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IEEE International Conference on Quantum Computing and Engineering (QCE)

A polarization diversity CV-QKD detection scheme for channels with strong polarization drift

Daniel Pereira, Nuno A. Silva, Armando N. Pinto

Published Oct 1, 2021• 9 citations
Publisher

Abstract

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.

DOI
10.1109/qce52317.2021.00082

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•Jun 1, 2023•0 citations
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.
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.
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.
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.
Discrete variables quantum key distribution (DV-QKD), with its well-studied and scrutinized BB84 protocol, benefits from being very attractive for highly secure communications. However, current detection schemes rely on the use of InGaAs SPDAs, which limits not only its use in high temperature environments, but also high secure communication rates. A possible approach is the use of coherent homodyne detection schemes for polarization encoding based DV-QKD combined. To deploy polarization encoding DV-QKD over standard optical fiber high speed networks, the polarization drift suffered due to birefringence over the channel must be compensated. In this work, we use a machine learning (ML) polarization tracking and compensation algorithm combined with a coherent homodyne receiver, thus allowing the deployment of high-speed polarization encoding based DV-QKD in standard optical fibers. The ML-algorithm predicts the SOP evolution keeping the error rate below 1 %. In this way, the overhead to polarization monitoring is reduced leading to a secure key exchange rate (SKR) of 79 Mbps for a communication over 40 km optical fiber.
Abstract Continuous variable quantum key distribution (CV-QKD) is a promising emerging technology for the distribution of secure keys for symmetric encryption. It can be readily implemented using commercial off-the-shelf optical telecommunications components. A key requirement of the CV-QKD receiver is the ability to measure the quantum states at the correct time instance and rate using the correct orthogonal observables, referred to as synchronization. We propose a digital synchronization procedure for a modern CV-QKD system with locally generated local oscillator for coherent reception. Our proposed method is modulation format independent allowing it to be used in a variety of CV-QKD systems. We experimentally investigate its performance with a Gaussian modulated CV-QKD system operating over a 10–20 km span of standard single mode fibre. Since the procedure does not require hardware modifications it paves the way for cost-effective QKD solutions that can adapt rapidly to changing environmental conditions.
Quantum key distribution is information-theoretically secure and a promising candidate for security against quantum computers. However, experimental implementations might deviate from assumptions made in the security proofs and leave the system vulnerable. Measurement device-independent QKD (MDI-QKD) scheme closes all the loopholes on the detection side, which is the most susceptible part of QKD. Therefore, the state preparation part of QKD systems becomes the only source of imperfections that needs to be considered in the security proofs of MDI-QKD. We examine our polarization encoding module and how imperfection in its components can affect state preparation. Our experiment demonstrates the feasibility of generating a secure key with imperfect states prepared by off-the-shelf devices. Another practical challenge that we address in implementing fibre-based MDI-QKD is polarization compensation. Polarization variations due to temperature fluctuations and fibre birefringence are inevitable and require polarization stabilization. Before this work, polarization compensation schemes for QKD either reduced the key sharing cycle or required extra equipment, making them less scalable or commercially viable and preventing the wide adaptation of polarization encoding. We propose and implement a novel polarization compensation scheme in the MDI-QKD systems that avoids the abovementioned drawbacks by using part of discarded detections. Our scheme evaluates the polarization drift in real-time based on single measurements corresponding to decoy intensities. We successfully implemented this active polarization compensation for four hours over 40 km of buffered fibre spool without thermal insulation or vibrational isolation. This thesis is a decisive step toward making polarization encoding MDI-QKD practical and secure, paving the way to realizing a polarization encoding MDI-QKD network. Future networks could be hybrid (ground and satellite links), and polarization encoding is a suitable candidate if an efficient stabilization scheme for fibre links is present. To this aim, our novel compensation scheme solely relies on recycling detections and retains a simple structure for a user node so that expanding the network could be feasible in terms of cost and practicality. Also, it does not reduce the key-sharing cycle, which is essential in networks where the detection system is shared, and each user has limited access time.