure the photons and obtain secret information. The type of protocol in this category is a protocol that utilized the entanglement of photons for encryption and decryption instead of using the state of the photon. Some of the protocols that categorized as this type of QKD protocols class are as follows:  
* E91 protocol is proposed in 1991 by [PERSON] [94]. This QKD protocol is categorized as an entanglement-based protocol. E91 is developed by applying entangled photon pairs in the protocol. Since the protocol is using the principle of entanglement photon, the source of the photons can come from either [PERSON] or [PERSON].