# Visible Light Communication System Technology Review: Devices, Architectures, and Applications
## 8 Quantum Communication  
Quantum communication is a way to transfer quantum states from the transmitter ([PERSON]) to the receiver ([PERSON]). Quantum Key Distribution (QKD) is a well-known case of quantum cryptography, first proposed by [PERSON] and [PERSON] in 1984 [141; 142], also known as the BB84 protocol, and was first experimentally demonstrated in 1991 [143]. Different photon states, such as polarization, period, frequency, phase, and space, can realize various QKD protocols. The essence of BB84 is to pressure eavesdropper ([PERSON]) to perform a quantum measurement that will expose her existence, by sending a series of individual photons to [PERSON]; many of the photons never arrive or are not detected. Of the resulting bits, some are kept and used as key material, the remaining are used to hunt for [PERSON]. A schematic of a polarization-based BB84 QKD protocol and the four polarization states that were applied are shown in Figure 20. A polarization-based BB84 QKD protocol is often used in free-space optical (FSO), since when used over optical fiber the polarization-dependent loss, polarization mode dispersion, and the fiber loss restricted the transmission distance [144]. The beam steering system allows [PERSON] to direct the beam to [PERSON] without physically moving the system, and can be realized by a liquid crystal spatial light modulator (LC-SLM), LCoS-SLM, and micro-electromechanical systems (MEMs) [7; 145]. The single photon source required by the BB84 protocol is realized by a high attenuation laser, which can generate weak coherent state (WCS) pulses [146]. The weak coherent state output by the highly attenuated laser source can guarantee that the coherent state has a high probability that there is no more than one photon. The number state \(|n\rangle\) can be expressed in terms of the ground state (\(|0\rangle\)) with the creation operator by Equation (24):  
\[|n\rangle=\frac{\left(a^{\dagger}\right)^{n}|0\rangle}{\left(n!\right)^{1/2}} \tag{24}\]  
where \(a\) is the annihilation operator. The density operator of a coherent state is given by Equation (25):  
\[\rho=|\alpha\rangle\langle\alpha|=e^{-|a|^{2}}\sum_{n=0}^{+\infty}\sum_{m=0}^ {+\infty}(n!)^{-\frac{1}{2}}(m!)^{-\frac{1}{2}}\alpha^{n}(\alpha^{*})^{m}|a \rangle\langle m| \tag{25}\]  
where \(a\) is a complex eigenvalue of the annihilation operator, and \(|\alpha\rangle\) is the coherent state vector. The probability that n-photons can be found in coherent state \(|\alpha\rangle\) follows the Poisson distribution shown in Equation (26):  
\[P(n)=|\langle n|\alpha\rangle|^{2}=e^{-n}\frac{\mu^{n}}{n!} \tag{26}\]  
where \(\mu=|\alpha|^{2}\) is the photon number. Considering that the chance of more than one photon is sent is not zero, by applying a photon number splitting (PNS) attack, it is possible for [PERSON] to obtain information. To solve the problem above, [PERSON] et al. proposed a decoy-state-based quantum key distribution in 2005 [147]. In decoy-state QKD, [PERSON] and [PERSON] can detect whether [PERSON] is capturing photons when more than one photon is sent since the average number of photons sent is increased during random timeslots. The quantum mechanics laws guarantee QKD's security. According to the method utilized on [PERSON]'s side, there are two common QKD schemes: discrete variable (DV)-QKD and continuous variable (CV)-QKD, which utilize a single-photon detector (SPD) and homodyne/heterodyne detection to measure field quadrature on [PERSON]'s side, respectively. Different QKD schemes can also be classified as entanglement assist or preparation and measurement types. For DV-QKD, the unclonability theorem and in distinguishability theorem of arbitrary quantum states make absolute security possible. The unclonability theorem declares that any quantum state is unique and cannot be copied, which means that even with the help of the most powerful computer in the world, [PERSON] cannot clone a non-orthogonal quantum state [148].