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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Characterization and Validation of CV-QKD Systems Traceable to the SI Units

Luiz C. Correa Pinto Filho, Jesper B. Christensen, Mikael Lassen

Published Jun 23, 2025• 0 citations
Publisher

Abstract

Accurate characterization and validation is essential for the performance and security of quantum communication systems. For example, in quantum random number generators (QRNGs) based on vacuum state measurements [1], [2], rigorous system characterization minimizes parameter uncertainties that could degrade output quality by causing deviations from the stochastic model underlying random number generation. Similarly, continuous-variable quantum key distribution (CV-QKD) systems, particularly those using Gaussian-modulated coherent states, rely on precise detection efficiency characterization to maintain security. Inaccurate loss assessment can introduce exploitable vulnerabilities, leaving these systems open to eavesdropping. Despite advancements in quantum technologies, a lack of metrologically sound, traceable standards for key system parameters limits their reliability and commercialization.

DOI
10.1109/cleo/europe-eqec65582.2025.11111672

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Accurate characterization and validation is essential for the performance and security of quantum communication systems. For example, in quantum random number generators (QRNGs) based on vacuum state measurements [1], [2], rigorous system characterization minimizes parameter uncertainties that could degrade output quality by causing deviations from the stochastic model underlying random number generation. Similarly, continuous-variable quantum key distribution (CV-QKD) systems, particularly those using Gaussian-modulated coherent states, rely on precise detection efficiency characterization to maintain security. Inaccurate loss assessment can introduce exploitable vulnerabilities, leaving these systems open to eavesdropping. Despite advancements in quantum technologies, a lack of metrologically sound, traceable standards for key system parameters limits their reliability and commercialization.
Continuous-variable quantum key distribution (CV-QKD) has emerged as a promising approach for secure quantum communication, offering advantages such as high key generation rates, compatibility with standard telecommunication infrastructure, and potential for integration on photonic chips. This review provides an accessible introduction to the theory of CV-QKD, aimed at researchers entering this rapidly developing field. We focus on fundamental concepts, key protocols, and security analysis essential for understanding CV-QKD systems, with a special emphasis on prepare-and-measure protocols using coherent states under asymptotic security conditions. We explain their equivalence to entanglement-based protocols and detail the security proof framework against collective attacks, encompassing both Gaussian and discrete modulation schemes. We also briefly address more advanced topics, including measurement-device-independent CV-QKD and finite-size security analysis. This work is motivated by Brazil's growing investment in quantum communication technologies. By presenting a clear learning path from basic concepts to advanced topics, this work aims to equip newcomers with the essential tools to engage with current research in CV-QKD, thereby supporting the training of a new generation of researchers in this strategic field.
Quantum Key Distribution (QKD) offers secure communication by leveraging core principles of quantum mechanics.Characterization of QKD system is fundamental for comparing the experimental results provided by researchers.Unfortunately, there is no standardized or unified approach; therefore, different researchers present results that provide heterogeneous information, making comparisons difficult.A systematic analysis of the literature is necessary to identify common approaches from a metrological perspective.This paper provides an overview of measurements used to characterize quantum channels, with a focus on parameters such as attenuation, polarization effects, and timing stability.To support this analysis, real-world case studies are examined.
Accurate calibration of coherent optical receivers is essential for reliable performance assessment in coherent communications, precision and quantum sensing, and continuous-variable quantum key distribution (CV-QKD), where the effective detection efficiency directly impacts channel parameter estimation. We present a methodology traceable to the International System of Units (SI) to determine the effective heterodyne detection efficiency of balanced receivers using shot-noise-referenced measurements. The protocol relies on two observables acquired with an electrical spectrum analyzer: the heterodyne beat-note power and the local oscillator shot-noise variance, with explicit treatment of the analyzer's equivalent noise bandwidth (ENBW). The photon flux in the signal path is referenced to SI units via calibrated radiometric standards. We first validate the protocol on a free-space receiver, demonstrating consistency with an independently constructed optical loss chain across a wide range of signal powers and under controlled, calibrated attenuation. Extending the same estimator to a fiber-coupled, polarization-maintaining balanced receiver confirms that the protocol is robust for practical coherent-receiver architectures and intermediate frequencies in the MHz range. These results establish a traceable, uncertainty-bounded framework for real-time receiver calibration, providing a practical route for CV-QKD and other coherent optical systems.
Continuous variable quantum key distribution (CV-QKD) and continuous variable quantum random number generation (CV-QRNG) are critical technologies for secure communication and high-speed randomness generation, exploiting shot-noise-limited coherent detection for their operation. Integrated photonic solutions are key to advancing these protocols, as they enable compact, scalable, and efficient system implementations. We introduce femtosecond laser micromachining (FLM) on borosilicate glass as a platform for producing photonic integrated circuits (PICs) realizing coherent detection suitable for quantum information processing. Employing off-chip detectors, we exploit the specific features of FLM to produce a PIC designed for CV-QKD and CV-QRNG applications. The PIC features fully adjustable optical components that achieve precise calibration and reliable operation under protocol-defined conditions. The device exhibits low insertion losses (≤1.28 dB), polarization-insensitive operation, and a common-mode rejection ratio exceeding 73 dB. These characteristics allowed the experimental realization of a source-device-independent CV-QRNG with a secure generation rate of 42.74 Gbit/s and a quadrature phase-shift-keying-based CV-QKD system achieving a secret key rate of 3.2 Mbit/s. Our results highlight the potential of FLM technology as an integrated photonic platform, paving the way for scalable and high-performing quantum communication systems.
As in conventional communication systems, test and measurement play important roles in quantum key distribution (QKD) systems. Besides the observation that QKD protocols estimate the bound of information leakage from the measurement results on the transmission channel, test of quantum apparatus is necessary to ensure that the assumptions behind the security proof are satisfied in practice. Moreover, precise characterization of the device imperfection improves the final key rate, because one can specify the effect of the errors originated from the devices and sbtract it. However, careful consideration is required to guarantee that the test and measurement procedure will not open a loop-hole to the eavesdroppers.