arXiv subjects

Marcos Curty

Publications and source records attributed to Marcos Curty.

At least 37 records · Page 2Linked to original sources

arXiv2025

Numerical security analysis for quantum key distribution with partial state characterization

Numerical security proofs offer a versatile approach for evaluating the secret-key generation rate of quantum key distribution (QKD) protocols. However, existing methods typically require perfect source characterization, which is unrealistic in practice due to the presence of inevitable encoding imperfections and side channels. In this paper, we introduce a novel security proof technique based on semidefinite programming that can evaluate the secret-key rate for both prepare-and-measure and measurement-device-independent QKD protocols when only partial information about the emitted states is available, significantly improving the applicability and practical relevance compared to existing numerical techniques. We demonstrate that our method can outperform current analytical approaches addressing partial state characterization in terms of achievable secret-key rates, particularly for protocols with non-qubit encoding spaces. This represents a significant step towards bridging the gap between theoretical security proofs and practical QKD implementations.

quant-ph↗

arXiv2025

Quantum key distribution with imperfectly isolated devices

Most security proofs of quantum key distribution (QKD) assume that there is no unwanted information leakage about the state preparation process. However, this assumption is impossible to guarantee in practice, as QKD systems can leak information to the channel due to device imperfections or the active action of an eavesdropper. Here, we solve this pressing issue by introducing a security proof in the presence of information leakage from all state preparation settings for arguably the most popular QKD scheme, namely the decoy-state BB84 protocol. The proof requires minimal experimental characterization, as only a single parameter related to the isolation of the source needs to be determined, thus providing a clear path for bridging the gap between theory and practice. Moreover, if information about the state of the side channels is available, this can be readily incorporated into the analysis to further improve the resulting performance.

quant-ph↗

arXiv2025

Quantum key distribution overcoming practical correlated intensity fluctuations

Intensity correlations between neighboring pulses open a prevalent yet often overlooked security loophole in decoy-state quantum key distribution (QKD). As a solution, we present and experimentally demonstrate an intensity-correlation-tolerant QKD protocol that mitigates the negative effect that this phenomenon has on the secret key rate according to existing security analyses. Compared to previous approaches, our method significantly enhances the robustness against correlations, notably improving both the maximum transmission distances and the achievable secret key rates across different scenarios. By relaxing constraints on correlation parameters, our protocol enables practical devices to counter intensity correlations. We experimentally demonstrate this first practical solution that directly overcomes this security vulnerability, establish the feasibility and efficacy of our proposal, taking a major step towards loophole-free and high-performance QKD.

quant-ph↗

arXiv2024

Loss-tolerant quantum key distribution with detection efficiency mismatch

Current implementations of quantum key distribution (QKD) typically rely on prepare-and-measure (P&M) schemes. Unfortunately, these implementations are not completely secure, unless security proofs fully incorporate all imperfections of real devices. So far, existing proofs have primarily focused on imperfections of either the light source or the measurement device. In this paper, we establish a security proof for the loss-tolerant P&M QKD protocol that incorporates imperfections in both the source and the detectors. Specifically, we demonstrate the security of this scheme when the emitted states deviate from the ideal ones and Bob's measurement device does not meet the basis-independent detection efficiency condition. Furthermore, we conduct an experiment to characterise the detection efficiency mismatch of commercial single-photon detectors as a function of the polarisation state of the input light, and determine the expected secret key rate in the presence of state preparation flaws when using such detectors. Our work provides a way towards guaranteeing the security of actual implementations of widely deployed P&M QKD.

quant-ph↗

arXiv2024

Characterising higher-order phase correlations in gain-switched laser sources with application to quantum key distribution

Multi-photon emissions in laser sources represent a serious threat for the security of quantum key distribution (QKD). While the decoy-state technique allows to solve this problem, it requires uniform phase randomisation of the emitted pulses. However, gain-switched lasers operating at high repetition rates do not fully satisfy this requirement, as residual photons in the laser cavity introduce correlations between the phases of consecutive pulses. Here, we introduce experimental schemes to characterise the phase probability distribution of the emitted pulses, and demonstrate that an optimisation task over interferometric measures suffices in determining the impact of arbitrary order correlations, which ultimately establishes the security level of the implementation according to recent security proofs. We expect that our findings may find usages beyond QKD as well.

quant-ph↗

arXiv2024

Security of practical modulator-free quantum key distribution

Recent advancements in quantum key distribution have led to the development of various modulator-free transmitters. Among their advantages, these transmitters offer enhanced security against Trojan-horse attacks. However, practical implementations emit residual pulses that, while not used in the quantum communication, still carry information about Alice's settings. While the intensity of these pulses can be attenuated with an intensity modulator, the extinction ratio of these devices is always finite, and therefore it remains crucial to account for the residual information leakage at the security-proof level. In this work, we analyze the security of these transmitters and evaluate their performance. We find that the secret-key rate of the protocol is severely affected when the information leakage is not sufficiently attenuated, which highlights the importance of accounting for such type of imperfections.

quant-ph↗

arXiv2024

Hacking coherent-one-way quantum key distribution with present-day technology

Recent results have shown that the secret-key rate of coherent-one-way (COW) quantum key distribution (QKD) scales quadratically with the system's transmittance, thus rendering this protocol unsuitable for long-distance transmission. This was proven by using a so-called zero-error attack, which relies on an unambiguous state discrimination (USD) measurement. This type of attack allows the eavesdropper to learn the whole secret key without introducing any error. Here, we investigate the feasibility and effectiveness of zero-error attacks against COW QKD with present-day technology. For this, we introduce two practical USD receivers that can be realized with linear passive optical elements, phase-space displacement operations and threshold single-photon detectors. The first receiver is optimal with respect to its success probability, while the second one can impose stronger restrictions on the protocol's performance with faulty eavesdropping equipment. Our findings suggest that zero-error attacks could break the security of COW QKD even assuming realistic experimental conditions.

quant-ph↗

arXiv2024

Quantum key distribution with unbounded pulse correlations

A prevalent issue in practical applications of quantum key distribution (QKD) is the emergence of correlations among the emitted signals. Although recent works have proved the security of QKD in the presence of this imperfection, they rest on the premise that pulse correlations are of finite length. However, this assumption is not necessarily met in practice, since the length of these correlations could be potentially unbounded. Indeed, the first emitted pulse could be correlated with the last one, even if very faintly. Still, intuitively, there should exist a pulse separation threshold after which these correlations become so small as to be essentially negligible, rendering them inconsequential from a security standpoint. Building on this insight, we introduce a general formalism designed to extend existing security proofs to the practically relevant scenario in which pulse correlations have an unbounded length. This approach significantly enhances the applicability of these proofs and the robustness of QKD's implementation security.

quant-ph↗

arXiv2024

Security of quantum key distribution with imperfect phase randomisation

The performance of quantum key distribution (QKD) is severely limited by multiphoton emissions, due to the photon-number-splitting attack. The most efficient solution, the decoy-state method, requires that the phases of all transmitted pulses are independent and uniformly random. In practice, however, these phases are often correlated, especially in high-speed systems, which opens a security loophole. Here, we address this pressing problem by providing a security proof for decoy-state QKD with correlated phases that offers key rates close to the ideal scenario. Our work paves the way towards high-performance secure QKD with practical laser sources, and may have applications beyond QKD.

quant-ph↗

arXiv2023

A fully passive transmitter for decoy-state quantum key distribution

A passive quantum key distribution (QKD) transmitter generates the quantum states prescribed by a QKD protocol at random, combining a fixed quantum mechanism and a post-selection step. By avoiding the use of active optical modulators externally driven by random number generators, passive QKD transmitters offer immunity to modulator side channels and potentially enable higher frequencies of operation. Recently, the first linear optics setup suitable for passive decoy-state QKD has been proposed. In this work, we simplify the prototype and adopt sharply different approaches for BB84 polarization encoding and decoy-state generation. On top of it, we elaborate a tight custom-made security analysis surpassing an unnecessary assumption and a post-selection step that are central to the former proposal.

quant-ph↗

arXiv2023

Implementation security in quantum key distribution

The problem of implementation security in quantum key distribution (QKD) refers to the difficulty of meeting the requirements of mathematical security proofs in real-life QKD systems. Here, we provide a succint review on this topic, focusing on discrete variable QKD setups. Particularly, we discuss some of their main vulnerabilities and comment on possible approaches to overcome them.

quant-ph↗

arXiv2023

Experimental demonstration of fully passive quantum key distribution

The passive approach to quantum key distribution (QKD) consists of removing all active modulation from the users' devices, a highly desirable countermeasure to get rid of modulator side-channels. Nevertheless, active modulation has not been completely removed in QKD systems so far, due to both theoretical and practical limitations. In this work, we present a fully passive time-bin encoding QKD system and report on the successful implementation of a modulator-free QKD link. According to the latest theoretical analysis, our prototype is capable of delivering competitive secret key rates in the finite key regime.

quant-ph↗

arXiv2023

Finite-key security of passive quantum key distribution

The passive approach to quantum key distribution (QKD) consists of eliminating all optical modulators and random number generators from QKD systems, in so reaching an enhanced simplicity, immunity to modulator side channels, and potentially higher repetition rates. In this work, we provide finite-key security bounds for a fully passive decoy-state BB84 protocol, considering a passive QKD source recently presented. With our analysis, the attainable secret key rate is comparable to that of the perfect parameter estimation limit, in fact differing from the key rate of the active approach by less than one order of magnitude. This demonstrates the practicality of fully passive QKD solutions.

quant-ph↗

arXiv2023

Secret key rate bounds for quantum key distribution with non-uniform phase randomization

Decoy-state quantum key distribution (QKD) is undoubtedly the most efficient solution to handle multi-photon signals emitted by laser sources, and provides the same secret key rate scaling as ideal single-photon sources. It requires, however, that the phase of each emitted pulse is uniformly random. This might be difficult to guarantee in practice, due to inevitable device imperfections and/or the use of an external phase modulator for phase randomization, which limits the possible selected phases to a finite set. Here, we investigate the security of decoy-state QKD with arbitrary, continuous or discrete, non-uniform phase randomization, and show that this technique is quite robust to deviations from the ideal uniformly random scenario. For this, we combine a novel parameter estimation technique based on semi-definite programming, with the use of basis mismatched events, to tightly estimate the parameters that determine the achievable secret key rate. In doing so, we demonstrate that our analysis can significantly outperform previous results that address more restricted scenarios.

quant-ph↗

arXiv2023

Quantum key distribution with correlated sources

In theory, quantum key distribution (QKD) offers information-theoretic security. In practice, however, it does not due to the discrepancies between the assumptions used in the security proofs and the behaviour of the real apparatuses. Recent years have witnessed a tremendous effort to fill the gap, but the treatment of correlations among pulses has remained a major elusive problem. Here, we close this gap by introducing a simple yet general method to prove the security of QKD with arbitrarily long-range pulse correlations. Our method is compatible with those security proofs that accommodate all the other typical device imperfections, thus paving the way towards achieving implementation security in QKD with arbitrary flawed devices. Moreover, we introduce a new framework for security proofs, which we call the reference technique. This framework includes existing security proofs as special cases and it can be widely applied to a number of QKD protocols.

quant-ph↗

arXiv2022

Modified BB84 quantum key distribution protocol robust to source imperfections

The Bennett-Brassard 1984 (BB84) protocol is the most widely implemented quantum key distribution (QKD) scheme. However, despite enormous theoretical and experimental efforts in the past decades, the security of this protocol with imperfect sources has not yet been rigorously established. In this work, we address this shortcoming and prove the security of the BB84 protocol in the presence of multiple source imperfections, including state preparation flaws and side channels, such as Trojan-horse attacks, mode dependencies and classical correlations between the emitted pulses. To do so, we consider a modified BB84 protocol that exploits the basis mismatched events, which are often discarded in standard security analyses of this scheme; and employ the reference technique, a powerful mathematical tool to accommodate source imperfections in the security analysis of QKD. Moreover, we compare the achievable secret-key rate of the modified BB84 protocol with that of the three-state loss-tolerant protocol, and show that the addition of a fourth state, while redundant in ideal conditions, significantly improves the estimation of the leaked information in the presence of source imperfections, resulting in a better performance. This work demonstrates the relevance of the BB84 protocol in guaranteeing implementation security, taking us a step further towards closing the existing gap between theory and practice of QKD.

quant-ph↗

arXiv2022

Advances in device-independent quantum key distribution

Device-independent quantum key distribution (DI-QKD) provides the gold standard for secure key exchange. Not only it allows for information-theoretic security based on quantum mechanics, but it relaxes the need to physically model the devices, hence fundamentally ruling out many quantum hacking threats to which non-DI QKD systems are vulnerable. In practice though, DI-QKD is very challenging. It relies on the loophole-free violation of a Bell inequality, a task that requires high quality entanglement to be distributed between distant parties and close to perfect quantum measurements, which is hardly achievable with current technology. Notwithstanding, recent theoretical and experimental efforts have led to the first proof-of-principle DI-QKD implementations. In this article, we review the state-of-the-art of DI-QKD by highlighting its main theoretical and experimental achievements, discussing the recent proof-of-principle demonstrations, and emphasizing the existing challenges in the field.

quant-ph↗

arXiv2022

Fully-Passive Quantum Key Distribution

Passive implementations of quantum key distribution (QKD) sources are highly desirable as they eliminate side-channels that active modulators might introduce. Up till now, passive decoy-state and passive encoding BB84 schemes have both been proposed. Nonetheless, passive decoy-state generation and passive encoding have never been simultaneously implemented with linear optical elements before, which greatly limits the practicality of such passive QKD schemes. In this work, we overcome this limitation and propose a fully-passive QKD source with linear optics that eliminates active modulators for both decoy-state choice and encoding. This allows for highly practical QKD systems that avoid side-channels from the source modulators. The passive source we propose (combined with the decoy-state analysis) can create any arbitrary state on a qubit system and is protocol-independent. That is, it can be used for various protocols such as BB84, reference-frame-independent QKD, or the six-state protocol. It can also in principle be combined with e.g. measurement-device-independent QKD, to build a system without side-channels in either detectors or modulators.

quant-ph↗