# Secure Quantum Communication Technologies and Systems: From Labs to Markets
## 2 Quantum Key Distribution Systems and Networks
### QKD Networking  
Current QKD systems are conceived mainly over point-to-point optical fiber links. This implies the availability of dark fibers at an acceptable access cost and in a multi-user environment; and the deployment of key-regenerated meshed networks, where quantum traffic is inefficiently regenerated at every node, leading to wasted energy and bandwidth resources and negatively impacting on security. Moreover, a separate classical network is still necessary, as required by practical QKD systems not using one-time pad encryption. The ability to support both classical and quantum communication channels on a shared, reconfigurable, transparent wide-area optical network infrastructure is the ultimate condition for the commercial success of QKD systems, but it requires the coexistence of quantum and classical optical channels on the same fiber infrastructure. Examples of experiments running quantum and classical channels on the same fiber are reported in Table 1[45]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Date** & **State/Company** & **Distance (km)** & \begin{tabular}{c} **Rate/Wavelength** \\ **(Gbit/s)/nm** \\ \end{tabular} & \
\begin{tabular}{c} **Quantum** \\ **Wavelength (nm)** \\ \end{tabular} &
\begin{tabular}{c} **Code Rate** \\ **(bit/s)** \\ \end{tabular} \\ \hline
1997 &
\begin{tabular}{c} United Kingdom/ \\ British Telecom \\ \end{tabular} & 28 & 1.2/1550 & 1300 & - \\ \hline
2009 &
\begin{tabular}{c} Sweden/Gothesburg \\ University \\ \end{tabular} & 50 & -/1550 & 1550 & 11 \\ \hline
2012 &
\begin{tabular}{c} United Kingdom/ \\ Cambridge University \\ \end{tabular} & 50 & 1/1571–1611 & 1550 & 507 k \\ \hline
2016 &
\begin{tabular}{c} United Kingdom/ \\ Cambridge University \\ \end{tabular} & 50 & 100/1547 & 1529 & 1.2 M \\ \hline
2017 &
\begin{tabular}{c} China/China Telecom \\ Corporation \\ \end{tabular} & 80–117 & \(80\times 100/1550\) & 1310 & 1.6 k–1 k \\ \hline
2018 &
\begin{tabular}{c} China/China Unicom \\ \end{tabular} & 66 & 3600/1550 & 1310 & 4.5 k \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Examples of experiments running quantum and classical channels on the same fiber. Considering that the transmitted optical power in classical optical networks is orders of magnitude higher than for quantum communication, multiplexing classical and quantum signals on the same fiber can result in significant performance impairment for the quantum channel, due to insufficient isolation of optical filters or non-linear propagation effects. In the following, an overview is provided about the main design challenges and guidelines. The first issue is the link distance: optical fiber loss and photodetection noise limit the distance of the current generation of QKD systems to about 200 km, one order of magnitude less than the distance achieved by classical long-haul systems, which appears to be a challenging goal for QKD systems, even when considering the progress single-photon source, low-noise single-photon detector, and low-loss optical fiber technologies. Besides the distance challenge, in a mixed quantum-classical network, quantum information must be dynamically routed from the transmitter of any node to the receiver of any other node, as currently happens in classical optical networks, which use tunable lasers and reconfigurable optical add drop multiplexers (ROADMs) for this purpose. ROADMs are based on photonic devices, such as wavelength selective switches (WSSs) and arrayed waveguide gratings (AWGs), which in principle preserve the quantum information. However, their current performance is not specified for quantum channels. For example, the residual crosstalk among different wavelengths could be an issue for the highly sensitive quantum channel, as well as requiring the use of noisy optical amplifiers to compensate for the device loss.