In the context of HellasQCI Phase 0 activities, integration scenarios and feasibility analysis of DV-QKD systems into Fiber to the Home (FTTH) infrastructures were experimentally investigated for applications considered in urban metropolitan areas. In support of these experiments, a real-world Gigabit Passive Optical Network (GPON)-based Fiber to the Home (FTTH) network serving up to 32 users was installed in COSMOTE laboratories in Athens. During these studies, two publications were produced focusing on the HellasQCI usecase deployment phases for integrating and finally demonstrating QKD links over FTTH services.

The first IEEE JLT article [1] presents research focusing on mitigating noise generated within the quantum channel’s spectral passband, particularly from Spontaneous Raman Scattering in classical/quantum coexistence schemes. To address this challenge, a C-band quantum channel operating at 1550 nm was proposed, leveraging low-loss transmission properties in standard telecom fibers. Experimental measurements involving photon counting using single photon detectors and effective filtering techniques were employed to isolate the quantum passband and accurately assess noise levels, revealed insights into noise characteristics, particularly in scenarios with varying numbers of active users and splitter configurations. Simulated noise rates demonstrate the impact of feeder fiber lengths and splitting ratios on Secure Key Rates per user. The experimental findings showed that QKD system integration is feasible if FTTH networks and provided valuable insights into optimizing system configurations to maximize quantum-secure communication performance while addressing practical deployment challenges for securing data transmission in modern telecommunications networks.

In another publication to be presented in the upcoming OFC 2024 conference [2], members of the HQCI team from the NKUA and NTUA introduce a novel coexistence scheme integrating an O-band QKD system over the same FTTH GPON network installed in COSMOTE R&D Laboratory. The experimental setup involved up to 9 Optical Network Terminals (ONTs) to simulate a partial full load of a FTTH carrier-grade access network. In this setup, the quantum signal co-propagated with the 2.5 Gbps 1490nm GPON channel while counter propagated with the O-band GPON upstream channel. The aim of the experiment is to deploy a QKD link over a functional realistic FTTH network. The sensitivity of the quantum channel to transmission losses presents a challenge, affecting achievable distances and Secure Key Rates. The implementation reveals a ~3 dB SKR degradation with a single ONT, possibly due to back-reflected photons leaking from the bandpass filter of the QKD receiver. Interestingly, adding more users to the ONT results in an improvement in the QKD link’s performance in terms of SKR. This improvement is attributed to the Power Levelling Sequence upstream function of GPON, which adjusts power levels to reduce the optical dynamic range. The testbed operated successfully for a continuous period of 60 hours, serving a total of 9 users simultaneously. The average achieved SKR was 10.07 kbps, with a Quantum Bit Error Rate of 5.11%. This experiment underscores the potential of quantum-secure communications using real-life access network deployments.

[1] D. Zavitsanos et al., “Feasibility Analysis of QKD Integration in Real-World FTTH Access Networks,” in Journal of Lightwave Technology, vol. 42, no. 1, pp. 4-11, 1 Jan.1, 2024, doi: 10.1109/JLT.2023.3303908