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EPJ Quantum Technology

Robustness of continuous variable quantum key distribution under strong polarization drift

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

Published Oct 15, 2025• 0 citations
Publisher

Abstract

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 \(10^{-10}\) .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.

DOI
10.1140/epjqt/s40507-025-00417-3

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IEEE photonics journal•Feb 20, 2023•17 citations
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^{-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 10−1010^{-10}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.
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.
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.
We present a polarization diverse receiver architecture for continuous variables quantum key distribution. We demonstrate experimentally that our system is capable of reliably achieving theoretical security against collective attacks, even under very adverse random polarization drift scenarios, and is capable of functioning indefinitely without any user input. We show that, in a similar scenario, a standard CV-QKD system without polarization diversity cannot distribute keys securely. Our receiver architecture forgoes the need for any polarization matching calibrations or feedback loops and is usable in any locally generated local oscillator key distribution system. A theoretical study on the security impact of polarization drift is presented, based on which we show that, when using our polarization diverse receiver, it is possible to achieve the performance of a channel with zero polarization drift.
Quantum key distribution (QKD) has been proven to be theoretically unconditionally secure. However, any theoretical security proof relies on certain assumptions. In QKD, the assumption in the theoretical proof is that the security of the protocol is considered under the asymptotic case where Alice and Bob exchange an infinite number of signals. In the continuous-variable QKD (CV-QKD), the finite-size effect imposes higher requirements on block size and excess noise control. However, the local local oscillator (LLO) CV-QKD system cannot be considered time-invariant under long blocks, especially in cases of environmental disturbances. Thus, we propose an LLO CV-QKD scheme with time-variant parameter estimation and compensation. We first establish an LLO CV-QKD theoretical model under the temporal modes of continuous-mode states. Then, a robust method is used to compensate for arbitrary frequency shift and arbitrary phase drift in CV-QKD systems with longer blocks, which cannot be achieved under traditional time-invariant parameter estimation. Besides, the digital signal processing method predicated on high-speed reference pilots can achieve a time complexity of O (1). In the experiment, the frequency shift is up to 89.05 MHz/s and phase drift is up to 3.036 Mrad/s using a piezoelectric transducer (PZT) to simulate the turbulences in the practical channel. With a signal-to-interference ratio (SIR) of −51.67 dB , we achieve a secret key rate (SKR) of 0.29 Mbits/s with an attenuation of 16 dB or a standard fiber of 80 km. This work paves the way for future long-distance field-test experiments in the finite-size regime.
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.
The compensation process for the state of polarization (SOP) from quantum signal plays an important role in the practical implementation of continuous-variable quantum key distribution (CV-QKD). In contrast to the previous scheme using an expensive digital dynamic polarization controller, we choose a cheaper manual polarization controller interfaced with a digital algorithm based on Kalman filter to achieve compensation of the polarization loss under the condition that the SOP of the fiber is relatively stable. This paper also enumerates the details of other digital signal processing method to achieve final secret key rate. And a pilot-sequential Gaussian modulated coherent state (GMCS) continuous-variable quantum key distribution system with a local local oscillator (LLO) is experimentally implemented based on the analysis of excess noise. Finally, the experimental results show that the method of polarization loss compensation takes advantages in maintaining the stability of the final secret key rate. And a secret key rate of 110 kbps is achieved within 20 km optical fiber (with 4 dB loss) under the finite-size block of1×1061 \times {10^{6}}1×106during a more extended time in the laboratory.