The quantum key distribution system can provide information-theoretically secure symmetric keys for both communicating parties. With the development of quantum cryptography, the decoy BB84 protocol quantum key distribution system is gradually becoming practical. 

However, the huge difference between quantum signals and classical signals leads to the fact that quantum signals need to be exclusively transmitted through an optical fiber in the process of engineering implementation, which consumes a lot of optical fiber resources and the cost of network deployment is also high, which is not conducive to the deployment and construction of quantum key networks. .

Difficulties and main solutions of common fiber technology

Due to the huge difference in intensity between quantum light and classical light, the main problems of common fiber transmission are crosstalk and scattering noise. Crosstalk refers to the fact that the frequency components of the classical optical signal itself are strung into the quantum optical signal through the demultiplexer, which is caused by the insufficient isolation of the demultiplexer; for the crosstalk problem, the main solution is to increase the isolation of the demultiplexer. Reduce or eliminate crosstalk noise. Scattering noise refers to the noise generated by nonlinear effects in the fiber, mainly including: Rayleigh scattering, Raman scattering and four-wave mixing. The wavelength of Rayleigh scattering noise is very close to the wavelength of classical light, which can be filtered by wavelength division multiplexing technology without affecting the quantum signal; the frequency domain range of Raman scattering noise is wide, the classical C-band (O-band) The Raman noise generated by light can affect the O-band (C-band) quantum light. For the noise with the same wavelength as the quantum light, we can only take measures to suppress it, but cannot complete the filtering. For this noise, by using quantum light and classical light The method of far wavelength interval, frequency domain and time domain filtering [5], reducing the power of classical optical signal, selecting special optical fiber and other methods. Four-wave mixing noise is generally generated by the interaction of signals of different wavelengths in a multi-channel transmission system, and the influence of four-wave mixing noise can be avoided by selecting appropriate quantum light wavelengths.

Common fiber transmission scheme

The quantum key distribution system includes quantum signal, synchronization signal and classical negotiation signal. Quantum signal and synchronization signal are transmitted on a common fiber by means of wavelength division multiplexing, and classical negotiation signal adopts the network signal provided by classical communication equipment. The classic optical communication system mainly includes the backbone network, metropolitan area network and access network. The transmission distance of the backbone network is relatively long. Optical amplifiers are added to most of the trunk roads to extend the transmission distance, and the fiber resources are relatively low. The access network is mainly for It is the end user, the current demand for the quantum key distribution system is not high; and the current optical fiber resource is relatively scarce and the demand for the quantum key distribution system is relatively high is the metropolitan area network, so the first thing to consider is the metropolitan area network. Common fiber transmission problem. At present, the classic optical communication equipment used in the metropolitan area network is mostly Packet Transport Network (PTN) and Synchronous Digital System (SDH) optical transmission equipment. ZXCTN6200, ZXCTN6300; the SDH equipment used in the metropolitan area network includes Huawei's OSN2500 and OSN3500, and ZTE's ZXMP330 and ZXMP385. The optical communication module used is a dual-fiber bidirectional module. For short- and medium-distance optical modules, O-band transmission is usually selected for classic optical modules. For long-distance transmission (80km), C-band transmission is usually selected for classic optical modules. Taking a gigabit optical module as an example, if the optical interface type is 1000BASE-VX, the operating wavelength range is 1 260 to 1 360 nm, the average transmit optical power is -5 to 0 dBm, and the receive sensitivity is -22 dBm. Then, for the common fiber transmission of this classic optical signal, the backscattering noise tends to be stable after about 25km, while the co-scattering noise decreases with the fiber transmission after 25km, so the same-direction classical optical signal of bidirectional transmission is selected for common fiber. The quantum signal of the quantum key distribution system is transmitted in the C-band. In the frequency domain, an appropriate narrow-band filter is selected to suppress noise. In the time domain, the width of the single-photon detector gate signal is reduced to suppress noise and improve the quantum signal SNR; At the same time, the transmit power of the optical module can be reduced to ensure that the receive power is greater than -22dBm.

In the actual deployment of quantum key network, an independent network will also appear. At this time, there is no corresponding classical optical communication equipment. For this application scenario, classical optical communication needs to be configured independently to ensure the quantum key distribution system. The negotiation signal can work normally. Classic optical communication equipment can choose Ethernet optical fiber transceiver, in order to further save optical fiber resources, choose single-fiber bidirectional optical module, also choose O-band classical optical signal, quantum key distribution system quantum signal choose C-band, except in the frequency domain and In addition to time domain filtering, you can further purchase high receiving sensitivity optical modules to reduce the transmission power of the optical modules. The receiving sensitivity can generally reach -25dBm, which improves the signal-to-noise ratio of quantum signals.