Experimental feasibility analysis of quantum/classical coexistence over fibre and free space links

The authors present a novel approach to Quantum Key Distribution (QKD) research, emphasising cost-effectiveness and practicality using a single photon polarisation-encoded system employing mainly commercial off-the-shelf components. This study diverges from previous high-cost, high-end setups by exploring the viability of QKD in more accessible and realistic settings. Our approach focuses on practical measurements of the signal-to-noise ratio by analysing polarisation-encoded photonic qubits over various transmission scenarios. The authors introduce a simplified evaluation method that incorporates experimental measurements, such as noise sources and losses, into a semi-empirical theoretical framework. This framework simulates the standard DS-BB84 protocol to estimate Secure Key Rates (SKRs), offering an alternative approach on the evaluation of the practical implementation of QKD. Specifically, the authors examine the feasibility of QKD over a 2.2 km intra-campus fibre link in coexistence scenarios, identifying optimal Wavelength-Division Multiplexing allocations to minimise Raman noise, achieving an expected SKR of up to 300 bps. Additionally, the authors’ study extends to 40 m indoor and 100 m outdoor Free-Space Optical (FSO) links using low-cost components, where the authors recorded Quantum Bit Error Rate (QBER) values below 3.2%, allowing for possible SKRs up to 600 bps even in daylight operation. The converged fibre/FSO scenario demonstrated robust performance, with QBER values below 3.7% and an expected SKR of over 200 bps. Our research bridges the gap between high-end and economical QKD solutions, providing valuable insights into the feasibility of QKD in everyday scenarios, especially within metropolitan fibre based and FSO links. By leveraging cost-effective components and a simplified single photon exchange setup, the authors work paves the way for the effortless characterisation of deployed infrastructure, highlighting its potential in diverse settings and its accessibility for widespread implementation.