Marco Avesani

Quantum communication is advancing toward large-scale quantum networks, with quantum key distribution (QKD) serving as a key driving technology. However, seamless interoperability between fiber-based and free-space links remains a major challenge for heterogeneous quantum networks. Here we report, to the best of our knowledge, the first continuous-variable QKD (CV-QKD) system distributing secret keys using both coherent and squeezed states over a hybrid channel composed of a 620m free-space link followed by 2km of optical fiber, corresponding to a total loss of 20 dB. Daylight operation is enabled by intrinsic mode filtering provided by a locally generated local oscillator, eliminating the need for complex spectral or spatial filtering. In addition, we introduce an optimized binning strategy that mitigates free-space transmittance fluctuations, resulting in an average of 45% increase in the secure key rate. These results demonstrate the feasibility of CV-QKD across hybrid optical channels and highlight its potential as a plug-and-play solution for heterogeneous quantum networks integrating fiber and free-space infrastructure.

Continuous‑Variable Quantum Key Distribution (CV‑QKD) and Quantum Random Number Generation (CV‑QRNG) are crucial technologies relying on shot‑noise‑limited coherent detection to enable secure communication and high‑speed randomness generation. Integrated photonics plays a central role in advancing these technologies, offering compact, scalable, and efficient implementations. In this work, we introduce Femtosecond Laser Micromachining (FLM) on borosilicate glass as a novel platform for Photonic Integrated Circuits (PICs) tailored to coherent detection in quantum information processing. Using off‑chip detectors, we exploit the versatility of FLM to realize a PIC designed for CV‑QKD and CV‑QRNG. The device features fully tunable optical components, low insertion loss ($\leq$ 1.28 dB), polarization‑insensitive operation, and a Common‑Mode Rejection Ratio (CMRR) exceeding 73 dB. These capabilities enable the experimental demonstration of a Source‑device‑Independent CV‑QRNG with a secure rate of 42.74 Gbps and a QPSK‑based CV‑QKD system achieving a 3.2 Mbit/s secret key rate. Our results establish FLM as a promising integrated‑photonics platform for scalable, high‑performance quantum communication systems.

A wide range of applications require, by hypothesis, to have access to a private and genuine random source. Quantum Random Number Generators (QRNGs) are currently the sole technology capable of producing true randomness. Nevertheless, other factors must be considered when addressing real-world use cases, and the bulkiness of current implementations significantly limits their adoption. In this work, we present a high-performance source-device independent QRNG leveraging a custom-made integrated silicon photonic chip. The proposed scheme exploits the properties of a heterodyne receiver to enhance security and integration to promote spatial footprint reduction while simplifying its implementation. Such characteristics could represent a significant advancement toward the development of generators better suited to meet the demands of portable and space applications. Indeed, the system can deliver secure random numbers at a rate greater than 20 Gbps with a reduced encumbrance.

We propose Qubit4Sync, a synchronization method for Quantum Key Distribution (QKD) setups, based on the same qubits exchanged during the protocol and without requiring additional hardware other than the one necessary to prepare and measure the quantum states, in a similar fashion to the clock recovery used in classical communications. Our approach introduces a new cross-correlation algorithm achieving the lowest computational complexity, to our knowledge, for high channel losses. We tested the robustness of our scheme in a real QKD implementation, and we believe it may find application in other quantum communication protocols

Here we present a simple and robust polarization encoded QKD experiment where synchronization, polarization compensation and QKD are all performed with the same optical setup, without requiring any changes or any additional hardware, by exploiting only the transmission of quantum states. Furthermore, the developed polarization encoder exhibits high stability and the lowest intrinsic Quantum Bit Error Rate ever reported.

Space-based quantum key distribution would allow, in the near future, secure communications be- tween parties over continental distances, complementing short-range fiber-based quantum networks. However, further demonstrations of daylight operations over free-space channels and the full compatibility with the telecom-based fiber infrastructure are still necessary. Here we present the prototype for daylight QKD at 1550 nm we developed as a demonstrator for application of QKD both on ground and in Space. Our QKD source, exploiting integrated silicon photonics technology, allows to reach a QBER of 1% during the field-test performed over a 145 m link, and represents a promising resource to design quantum optical payloads to be implemented in future satellite missions.

Key exchange over an untrusted communication channel is a crucial step in modern cryptographic protocols, yet current approaches rely on computational assumptions to ensure their security. However, these assumptions may be undermined by an adversary possessing quantum computational capabilities. Quantum Key Distribution (QKD), both in its Discrete Variable (DV) and Continuous Variable (CV) format, is a way to counter this threat. By exploiting the principles of quantum mechanics, QKD enables the development of unconditionally secure protocols, providing a lasting solution that can withstand adversaries regardless of their computational resources. Between DV and CV, the latter is particularly appealing if speed and compatibility with the current fiber telecom infrastructure are considered. In fact, CV-QKD systems can benefit from coherent receivers and widely accessible fiber-based commercial telecom components. Despite this, several practical tasks must be addressed to enable such systems targeting field-deployed real-world scenarios. Among these are the integration of a Quantum Random Number Generator, real-time estimation of key system parameters necessary to compute the Secret Key Rate and network reconfigurability. This study presents a CV-QKD architecture integrating a coherent receiver at the transmitter, enabling secure quantum randomness generation and in-line estimation of key parameters for secret-key-rate computation, advancing the deployment over existing telecom networks.

Quantum Random Number Generators (QRNGs) are essential components for modern cryptography, as they are a practical source of true randomness, without which the security of such protocols cannot be guaranteed. Among the proposed schemes, Semi-Device-Independent QRNGs (SDI-QRNGs) based on photon-number constraints offer an appealing balance of security, speed, and experimental simplicity, but demonstrations have so far focused mainly on binary encoding and conditional min‑entropy certification. In this work, we report the first experimental implementation of a Continuous‑Variable SDI‑QRNG in a prepare‑and‑measure configuration building on the SDP‑based (Semi-Definite Programming) framework of Ref.~\cite{carceller2025}. This approach allows us to directly lower‑bound the conditional Shannon entropy, employ more complex modulation formats, and apply entropy‑accumulation techniques beyond the i.i.d. assumption. The experiment combines a low-loss integrated photonic heterodyne receiver with a simple transmitter built from commercial components, generating coherent‑state Quadrature-Phase-Shift-Keying (QPSK) modulation, to preserve experimental practicality while enabling high‑speed operation.

Intermodal quantum key distribution (QKD) provides a fundamental interface for scalable quantum networks. Nevertheless, long-distance implementations are typically compromised by turbulence-induced aberrations, which decrease coupling efficiency into single-mode fiber. Here, we report a real-time intermodal QKD field trial over an 18 km free-space link, connecting a remote terminal to an urban optical ground station. By utilizing an adaptive optics system for high-order aberration correction, we achieved efficient single-mode fiber coupling and a secure key rate of 200 bit/s using room-temperature detectors. Furthermore, we validate a turbulence-based coupling model that provides practical design guidelines for future interoperable quantum architectures.

We report, to the best of our knowledge, the first experimental demonstration of wavelength-division multiplexing between independent continuous-variable (CV) and discrete-variable (DV) quantum key distribution (QKD) systems operating simultaneously on the same optical link. We validate coexistence over a fiber and a daylight free-space channel, benchmarking the Secret Key Rate (SKR) versus channel attenuation while both systems operate simultaneously. We observe the expected CV-DV complementarity, with CV-QKD providing higher SKR at low loss and DV-QKD becoming advantageous in the high-loss regime. In free-space daylight, both systems sustain Mbit/s key rates under atmospheric fluctuations. In all scenarios analyzed, simultaneous operation introduces negligible multiplexing-induced penalty. These results provide a validation of hybrid CV-DV architectures for heterogeneous quantum communication infrastructures, where high-throughput metropolitan users and long-reach links can be simultaneously served on the same physical channel.

We propose the MacZac, a time-bin encoder with ultra-low intrinsic QBER (<2e-5) and high stability. The device is based on nested Sagnac and Mach–Zehnder interferometers and uses a single phase modulator for both decoy and state preparation, greatly simplifying the optical setup. The encoder does not require any active compensation or feedback system and it can be scaled for the generation of states with arbitrary dimension. We realized and tested the device performances as a stand alone component and in a complete QKD experiment.

We present a versatile hybrid encoder for quantum key distribution that supports both discrete variable (DV) and continuous variable (CV) protocols. The encoder, based on an iPOGNAC modulator, utilizes commercial off-the-shelf components and can be reconfigured for efficient polarization modulation in DV protocols or polarization-independent phase modulation in CV protocols. This innovative design enhances flexibility, enabling the selection of the most efficient protocol based on link parameters. We experimentally realized the proposed device and tested it with both DV and CV receivers to demonstrate its performance.

Intermodal quantum key distribution enables the integration of fiber networks and free-space channels, essential components for developing a global quantum network. We conducted a field trial of an intermodal quantum key distribution system, featuring two polarization-based transmitters and a single receiver. In this trial, the active channel was alternately switched between a 620-meter free-space link and a 17-kilometer deployed fiber in the metropolitan area of Padova. The free-space channel's performance was assessed in relation to atmospheric turbulence strength. The field trial, conducted over several hours in daylight, demonstrated the intermodal functionality between fiber and free-space channels. Our switching system offers a cost-effective solution for a trusted quantum key distribution network, minimizing the number of necessary devices across different network topologies.

Quantum correlations between measurements of two or more separated observers play a fundamental role in many applications, such as randomness generation or key distribution. Although security can be certified from correlations with minimal assumptions in the device-independent scenario, the performance of such protocols is currently limited. This limitation motivates the exploration of sequential measurements, that is, defined with precise temporal ordering, as a means of improving performance through the reuse of the quantum states. To date, the study of sequential quantum protocols has been modest, lacking a comprehensive mathematical framework to explore the properties of the obtainable correlations. In this study, we adopt a geometric perspective to investigate sequential quantum correlations, providing a general mathematical framework. Here, we analytically prove a Tsirelson-like boundary for sequential quantum correlations, expressed as a trade-off between the amount of nonlocality shared by each sequential user. This boundary is particularly beneficial for the generation of secure quantum randomness. Indeed, observing a correlation on it can certify the maximum attainable bits per state in the case of one remote party and two sequential parties. In contrast to all previous schemes, this can happen even if one of the sequential users does not share any nonlocality. We demonstrate that this quantum boundary can be reached with a simple qubit protocol and investigate numerically the robustness of randomness generation under realistic noise conditions, finding that it greatly improved compared to previous proposals. Our proof-of-concept photonic implementation of the protocol confirms experimentally that our approach certifies more bits per state compared to the standard Clauser-Horne-Shimony-Holt scenario for the same noise, affirming both feasibility and robustness. This study marks a significant advance in understanding sequential quantum correlations, offering valuable insights and new mathematical tools for further fundamental studies and practical applications of efficient device-independent protocols.

Over the last decades, Quantum Key Distribution (QKD) has risen as a promising solution for secure communications, a pressing subject in the aftermath of the security threat posed by Quantum Computers and the Shor's Algorithm. Offering a theoretically secure way to share secret keys between parties, QKD state of the art has witnessed remarkable progress in the last years. Nonetheless, although theoretically secure, QKD is not implementation-secure and until now, the study of physical vulnerabilities in QKD setups has mainly focused on the optical channel. The concept of attacking a cryptographic system via its physical characteristics and associated leakages, known as side-channel analysis, was firstly introduced in classical cryptography, with the seminal work of Paul Kosher. Since then, power and electromagnetic side-channel analysis have become a staple in classical cryptanalysis. However, these concepts have hardly been applied to QKD. In this work, we propose and implement a new method for side-channel analysis on QKD systems, by exploiting the power consumption of the electronic driver controlling the electro-optical components of the QKD transmitter. For high-rate transmission, QKD modules typically require electronic drivers, such as Field Programmable Gate Arrays (FPGAs). Here, we will show that the FPGA's power consumption can leak information about the QKD operation, and consequently the transmitted key. The analysis was performed on the QKD transmitter at the University of Padua. Our results are consistent and show critical information leakage, having reached a maximum accuracy of 73.35% in the prediction of transmitted random keys at 100 MHz repetition frequency.

We describe a QKD field trial running on urban fibers deployed in Padua, Italy. This is the first validation outside of the laboratory environment of a new low-error and calibration-free polarization encoder, called iPOGNAC, which we also present here. Our system is resource- and cost-effective, and can be installed quickly in an existing fiber network.

Semi-device independent (Semi-DI) quantum random number generators (QRNG) gained attention for security applications, offering an excellent trade-off between security and generation rate. This paper presents a proof-of-principle time-bin encoding semi-DI QRNG experiments based on a prepare-and-measure scheme. The protocol requires two simple assumptions and a measurable condition: an upper-bound on the prepared pulses' energy. We lower-bound the conditional min-entropy from the energy-bound and the input-output correlation, determining the amount of genuine randomness that can be certified. Moreover, we present a generalized optimization problem for bounding the min-entropy in the case of multiple input and outcomes, in the form of a semidefinite program (SDP). The protocol is tested with a simple experimental setup, capable of realizing two configurations for the ternary time-bin encoding scheme. The experimental setup is easy-to-implement and comprises commercially available off-the-shelf (COTS) components at the telecom wavelength, granting a secure and certifiable entropy source. The combination of ease-of-implementation, scalability, high security level and output-entropy, make our system a promising candidate for commercial QRNGs.

Randomness is a fundamental feature of quantum mechanics, which is an invaluable resource for both classical and quantum technologies. Practical quantum random number generators (QRNG) usually need to trust their devices, but their security can be jeopardized in case of imperfections or malicious external actions. In this work, we present a robust implementation of a Semi-Device-Independent QRNG that guarantees both security and fast generation rates. The system works in a prepare and measure scenario where measurement and source are untrusted, but a bound on the energy of the prepared states is assumed. Our implementation exploits heterodyne detection, which offers increased generation rate and improved long-term stability compared to alternative measurement strategies. In particular, due to the tomographic properties of heterodyne measurement, we can compensate for fast phase fluctuations via post-processing, avoiding complex active phase stabilization systems. As a result, our scheme combines high security and speed with a simple setup featuring only commercial-off-the-shelf components, making it an attractive solution in many practical scenarios.