# Quantum Key Distribution for 5G Networks: A Review, State of Art and Future Directions
## 4 Quantum Key Distribution Categories
### QKD Based on Single Photon  
Hilbert space is used in evaluating the criteria for classification for this category of protocols. For this category, quantum states are used to code the finite-dimensional of the entire QKD process. To distinguish the states, the information polarization direction that contains the photon or the phase of the photon is utilized. For example, BB84 is the most basic single photon protocol in this category. However, this protocol integrates Distributed Phase Reference (DPR) convention that is equal with the conventional correspondence protocols. For this category, to convey the information, these protocols use different states of a single photon. Consequently, other various states are used for the coding and encoding of the key distribution process. Some of the protocols categorized in this QKD protocols class are as follows:  
* BB84 protocol is proposed in 1984 by [PERSON] and [PERSON] [3]. BB84 is a QKD scheme that used quantum mechanics such as [PERSON]'s uncertainty principle to share the secret key between sender and receiver. Furthermore, it is also the first QKD scheme that described the usage of photon polarization state in transmitting the secret key using a quantum communication channel. BB84 is considered as PM QKD protocol and used a single photon in transmitting and distributing the secret key's random bits. In BB84, the photon is polarized using either of the four polarization states, which is called rectilinear basis and choosing either of the two conjugate bases, which is called vertical polarization and horizontal polarization. The same principle applies to the diagonal basis polarization states and their conjugate bases, which are called diagonal polarization and anti-diagonal polarization. The polarization bases are shown in Figure 3. For the implementation of the BB84 protocol, there are four steps involved which is Quantum Exchange, Key Sifting, Information Reconciliation, and Privacy Amplification. BB84 is theoretically proven in providing complete security, as the author discussed in [87; 88]. * B92 protocol is proposed in 1992 by [PERSON] [89]. This protocol is considered a prepare-and-measure-based QKD protocol. The B92 protocol is regarded as a simple protocol compares to the BB84 protocol, where the protocol can only choose either from two polarization states. In contrast, the BB84 protocol can choose either from four polarization states. For the B92 protocol, 0 degrees of the rectilinear basis is considered as bit 0, while 45 degrees of the diagonal basis is regarded as bit 1. Note  
Figure 2: Categories of QKD Protocols. that, single non-orthogonal basis could be utilized to encode and decode the QKD protocol while not affecting the eavesdropper detection capabilities. Furthermore, the difference between BB84 and B92 protocol is that the B92 protocol will not obtain the measurement if the receiver chooses the wrong basis. The circumstances are called an erasure in quantum mechanics [90]. * Six-State Protocol (SSP) is proposed in 1998 by [PERSON] [91]. This protocol is classified as a prepare-and-measure-based QKD protocol. The SSP protocol uses three measurement bases and six polarization states. This protocol can be assumed as advanced BB84 with additional measurement bases. The BB84 protocol utilizes four spin and half polarization states in the Poincare sphere that identical with \(\pm\)x and \(\pm\)y direction. Although the polarization state contains two additional states that correspond to \(\pm\)z, which makes it becoming six states, they are considered \(\pm\)x, \(\pm\)y, and \(\pm\)z in the Poincare sphere. The advantages of this protocol are that it has higher symmetrical compared to the BB84 protocol. * SARG04 protocol is proposed in 2004 by [PERSON] et al. [92]. In terms of photon source, this protocol is developed by utilizing attenuated laser pulse rather than a single-photon source. This protocol is categorized as a prepare-and-measure-based QKD protocol.