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arXiv:2408.17412 · Advanced Quantum Technologies

Hybrid Encoder for Discrete and Continuous Variable QKD

Mattia Sabatini, Tommaso Bertapelle, Paolo Villoresi, Giuseppe Vallone, Marco Avesani

Published May 20, 2025• 11 citations
arXivHTMLPDF

Abstract

Abstract Quantum key distribution (QKD) is emerging as a cutting‐edge application of quantum technology, gradually integrating into the industrial landscape. Many protocols employing discrete or continuous variables have been developed over time. Whereas the firsts usually excel in covering longer distances, the seconds are typically superior in producing higher secret key rates at short distances. Present efforts aim to create systems that can exploit both these strengths, foreseeing the future challenge regarding the realization of a quantum network consisting of multiple and heterogeneous interconnected nodes. Within such a context, a possible solution is systems able to efficiently toggle between discrete and continuous variable working modes with hybrid quantum state encoders. Therefore, this study presents a new hybrid encoder based on an iPOGNAC modulator, ensuring compatibility with Discrete‐Variable (DV) and Continuous‐Variable (CV) QKD systems that can be assembled entirely with commercial‐off‐the‐shelf components. The proposed scheme is the first supporting DV polarization protocols, thus making it an appealing candidate for space nodes of a future quantum network, given that polarization‐based protocols are well suited for space links.

DOI
10.1002/qute.202400522

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Abstract Quantum key distribution (QKD) is emerging as a cutting‐edge application of quantum technology, gradually integrating into the industrial landscape. Many protocols employing discrete or continuous variables have been developed over time. Whereas the firsts usually excel in covering longer distances, the seconds are typically superior in producing higher secret key rates at short distances. Present efforts aim to create systems that can exploit both these strengths, foreseeing the future challenge regarding the realization of a quantum network consisting of multiple and heterogeneous interconnected nodes. Within such a context, a possible solution is systems able to efficiently toggle between discrete and continuous variable working modes with hybrid quantum state encoders. Therefore, this study presents a new hybrid encoder based on an iPOGNAC modulator, ensuring compatibility with Discrete‐Variable (DV) and Continuous‐Variable (CV) QKD systems that can be assembled entirely with commercial‐off‐the‐shelf components. The proposed scheme is the first supporting DV polarization protocols, thus making it an appealing candidate for space nodes of a future quantum network, given that polarization‐based protocols are well suited for space links.
Quantum Key Distribution (QKD) offers unconditional security in principle. Many QKD protocols have been proposed and demonstrated to ensure secure communication between two authenticated users. Continuous variable (CV) QKD offers many advantages over discrete variable (DV) QKD since it is cost-effective, compatible with current classical communication technologies, efficient even in daylight, and gives a higher secure key rate. Keeping this in view, we demonstrate a discrete modulated CVQKD protocol in the free space which is robust against polarization drift. We also present the simulation results with a noise model to account for the channel noise and the effects of various parameter changes on the secure key rate. These simulation results help us to verify the experimental values obtained for the implemented CVQKD.
To overcome the limitations of both discrete variable (DV) and continuous variable (CV) QKD protocols, in this paper, a hybrid QKD protocol is proposed. In the proposed hybrid QKD protocol, Alice simultaneously performs discrete modulation (DM)-based encoding for CV-QKD subsystem and time-phase encoding for DV-QKD on a transmitter side and transmits such hybrid encoded pulse with optimized average number of photons per pulse. On receiver side, Bob employs a 1:2 optical space switch to select either DV-QKD receiver or CV-QKD receiver with the optimized probability of selection. Other compatible CV-QKD and DV-QKD protocols can also be used in hybrid QKD. Bob further performs the classical postprocessing applied to both subsystems so that resulting joint secure key is derived from both subsystems. The proposed hybrid QKD protocol significantly outperforms previously introduced both Gaussian modulation (GM)- and DM-based CV-QKD protocols as well as DV-QKD protocols in terms of both secret-key rate and achievable transmission distance.
To overcome the limitations of both discrete variable (DV) and continuous variable (CV) QKD protocols, in this paper, a hybrid DV-CV QKD protocol is proposed. In the proposed hybrid QKD protocol, Alice simultaneously performs discrete modulation (DM)-based encoding for CV-QKD subsystem and time-phase encoding for DV-QKD on a transmitter side. On receiver side, Bob employs a 1:2 optical space switch to select either DV-QKD receiver or CV-QKD receiver with the optimized probability of selection. Bob further performs the classical postprocessing applied to both subsystems so that resulting joint secure key is derived from both subsystems. The proposed hybrid QKD protocol significantly outperforms previously introduced both Gaussian modulation (GM)- and DM-based CV-QKD protocols as well as DV-QKD protocols in terms of both secret-key rate (SKR) and achievable transmission distance.
There exist several initiatives worldwide to deploy quantum key distribution (QKD) over existing fibre networks and achieve quantum-safe security at large scales. To understand the overall QKD network performance, it is required to transition from the analysis of individual links, as done so far, to the characterization of the network as a whole. In this work, we undertake this study by embedding QKD protocols on complex networks, which correctly model the existing fiber networks. We focus on networks with trusted nodes and on continuous-variable (CV) schemes, which have much higher key rates than their discrete-variable (DV) counterparts. In the effective CV network, however, many of the unique properties of complex networks, such as small-worldness and the presence of hubs, are lost due to the fast decay of the key rate with physical distance for CV systems. These properties can be restored when considering a hybrid network consisting of both CV and DV protocols, achieving at the same time high average rate and inter-connectivity. Our work opens the path to the study of QKD complex networks in existing infrastructures.
Establishing scalable, secure quantum networks requires advancing beyond conventional point-to-point quantum key distribution (QKD) protocols toward point-to-multipoint QKD protocols. Here, we generalize a well-established discrete-modulated continuous-variable (CV) QKD protocol from the point-to-point to the point-to-multipoint setting. We present a comprehensive security analysis across different scenarios and derive secret key rates for loss-only and noisy channels, both in the asymptotic and the composable finite-size regime. Experimentally, we validate the protocol in a passive optical network with 10 km access links, achieving a composable secure key rate of 2.185×10 −3 bits per symbol (0.273 Mbit/s) against independent and identically distributed collective attacks. Our results demonstrate that discrete-modulated CV-QKD can support access networks with multiple users while relying solely on cost-efficient, off-the-shelf telecommunication components, paving the way toward practical, scalable, and secure quantum networks.
Continuous-variable (CV) quantum key distribution (QKD) allows for quantum secure communication with the benefit of being close to classical coherent communication. In recent years, CV QKD protocols using a discrete number of displaced coherent states have been studied intensively as the modulation can be directly implemented with real devices with finite resolution. Until now, experiments only calculated key rates in the asymptotic regime. Here, we present a CV QKD system using discrete modulation that is especially designed for atmospheric channels. We use polarization encoding to exploit the nonbirefringent nature of the turbulent atmosphere. This allows to expand CV QKD networks beyond the existing fiber backbone. In a laboratory demonstration with a static 3-decibel loss channel, we implemented a recently developed security proof allowing to calculate composable finite-size key rates against independently and identically distributed collective attacks. We applied the full QKD protocol including a quantum random number generator, error correction, and privacy amplification to extract secret keys.