arXiv subjects

Alessandro Marcomini

Publications and source records attributed to Alessandro Marcomini.

6 recordsLinked to original sources

arXiv2026

Phase-correlation-free quantum key distribution source operating at gigahertz rates

Phase randomization is essential for the security of practical decoy-state quantum key distribution (QKD) systems. Commonly, implementations rely on laser sources which are either actively phase-randomized, or gain-switched. However, at high repetition rates these show correlations, which can ultimately compromise security and performance. We present a 1.25 GHz phase-randomized QKD source based on a super-luminescent light emitting diode (SLED) operating in the C-band as a compact and cost-effective alternative. The source generates $\sim100$ ps optical pulses with $400$ ps pulse-to-pulse separation, compatible with high-speed time-bin encoding. Interferometric measurements demonstrate $>99\%$ visibility between adjacent time bins, confirming strong first-order coherence within the same quantum signals, while the spontaneous-emission-driven nature of the SLED ensures intrinsic global phase randomization between adjacent signals. This work establishes a scalable SLED-based platform for high-speed prepare-and-measure QKD systems.

quant-ph↗

arXiv2026

Security of decoy-state quantum key distribution with correlated bit-and-basis encoders

Practical quantum key distribution (QKD) modulators inevitably introduce correlations, causing the state emitted in a given round to depend on the setting choices made in previous rounds. These correlations break the round-by-round independence structure on which many widely used security proof techniques rely, leaving a significant gap between available theoretical guarantees and the reality of practical implementations. In this work, we develop a fully analytical finite-key security proof for decoy-state BB84 against general coherent attacks that rigorously incorporates correlations introduced by Alice's bit-and-basis encoder, while requiring only partial characterization of such correlations. Crucially, it does so without requiring the round partitioning of the data used for phase-error estimation introduced in previous works, showing that this technique is not indispensable for dealing with correlated encoders, and resulting in improved finite-key performance. Moreover, we also analyze the case of ideal single-photon sources, and our results reveal a counterintuitive effect: in the presence of strong encoding correlations, decoy-state BB84 can outperform single-photon implementations.

quant-ph↗

arXiv2026

Gate Optimization via Efficient Two-Qubit Benchmarking for NV Centers in Diamond

High-fidelity gate implementation requires sophisticated control pulses that steer the quantum system to undergo the desired transformation. Quantum Optimal Control allows to derive these control pulses in an open-loop fashion based on numerical simulations. However, their precision can be limited by incomplete knowledge of the system. Closed-loop optimization overcomes this limitation by incorporating feedback from measurements, provided a suitable and efficient measure of the gate performance can be defined. In this article, we present an efficient method to evaluate the performance of a two-qubit gate by preparation and measurement of only two quantum states, enabling experimental closed-loop optimization with a metric previously believed to be limited to open-loop control. We tailor the approach to nitrogen-vacancy centers in diamond and, through numerical simulations, demonstrate how the method can optimize a two-qubit gate while reducing the number of required measurements by two orders of magnitude compared to standard process tomography under realistic experimental settings.

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arXiv2025

Reference-Beam Attacks against Twin-Field Quantum Key Distribution using Optical Injection Locking

Twin-Field Quantum Key Distribution (TF-QKD) has become a leading protocol to bring quantum communications to the national scale. The protocol requires the establishment of a shared phase and frequency reference between distant parties, which is commonly achieved by using an external reference laser in an Optical Injection Locking (OIL) architecture. In this work, we analyze the side channels in OIL-based TF-QKD that may arise from adversarial manipulation of the various degrees of freedom of this untrusted reference beam. We experimentally demonstrate two realistic attack scenarios: fast intensity modulation of the reference laser, and additional signals embedded in the reference light exploiting wavelengths undetectable by conventional monitoring techniques. These attacks can allow a potential eavesdropper to deterministically increase the mean photon number of the sources, or circumvent the decoy-state technique, respectively. To counter these vulnerabilities, we propose practical and highly effective countermeasures that reinforce the security of TF-QKD systems without significant additional complexity or performance degradation.

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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.

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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.

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