# Secure Quantum Communication Technologies and Systems: From Labs to Markets
## 2 Quantum Key Distribution Systems and Networks
### QKD Networking
The principle operation of a ROADM-based quantum-classical dense wavelength division multiplexing (DWDM) network operating at 1550 nm is reported in [46], where a quantum channel coexists with two simultaneous 200 GHz spaced classical telecom channels. However, as reported in the same paper, Raman and four-wave mixing impairments severally limit the application space of such systems, operating entirely in the 1550 nm window [47, 48]. A list of high-level design features to ensure the coexistence of quantum and classical optical channels on the same network infrastructure is provided below:  
* High-isolation (>100 dB) wavelength division multiplexing (WDM) of quantum and classical channels to remove crosstalk generated by classical channels and spontaneous emission noise generated by optical amplifiers from the quantum channel band. * A proper wavelength plan to minimize the transfer of linear and non-linear noise from classical channels into quantum channels. * Optical bypass of quantum channels in optical amplifiers and other non-quantum-compatible devices. * Signal-format transparent and independent optical switching for quantum and classical channels. In systems where quantum and classical signals coexist in the same fiber, the noise due to the classical channels must be measured with high accuracy. As a rule of thumb, any in-band noise contribution should be less than the quantum system detector dark noise. For Indium Gallium Arsenide Phosphide (InGaAsP) avalanche photo-diodes (APDs) in Geiger mode, the dark count probability can be as low as \(10^{-7}\) over one nanosecond. In order to avoid a negative impact on the performance of a QKD system, the average noise optical power in the quantum band should be less than \(-138\) dBm, corresponding to \(1.24\times 10^{7}\) photons/ns in the 1550 nm band [47]. This rule does not apply to future single-photon detectors, designed to have almost no intrinsic dark counts: in this case, detectors themselves can also be used to measure the noise level with high accuracy. The noise level must be much lower than the level considered in classical optical communications. Spontaneous emission noise from lasers and optical amplifiers are two examples of noise sources from a classical system. Its typical values, in the order of hundreds of photons/ns at 1550 nm, are incompatible with single-photon quantum communication systems, but they decrease to \(10^{-2}\)-\(10^{-3}\) photons/ns in the 1300 nm region, making quantum-classical coexistence possible using high-isolation optical filters. Deep notch filters used to suppress the noise before the quantum channel insertion, having an adjacent channel isolation of about 75 dB, are an alternative to band duplexers to allow the coexistence of quantum and classical channels in the same 1550 nm band, having the advantage of lower fiber attenuation. However, they need an active control of the central frequency to compensate for any offset from the channel frequency (e.g., due to thermal drift). The filter specifications are even more stringent at the optical demultiplexer, placed before the receiver, where an isolation of about 120 dB is required [49]. Optical filters cannot remove in-band noise, such as that generated by light scattering of classical optical channels, as they propagate in the fiber. Rayleigh and Brillouin scattering add a significant noise contribution only when the quantum channel is very near to the conventional channel and can be mitigated by allocating a frequency gap of at least 100 GHz. Raman scattering noise, which is approximately 200 nm wide, can lead instead to a severe performance impairment and requires a proper wavelength allocation plan and channel optical power control. Coexistence of a quantum channel in the 1300 nm band with four classical DWDM channels with an aggregate power of 2 dBm in the 1550 nm band was demonstrated for a system with a dark count level of 10\({}^{-3}\) photons/ns over 25 km of standard single mode fiber (SSMF) [50].