The influence of approaches for measuring the spectral characteristics of fiber-optic elements of quantum communication systems on the key generation rate

A method for estimating insertion losses introduced by spectrally selective elements is proposed, which enables an increase in the dynamic range of measurements and subsequent calculations of the performance characteristics for a Continuous Variable Quantum Key Distribution system. In contrast to common approaches that employ supercontinuum sources, the proposed method provides more accurate assessment of the system’s resilience against the Trojanhorse attack. The essence of the method consists in using a tunable laser source with additional filtering incorporated into the optical setup for insertionloss measurements. The paper presents a comparative analysis of loss measurement results obtained with a supercontinuum source and a tunable laser source. For the tunable source, the addition of extra filtering in the optical scheme is proposed. It is demonstrated that the maximum loss values measured with the tunable source exceed those obtained with the supercontinuum source by 20 dB. The experiments reveal that employing the tunable source together with additional filtering enhances the dynamic range of insertionloss measurements for Coarse Wavelength Division Multiplexing filters. Using the Trojanhorse attack as a case study, it is shown that additional filtering in the tunablesource configuration yields more accurate transmittance values. This, in turn, affects the formulation of requirements for the implementation of countermeasures, aiming to maintain the quantum key generation rate as close as possible to the case without the attack.