Alessandro Marcomini

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.

Phase randomization is essential for the security of practical quantum key distribution (QKD) systems. Commonly, implementations rely on laser sources (either actively phase-randomized, or gain-switched). However, at high repetition rates these show correlations, which can ultimate compromise security and performance. We present a 1.25 GHz fully phase-randomized QKD source based on a super luminescent diode (SLED) operating in the C-band as a compact and cost-effective alternative. The source generates ∼ 100 ps optical pulses with 400 ps time-bin separation, compatible with high-speed time-bin encoding. Interferometric measurements demonstrate > 99% visibility between adjacent time bins, confirming strong first-order coherence within a qubit, while the spontaneous-emission-driven nature of the SLED ensures intrinsic pulse to pulse phase randomization. The broadband architecture further enables operation across multiple ITU channels, supporting wavelength-multiplexed QKD from a single emitter. This work establishes a scalable SLED-based platform for high-speed time-bin QKD systems.

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 work, 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.

Quantum key distribution (QKD) protocols leverage quantum mechanics to achieve information theoretically secure communication, yet real-world implementations must address experimental limitations, particularly phase correlations in weak coherent laser pulses (WCPs). High-speed gain-switching lasers, commonly used in QKD, can exhibit residual photons causing phase correlations between consecutive pulses, challenging the perfect phase randomization assumption crucial for the decoy-state BB84 protocol. Theoretical work has proposed security proofs that require knowledge of how closely each phase's probability distribution approximates uniformity, which is complex to estimate experimentally. In this study we introduce an experimental method to characterise phase correlations of any length under realistic conditions by modelling the phase generation process within the laser cavity. Additionally, we experimentally benchmark this practical routine for measuring second-order correlations using a double Michelson interferometer with tunable amplitude attenuators, allowing comprehensive characterisation of the phase generation process and accurate measurement of the phase probability distribution, thus enhancing the security of QKD systems.

Decoy-state quantum key distribution (QKD) represents nowadays the best countermeasure to attacks exploiting multi-photon emissions in realistic sources. A fundamental requirement is the uniform and independent distribution of phases of the transmitted pulses. However, this can not be true for lasers working under high-speed gain-switching conditions, as residual photons in the cavity can induce phase correlations across consecutive pulses. A security proof robust against such imperfections has been recently proposed, which requires knowledge of a parameter that quantifies how close the conditional distribution of each phase is to a uniform distribution. In this work we propose an experimental method to characterise this parameter in realistic setup conditions and we extend the application to the case of arbitrary length of correlations, aiming to enable experimental verification of the implementation security.