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arXiv:2510.20528

Feasibility of entanglement-based QKD protocols with SPDC and QD sources

Mariia Gumberidze, Vladyslav C. Usenko

Published Oct 23, 2025• 0 citations
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Abstract

We theoretically analyze the feasibility of entanglement-based quantum key distribution (QKD) protocols considering widely used spontaneous parametric down-conversion (SPDC) and novel quantum dot (QD) sources. We account for multiphoton emission in SPDC sources and fine-structure splitting (FSS) in QD. In addition, we incorporate imperfect detection, including dark counts and limited efficiency. For SPDC sources, we confirm that the presence of vacuum and multiphoton pairs renders them unsuitable for secure device-independent (DI) QKD implementations under standard detection strategies. Conversely, in the case of QD sources, accounting for the effects of FSS, results in reduced performance of protocols. Our findings are crucial for the practical implementation of entanglement-based QKD protocols using realistic sources and detectors.

DOI
10.48550/arxiv.2510.20528

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We theoretically analyze the feasibility of entanglement-based quantum key distribution (QKD) protocols considering widely used spontaneous parametric down-conversion (SPDC) and novel quantum dot (QD) sources. We account for multiphoton emission in SPDC sources and fine-structure splitting (FSS) in QD. In addition, we incorporate imperfect detection, including dark counts and limited efficiency. For SPDC sources, we confirm that the presence of vacuum and multiphoton pairs renders them unsuitable for secure device-independent (DI) QKD implementations under standard detection strategies. Conversely, in the case of QD sources, accounting for the effects of FSS, results in reduced performance of protocols. Our findings are crucial for the practical implementation of entanglement-based QKD protocols using realistic sources and detectors.
We theoretically analyze the feasibility of entanglement-based quantum key distribution (QKD) protocols [1], [2], considering both spontaneous parametric down-conversion (SPDC) and quantum dot (QD) sources. We account for multiphoton emission in SPDC sources and fine-structure splitting (FSS) in QD. In addition, we incorporate imperfect detection, including dark counts and limited efficiency. For SPDC sources, we confirm the presence of vacuum and multiphoton pairs renders them unsuitable for secure device-independent (DI) QKD implementations. Conversely, with QD sources, accounting for the effects of FSS results in reduced secret key rates compared to ideal conditions. Our findings are crucial for the practical implementation of entanglement-based QKD protocols using realistic sources and detectors.
We theoretically analyse the relation between non-Gaussianity and security of entanglement-based quantum key distribution (QKD) protocols, namely device-independent (DI) and entanglement-based BB848484. A similar analysis has already been made for prepare-and-measure (P\&M) protocols \cite{Lasota2017}. In addition, we consider imperfect detection with dark counts and limited efficiency. We assume a perfect source of entangled Bell states as produced by quantum-dot type sources, depolarisation in the channel and different noise statistics, namely thermal and Poissonian. We consider single-photon avalanche photodiodes (SPAD) and photon number resolving detectors (PNRD) and use their respective criteria for non-Gaussianity. The results show cross-regions for both security and non-Gaussianity, hence, the possibility to conclude about the suitability of a given channel for secret key distribution. Our results can be useful as a pre-check for the implementation of QKD protocols.
We theoretically analyze the feasibility of entanglement-based quantum key distribution (QKD) protocols-both device-independent QKD (DI-QKD) and entanglement-based BB84 (EPR-BB84)-using widely employed spontaneous parametric down-conversion (SPDC) and what we believe to be a novel semiconductor quantum-dot (QD) sources. Within a unified photodetection-theory framework, we jointly evaluate the Bell parameter, quantum bit error rate (QBER), and asymptotic Devetak-Winter secret key rate for both source types, accounting for multiphoton emission in SPDC, fine-structure splitting (FSS) in the QD, and imperfect detection including finite efficiency and dark counts. Our results demonstrate that, despite the detrimental effect of FSS, QD sources achieve sufficient Bell violation and positive secret key rates under realistic experimental conditions. In contrast, within the standard CHSH-based DI-QKD framework and the detection and binning strategies, bare SPDC sources do not yield a positive secret key rate, despite the feasibility of loophole-free Bell tests with such sources. Our findings are crucial for the practical implementation of entanglement-based QKD protocols using realistic sources and detectors.
Abstract The paper analyzes security aspects of practical entanglement-based quantum key distribution (QKD), namely, BBM92 or entanglement-based BB84 protocol. Similar to prepare-and-measure QKD protocols, practical implementations of the entanglement-based QKD have to rely upon non-ideal photon sources. A typical solution for entanglement generation is the spontaneous parametric down-conversion. However, this process creates not only single photon pairs, but also quantum states with more than two photons, which potentially may lead to security deterioration. We show that this effect does not impair the security of entanglement-based QKD systems. We also review the available security proofs and show that properties of the entanglement source have nothing to do with security degradation.
A workflow for evaluation of entanglement source quality is proposed. Based on quantum state density matrices obtained from theoretical models and experimental data, we make an estimate of a potential performance of a quantum entanglement source in quantum key distribution protocols. This workflow is showcased for continuously pumped spontaneous parametric down-conversion (SPDC) source, where it highlights the trade-off between entangled pair generation rate and entanglement quality caused by multiphoton nature of the generated quantum states. We employ this characterization technique to show that secure key rate of down-converted photon pairs is limited to 0.029 bits per detection window due to intrinsic multiphoton contributions. We also report that there exists one optimum gain for continuous-wave down-conversion sources. We find a bound for secure key rate extracted from SPDC sources and make a comparison with perfectly single-pair quantum states, such as those produced by quantum dots.
Quantum key distribution (QKD) is at the heart of future secure quantum communication networks as it can enhance security of classical communication strategies. Although prepare-and-measure protocols are the most widely used in practical applications, entangled-based QKD is promising for further improving the degree of security and developing free-trusted node networks. However, free-space link implementations for single photon transmission suffer daylight operations and few works have been demonstrated in this regime. Furthermore, entangled-based QKD in air link has been demonstrated only using spontaneous parametric down-conversion (SPDC) sources and never tested using quantum dot devices, despite they potentially overcome SPDC performances in terms of effi-ciency and fidelity of the QKD protocol.