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International Conference on Transparent Optical Networks

Polarization Multiplexing CV-QKD with Polarization Drift Pre-Compensation

Margarida Almeida, Íris Guilherme, Armando N. Pinto, Nuno A. Silva

Published Jul 6, 2025• 0 citations
Publisher

Abstract

Practical implementations of Continuous Variable (CV) Quantum Key Distribution (QKD) systems can be performed over the existing optical fiber telecommunication infrastructure. However, optical fibers suffer from random variations in the state of polarization affecting the performance of the system. In this work, we study the impact of the polarization drift on the performance of a CV-QKD setup with a polarization diversity receiver, in which the polarization drift is pre-compensated on Alice’s side. We show that pre-compensating for the polarization drift can successfully realign Alice’s and Bob’s raw signal, without affecting the excess noise, up to a polarization drift variance of 10−10, compatible with most practical implementations.

DOI
10.1109/icton67126.2025.11125191

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Practical implementations of Continuous Variable (CV) Quantum Key Distribution (QKD) systems can be performed over the existing optical fiber telecommunication infrastructure. However, optical fibers suffer from random variations in the state of polarization affecting the performance of the system. In this work, we study the impact of the polarization drift on the performance of a CV-QKD setup with a polarization diversity receiver, in which the polarization drift is pre-compensated on Alice’s side. We show that pre-compensating for the polarization drift can successfully realign Alice’s and Bob’s raw signal, without affecting the excess noise, up to a polarization drift variance of 10−10, compatible with most practical implementations.
Abstract The practical deployment of Continuous Variables Quantum Key Distribution (CV-QKD) systems benefits from existing optical fiber telecommunication infrastructures. However, optical fibers introduce random variations in the state of polarization, which degrades the system's performance.We consider a CV-QKD system featuring a polarization diversity heterodyne receiver and the constant modulus algorithm (CMA) to compensate for the polarization drifts in the quantum channel. Our setup can effectively realign Alice's quantum signal with Bob's local oscillator for polarization drift variances below 10−1010^{-10}10−10 .This value is compatible with most experimental implementations, allowing for accurate estimation of the channel transmission and excess noise parameters. This validates the approximation of real-world implementations under polarization drift to an ideal scenario without polarization drift, ensuring that the security proof remains applicable for polarization drift variances below $ 10^{-10} $.Our results establish operational limits for passive polarization drift compensation using a polarization diversity receiver combined with digital CMA.This enables long-term stability in \linebreak CV-QKD systems, eliminating the need for active polarization controllers and manual adjustments.
Continuous-variable quantum key distribution (CV-QKD) provides a theoretical unconditionally secure solution to distribute symmetric keys among users connected through a communication channel. However, polarization drift in standard optical fibres can pose a considerable impairment, as it will degrade the efficiency of the coherent detection scheme if not compensated. In this work, we present a novel receiver configuration for CV-QKD that allows for passive polarization drift compensation. The presented results show that our detection system can effectively compensate random polarization drift even in very adverse scenarios.
Integrating Quantum Key Distribution (QKD) systems in existing optical infrastructure towards hybrid classical/quantum communication networks has arisen as a top priority for fiber infrastructure owners [1] . In this direction, important steps have been made using multiplexing techniques such as the recently reported record joint transmission of Continuous Variable-QKD (CV-QKD) with Wavelength Division Multiplexing (WDM) 18.3Tbps coherent data channels [2] . However, moving towards practical implementations where real-world deployments such as filtering nodes and lasers sources suffer from few GHz frequency drifts, a thorough study and a precise quantification of the impact of setups misalignments and the subsequent evaluation of the tolerance of QKD links are yet to be investigated. In this work, we attempt to focus on this setup misalignment by addressing the impact of filtering penalty in the WDM co-existence of QKD with classical bright signals.
In a commercial fiber-based quantum key distribution (QKD) system, the state of polarization (SOP) of the optical fields is inevitably disturbed by random birefringence of the standard single-mode fiber due to an external complex environment. We analyze theoretically the effect of SOP fluctuations on the continuous-variable (CV) QKD system and experimentally verify its validity. To overcome the influence of the polarization variations, a self-adaptive gradient algorithm is proposed to achieve high-speed polarization controlling under pulsed light with field programmable gate array hardware. For single random polarization scrambling, the polarization extinction ratio achieved is over 30 dB and the average time of polarization control is 827 $\ensuremath{\mu}\mathrm{s}$. In order to eliminate further the effect of the occasional failures of polarization control, we properly filter the raw keys in terms of the relative phase fluctuations between the signal and the local oscillator. By combining this approach with the high-speed polarization control, we demonstrate a CV QKD under a continuous polarization scrambling of 314 rad/s. Our results provide useful references for practical application of fiber-based CV QKD in a field environment.
Continuous variable-quantum key distribution (CV-QKD) based on coherent detection has potential to be a cost-effective solution to deploy secure keys over optical networks. This is due to its compatibility with mature optical communication technologies, such as homodyne and heterodyne detection schemes. Nevertheless, realistic implementations of CV-QKD systems suffer from imperfections in the practical devices used to generate and detect the coherent states. Those imperfections deteriorate the performance of the CV-QKD system and can be exploited by an eavesdropper to steal key information without being detected. In this work, we discuss the impact of practical device imperfections on the excess noise of the CV-QKD and on the achievable transmission distance. We also analyze the impact of detection imperfections on the measurement of the shot noise, and its impact on CV-QKD system security.
Quantum key distribution (QKD) is a quantum communication technology that offers absolute security. Among its various implementations, continuous-variable (CV) QKD has garnered significant attention due to its high secret key generation rates and compatibility with conventional optical telecommunication infrastructure, which operates under ambient conditions. However, chip-based integration is crucial for realizing practical and scalable CV-QKD networks, and photonic integrated circuit technology is a potential solution, leveraging the on-chip integration of essential optical components onto silicon photonic circuits. Therefore, this work presents a compact, stable, and cost-effective quantum photonic CV-QKD system on a chip. Our architecture combines time- and polarization-multiplexing with high-extinction-ratio pulse generation to mitigate excess noise. At the same time, a real-time feedback control mechanism compensates for polarization drift, ensuring robust system performance. Fiber-based evaluations reveal secret key generation rates of 2.55 Mbps over 25 km and 0.737 Mbps over 50 km with 12 h stable operation, establishing, to our knowledge, a new benchmark for integrated photonic CV-QKD systems with both the transmitter and receiver on-chip. These results mark a significant milestone in on-chip quantum communication, paving the way for widespread adoption of photonic integrated circuits in future metropolitan quantum-secure networks.