# Research on Key Technologies of Quantum-Safe Metro-Optimized Optical Transport Networks
## 1 Introduction  
In China, vast amounts of data are transmitted via metropolitan-optimized optical transport networks (M-OTNs), coinciding with a surge in network security threats. Traditional public key cryptography provides a security barrier against eavesdropping but is vulnerable to \"collect first, decrypt later\" attacks facilitated by quantum computers. Quantum key distribution (QKD) technology, which transmits single photons between communicating parties, ensures secure key agreement and detects potential eavesdropping attempts, thereby protecting data integrity [1, 2, 3]. Integrating QKD into M-OTNs can mitigate these security risks.  
China Telecom has developed M-OTNs based on optical service units (OSUs), enabling flexible transport for various government and service applications through the adaptive mapping of optical service units. OSUs have been introduced to address the issues of low bandwidth utilization and insufficient service flexibility in traditional OTNs within metropolitan networks [4]. Additionally, an OSU-OTN architecture and service model have been designed specifically for government and enterprise dedicated networks, ensuring high security, reliability, and multi-service support [5]. An OSU routing optimization algorithm has also been developed to mitigate traffic load imbalance in optical inter-satellitelinks (OISLs) [6]. However, M-OTNs face challenges such as optical network eavesdropping and weak signal interception, particularly due to the increasing risk of eavesdropping in elastic optical networks, which results from flexible spectrum allocation [7], full-stack security threats, and hierarchical protection requirements [8]. Furthermore, a non-invasive traffic monitoring method using a temporary optical coupler based on fiber bending has been proposed for security analysis [9]. The advent of quantum computing poses significant threats to classical cryptographic systems, necessitating quantum-enhanced security paradigms. Current quantum communication solutions include continuous-variable QKD (CV-QKD), discrete-variable QKD (DV-QKD), and quantum secure direct communication (QSDC) [10; 11; 12]. These methods address vulnerabilities exposed by quantum computational attacks through different physical encoding mechanisms. However, QSDC operates at the Kbps level, while DV-QKD and CV-QKD achieve Mbps-level communication rates. The authors of [13] proposed teleportation principles in classical optical networks, demonstrating simultaneous quantum state transmission and 400 Gbps C-band data traffic over 30.2 km of optical cable. Additionally, a nonlinear all-optical encryption technique utilizing multi-photon absorption in carbon nanotubes has been proposed. This technology achieves high-security physical-layer encryption through dynamic optical intensity threshold adjustment, although it faces challenges in material and system integration [14]. The integration of QKD with transport layer security (TLS) 1.3 has been introduced to enhance optical network security against quantum threats [15].  
To meet the communication rate requirements of 10 Mbps to 10 Gbps in M-OTNs, this paper proposes and experimentally validates a method combining the real-time optical signal time-domain scrambling/descrambling of OSU optical signals with decoy-state quantum key distribution (DS-QKD). The approach uses tunable Fabry-Perot cavities (FPCs) to process OSU optical signals in real time, synchronized with a running key. The DS-QKD system employs the decoy-state BB84 protocol and polarization coding to distribute seed keys. Experimental results confirm secure end-to-end OTU2 (10.709 Gbps) data transmission in access-side M-OTNs, with more than 100 km of optical data transmission over conventional single-mode fiber and a power loss of only 1.77 dB. This method offers an effective and adaptable solution for high-speed optical signal scrambling/descrambling.