Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2507.15446 · Scientific Reports

Realistic vulnerabilities of decoy-state quantum key distribution

Ivan S. Sushchev, Kirill E. Bugai, S. N. Molotkov, Daniil S. Bulavkin, Anna S. Sidelnikova, Dmitri Melkonian, Veronika M. Vakhrusheva, R. Yu. Lokhmatov, Dmitriy A. Dvoretskiy

Published Dec 29, 2025• 4 citations
arXivHTMLPDF

Abstract

We analyze realistic vulnerabilities of decoy-state quantum key distribution (QKD) arising from the combination of laser damage attack (LDA) and unambiguous state discrimination (USD). While decoy-state QKD is designed to protect against photon-number-splitting and beam-splitting attacks by accurately estimating the single-photon fraction, it relies on stable attenuation to prepare pulses with fixed mean-photon numbers. An eavesdropper (Eve) can exploit LDA to irreversibly alter the optical components on Alice's side, effectively increasing the mean-photon numbers beyond the decoy-state security regime. We show that once the alteration exceeds a critical threshold-on the order of 10-20 dB-Eve can implement an efficient USD-based intercept-resend strategy using current off-the-shelf technology, thus obtaining the entire secret key. Numerical simulations confirm that for sufficiently elevated mean-photon numbers, Eve's conclusive measurement outcomes skew the decoy-state statistics, yet remain undetected by standard security checks. We further demonstrate how a modified USD setup employing an additional beam splitter can reduce the required threshold, facilitating Eve's attack. Additionally, we introduce the pseudo-photon-number resolution USD attack, which allows Eve to emulate all observable gains at Bob's side so that she remains fully undetectable even with advanced statistical checks. Our findings emphasize the need for robust safeguards against high-power laser damage in QKD systems, including careful hardware selection, rigorous testing under high-power illumination, and real-time monitoring to ensure the integrity of the decoy-state protocol.

DOI
10.1038/s41598-025-29144-6

Related papers

Quantum-Key Distribution using Decoy Pulses to Combat Photon-Number Splitting by Eavesdropper: An Event-by-Event Impairment Enumeration Approach for Performance Evaluation and Design

Debasish Datta

Quantum-key distribution (QKD) schemes employing quantum communication links are typically based on the transmission of weak optical pulses over optical fibers to setup a secret key between the transmitting and receiving nodes. Alice transmits optically a random bit stream to the receiver (Bob) through the photon polarizations or the quadrature components of the lightwaves associated with the photons, with a secret key remaining implicitly embedded therein. However, during the above transmission, some eavesdropper (Eve) might attempt to tap the passing-by photons from the optical fiber links to extract the key. In one of the popular QKD schemes, along with signal pulses, some additional decoy pulses are transmitted by Alice, while Eve might use photon-number splitting (PNS) for eavesdropping. In a typical PNS scheme, (i) the optical pulses with single photon are blocked by Eve, (ii) from the optical pulses with two photons, one photon is retained by Eve to carry out eavesdropping operation and the other is retransmitted to Bob, and (iii) all other pulses with more than two photons are retransmitted by Eve to Bob without retaining any photon from them. Extensive theoretical research has been carried out on such QKD schemes, by employing information-theoretic approach along with computer simulations and experimental studies. We present a novel event-by-event impairment enumeration approach to evaluate the overall performance of one such QKD scheme analytically with due consideration to the physical layer of the quantum communication links. The proposed approach monitors the impairments of the propagating optical pulses event-by-event at all possible locations along the optical fiber link using statistical approach, and provides estimates of the realizable key generation rate, while assuring an adequate yield ratio between signal and decoy pulses for the detection of possible eavesdropping.

arXiv:2501.18394•Jan 30, 2025•0 citations

The influence of an eavesdropper in point-to-multipoint QKD based on passive beam splitters on the photon number yields

Mira Stephan, Fabian Klingmann, René Kirrbach +1 more

In theory, quantum key distribution (QKD) provides unconditionally secure keys due to the laws of quantum mechanics. However, real-world QKD systems often use strongly attenuated lasers which enables the photon number splitting (PNS) attack. Decoy state protocols weaken an eavesdropper’s chance to perform the compromising PNS attack undetected. In small multi-user QKD networks the photons can be distributed to the users via passive beam splitters. This approach lowers the sender-to-receiver QKD channel efficiency (relative to a standard point-to-point QKD system) and reduces the mean photon number in the sender-to-receiver channel. For this contribution, we investigated the performance of decoy state protocols in a passive multi-user system. We compared this point-to-multipoint setup to a standard point-to-point system by checking the photon number dependent yields, hereby showing that with some restrictions such a system would be safe against the PNS attack.

Quantum Communications and Quantum Imaging•Oct 4, 2024•1 citation

Quantum soft filtering for the improved security analysis of the coherent one-way quantum-key-distribution protocol

Dmitry Anatolevich Kronberg, Anastasiia S. Nikolaeva, Yury Kurochkin +1 more

A precise security analysis of practical quantum key distribution (QKD) systems is an important step for improving their performance. Here we consider a class of quantum soft filtering operations, which generalizes the unambiguous state discrimination (USD) technique. These operations can be applied as a basis for a security analysis of the original coherent one-way (COW) QKD protocol since their application interpolates between beam-splitting (BS) and USD attacks. We demonstrate that a zero-error attack based on quantum soft filtering operations gives a larger amount of the information for Eve at a given level of losses. We calculate the Eve information as a function of the channel length. The efficiency of the proposed attack highly depends on the level of the monitoring under the maintenance of the statistics of control (decoy) states, and best-case results are achieved in the case of the absence of maintenance of control state statistics. Our results form additional requirements for the analysis of practical QKD systems based on the COW QKD protocol and its variants by providing an upper bound on the security.

arXiv:1910.06167•Physical Review A•Mar 20, 2020•26 citations

Quantum key distribution with distinguishable decoy states

Anqi Huang, Shi‐Hai Sun, Zhihong Liu +1 more

The decoy-state protocol has been considered to be one of the most important methods to protect the security of quantum key distribution (QKD) with a weak coherent source. Here we test two experimental approaches to generating the decoy states with different intensities: modulation of the pump current of a semiconductor laser diode, and external modulation by an optical intensity modulator. The former approach shows a side channel in the time domain that allows an attacker to distinguish s signal state from a decoy state, breaking a basic assumption in the protocol. We model a photon-number-splitting attack based on our experimental data, and show that it compromises the system's security. Then, based on the work of Tamaki et al. [New J. Phys. 18, 065008 (2016)], we obtain two analytical formulas to estimate the yield and error rate of single-photon pulses when the signal and decoy states are distinguishable. The distinguishability reduces the secure key rate below that of a perfect decoy-state protocol. To mitigate this reduction, we propose to calibrate the transmittance of the receiver (Bob's) unit. We apply our method to three QKD systems and estimate their secure key rates.

arXiv:1711.00597•Physical Review A•Jul 27, 2018•95 citations

The rationality about the assumption that the signal and decoy states are indistinguishable in decoy-state quantum key distribution

Chen, Xiao-Ming, Lei Chen, Yan, Ya-Long

Decoy-state quantum key distribution (QKD) has become the most efficient method to resist the photon-number-splitting (PNS) attack and estimate the secure key rate. The decoy-state method has many assumptions, among which a critical one is that an eavesdropper (Eve) cannot distinguish between the signal and decoy states. However, a rigorous proof of the rationality about this assumption is not yet available so far. In fact, due to the difference of photon-number probability distribution between the signal and decoy states, Eve is able to distinguish the two states with a certain probability. In this work, we adopt the Bayesian decision to distinguish the signal and decoy states in one-decoy-state QKD, and perform different PNS attack strategies for the two states according to the previous decision. The numerical simulations indicate that the attack effect is not obvious or even failed. Thus, it is reasonable to assume that the signal and decoy states are indistinguishable in decoy-state QKD. In addition, we also provide the method to set the intensities of signal and decoy states properly, which can not only reduce the preparation cost and improve the communication efficiency, but also avoid the attack from Eve using the intensity difference between the signal and decoy states.

arXiv:2110.08504•Oct 16, 2021•0 citations

Enhancing practical security of quantum key distribution with a few decoy states

Jim Harrington, Ettinger, J. Mark, Richard Hughes +1 more

Quantum key distribution establishes a secret string of bits between two distant parties. Of concern in weak laser pulse schemes is the especially strong photon number splitting attack by an eavesdropper, but the decoy state method can detect this attack with current technology, yielding a high rate of secret bits. In this Letter, we develop rigorous security statements in the case of finite statistics with only a few decoy states, and we present the results of simulations of an experimental setup of a decoy state protocol that can be simply realized with current technology.

arXiv:quant-ph/0503002•Mar 1, 2005•33 citations
We analyze realistic vulnerabilities of decoy-state quantum key distribution (QKD) arising from the combination of laser damage attack (LDA) and unambiguous state discrimination (USD). While decoy-state QKD is designed to protect against photon-number-splitting and beam-splitting attacks by accurately estimating the single-photon fraction, it relies on stable attenuation to prepare pulses with fixed mean-photon numbers. An eavesdropper (Eve) can exploit LDA to irreversibly alter the optical components on Alice's side, effectively increasing the mean-photon numbers beyond the decoy-state security regime. We show that once the alteration exceeds a critical threshold-on the order of 10-20 dB-Eve can implement an efficient USD-based intercept-resend strategy using current off-the-shelf technology, thus obtaining the entire secret key. Numerical simulations confirm that for sufficiently elevated mean-photon numbers, Eve's conclusive measurement outcomes skew the decoy-state statistics, yet remain undetected by standard security checks. We further demonstrate how a modified USD setup employing an additional beam splitter can reduce the required threshold, facilitating Eve's attack. Additionally, we introduce the pseudo-photon-number resolution USD attack, which allows Eve to emulate all observable gains at Bob's side so that she remains fully undetectable even with advanced statistical checks. Our findings emphasize the need for robust safeguards against high-power laser damage in QKD systems, including careful hardware selection, rigorous testing under high-power illumination, and real-time monitoring to ensure the integrity of the decoy-state protocol.
Quantum-key distribution (QKD) schemes employing quantum communication links are typically based on the transmission of weak optical pulses over optical fibers to setup a secret key between the transmitting and receiving nodes. Alice transmits optically a random bit stream to the receiver (Bob) through the photon polarizations or the quadrature components of the lightwaves associated with the photons, with a secret key remaining implicitly embedded therein. However, during the above transmission, some eavesdropper (Eve) might attempt to tap the passing-by photons from the optical fiber links to extract the key. In one of the popular QKD schemes, along with signal pulses, some additional decoy pulses are transmitted by Alice, while Eve might use photon-number splitting (PNS) for eavesdropping. In a typical PNS scheme, (i) the optical pulses with single photon are blocked by Eve, (ii) from the optical pulses with two photons, one photon is retained by Eve to carry out eavesdropping operation and the other is retransmitted to Bob, and (iii) all other pulses with more than two photons are retransmitted by Eve to Bob without retaining any photon from them. Extensive theoretical research has been carried out on such QKD schemes, by employing information-theoretic approach along with computer simulations and experimental studies. We present a novel event-by-event impairment enumeration approach to evaluate the overall performance of one such QKD scheme analytically with due consideration to the physical layer of the quantum communication links. The proposed approach monitors the impairments of the propagating optical pulses event-by-event at all possible locations along the optical fiber link using statistical approach, and provides estimates of the realizable key generation rate, while assuring an adequate yield ratio between signal and decoy pulses for the detection of possible eavesdropping.
In theory, quantum key distribution (QKD) provides unconditionally secure keys due to the laws of quantum mechanics. However, real-world QKD systems often use strongly attenuated lasers which enables the photon number splitting (PNS) attack. Decoy state protocols weaken an eavesdropper’s chance to perform the compromising PNS attack undetected. In small multi-user QKD networks the photons can be distributed to the users via passive beam splitters. This approach lowers the sender-to-receiver QKD channel efficiency (relative to a standard point-to-point QKD system) and reduces the mean photon number in the sender-to-receiver channel. For this contribution, we investigated the performance of decoy state protocols in a passive multi-user system. We compared this point-to-multipoint setup to a standard point-to-point system by checking the photon number dependent yields, hereby showing that with some restrictions such a system would be safe against the PNS attack.
A precise security analysis of practical quantum key distribution (QKD) systems is an important step for improving their performance. Here we consider a class of quantum soft filtering operations, which generalizes the unambiguous state discrimination (USD) technique. These operations can be applied as a basis for a security analysis of the original coherent one-way (COW) QKD protocol since their application interpolates between beam-splitting (BS) and USD attacks. We demonstrate that a zero-error attack based on quantum soft filtering operations gives a larger amount of the information for Eve at a given level of losses. We calculate the Eve information as a function of the channel length. The efficiency of the proposed attack highly depends on the level of the monitoring under the maintenance of the statistics of control (decoy) states, and best-case results are achieved in the case of the absence of maintenance of control state statistics. Our results form additional requirements for the analysis of practical QKD systems based on the COW QKD protocol and its variants by providing an upper bound on the security.
The decoy-state protocol has been considered to be one of the most important methods to protect the security of quantum key distribution (QKD) with a weak coherent source. Here we test two experimental approaches to generating the decoy states with different intensities: modulation of the pump current of a semiconductor laser diode, and external modulation by an optical intensity modulator. The former approach shows a side channel in the time domain that allows an attacker to distinguish s signal state from a decoy state, breaking a basic assumption in the protocol. We model a photon-number-splitting attack based on our experimental data, and show that it compromises the system's security. Then, based on the work of Tamaki et al. [New J. Phys. 18, 065008 (2016)], we obtain two analytical formulas to estimate the yield and error rate of single-photon pulses when the signal and decoy states are distinguishable. The distinguishability reduces the secure key rate below that of a perfect decoy-state protocol. To mitigate this reduction, we propose to calibrate the transmittance of the receiver (Bob's) unit. We apply our method to three QKD systems and estimate their secure key rates.
Decoy-state quantum key distribution (QKD) has become the most efficient method to resist the photon-number-splitting (PNS) attack and estimate the secure key rate. The decoy-state method has many assumptions, among which a critical one is that an eavesdropper (Eve) cannot distinguish between the signal and decoy states. However, a rigorous proof of the rationality about this assumption is not yet available so far. In fact, due to the difference of photon-number probability distribution between the signal and decoy states, Eve is able to distinguish the two states with a certain probability. In this work, we adopt the Bayesian decision to distinguish the signal and decoy states in one-decoy-state QKD, and perform different PNS attack strategies for the two states according to the previous decision. The numerical simulations indicate that the attack effect is not obvious or even failed. Thus, it is reasonable to assume that the signal and decoy states are indistinguishable in decoy-state QKD. In addition, we also provide the method to set the intensities of signal and decoy states properly, which can not only reduce the preparation cost and improve the communication efficiency, but also avoid the attack from Eve using the intensity difference between the signal and decoy states.
Quantum key distribution establishes a secret string of bits between two distant parties. Of concern in weak laser pulse schemes is the especially strong photon number splitting attack by an eavesdropper, but the decoy state method can detect this attack with current technology, yielding a high rate of secret bits. In this Letter, we develop rigorous security statements in the case of finite statistics with only a few decoy states, and we present the results of simulations of an experimental setup of a decoy state protocol that can be simply realized with current technology.