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Alice Meda

Publications and source records attributed to Alice Meda.

16 recordsLinked to original sources
arXiv2026

Spectral evolution of two-photon emission in microresonators

High-Q silicon nitride microresonators are versatile sources for generating photon pairs via four-wave mixing. We investigate the spectral coherence of this process, tracking the transition from the spontaneous quantum regime to the onset of optical parametric oscillation. By combining time-correlation measurements with phase-sensitive measurements, we continuously monitor the emission linewidth as it evolves from a cavity-lifetime-limited linewidth toward the pump-linewidth scale. This characterization is essential for optimizing integrated sources for scalable quantum networks.

quant-ph↗
arXiv2024

QKD protected fiber-based infrastructure for time dissemination

In this study, we demonstrate the possibility to protect, with Quantum Key Distribution (QKD), a critical infrastructure as the fiber-based one used for time and frequency (TF) dissemination service. The proposed technique allows to disseminate secure and precise TF signals between two fiber-opticconnected locations, on a critical infrastructure, using both QKD and White Rabbit technique. This secure exchange allows the secret sharing of time information between two parties for the synchronization of distant clocks with the highest stability and traceable to the Italian time scale. When encrypted, time signals would reveal to a third party no useful information about the synchronization status, providing a time stability two orders of magnitude worsened.

quant-ph↗
arXiv2024

Detection Efficiency Characterization for Free-Space Single-Photon Detectors: Measurement Facility and Wavelength-Dependence Investigation

In this paper, we present a new experimental apparatus for the measurement of the detection efficiency of free-space single-photon detectors based on the substitution method. For the first time, we extend the analysis to account for the wavelength dependence introduced by the transmissivity of the optical window in front of the detector's active area. Our method involves measuring the detector's response at different wavelengths and comparing it to a calibrated reference detector. This allows us to accurately quantify the efficiency variations due to the optical window's transmissivity. The results provide a comprehensive understanding of the wavelength-dependent efficiency, which is crucial for optimizing the performance of single-photon detectors in various applications, including quantum communication and photonics research. This characterization technique offers a significant advancement in the precision and reliability of single-photon detection efficiency measurements.

quant-ph↗
arXiv2023

Phase Noise in Real-World Twin-Field Quantum Key Distribution

The impact of noise sources in real-world implementations of Twin-Field Quantum Key Distribution (TF-QKD) protocols is investigated, focusing on phase noise from photon sources and connecting fibers. This work emphasizes the role of laser quality, network topology, fiber length, arm balance, and detector performance in determining key rates. Remarkably, it reveals that the leading TF-QKD protocols are similarly affected by phase noise despite different mechanisms. This study demonstrates duty cycle improvements of over a factor of two through narrow-linewidth lasers and phase-control techniques, highlighting the potential synergy with high-precision time/frequency distribution services. Ultrastable lasers, evolving toward integration and miniaturization, offer promise for agile TF-QKD implementations on existing networks. Properly addressing phase noise and practical constraints allows for consistent key rate predictions, protocol selection, and layout design, crucial for establishing secure long-haul links for the Quantum Communication Infrastructures under development in several countries.

quant-ph↗
arXiv2023

Light Field Ghost Imaging

Techniques based on classical and quantum correlations in light beams, such as ghost imaging, allow us to overcome many limitations of conventional imaging and sensing protocols. Despite their advantages, applications of such techniques are often limited in practical scenarios where the position and the longitudinal extension of the target object are unknown. In this work, we propose and experimentally demonstrate a novel imaging technique, named Light Field Ghost Imaging, that exploits light correlations and light field imaging principles to enable going beyond the limitations of ghost imaging in a wide range of applications. Notably, our technique removes the requirement to have prior knowledge of the object distance allowing the possibility of refocusing in post-processing, as well as performing 3D imaging while retaining all the benefits of ghost imaging protocols.

physics.optics↗
arXiv2021

Unexpected detection rate dependence of the intrinsic detection efficiency in single-photon detectors based on avalanche diodes

Single-photon detectors are a pivotal component in photonic quantum technologies. A precise and comprehensive calibration of the intrinsic detection efficiency is of utmost importance to ensure the proper evaluation of the performance in view of the specific technological application of interest, such as the protection against security breaches in quantum cryptographic solutions. Here we report on a systematic study on and comprehensive analysis of the estimation of the intrinsic detection efficiency of two commercial single-photon detectors based on single-photon avalanche diodes (SPADs) for various mean photon numbers and at high laser pulse repetition rates using different techniques. We observed an unexpected and signifucant drop in intrinsic detection efficiency at detection rates of 10 % and higher relative to the maximum detection rate. It is demonstrated that for data analysis a statistical model for the detection rate conveniently can be used if no timestamped data are available. We conclude that the full characterization of single-photon detectors used in critical applications should include the sensitivity of their intrinsic detection efficiency to high event rates.

quant-ph↗
arXiv2021

Detection of ultra-weak laser pulses by free-running single-photon detectors: modeling dead time and dark counts effects

In quantum communication systems, the precise estimation of the detector's response to the incoming light is necessary to avoid security breaches. The typical working regime uses a free-running single-photon avalanche diode in combination with attenuated laser pulses at telecom wavelength for encoding information. We demonstrate the validity of an analytical model for this regime which considers the effects of dark counts and dead time on the measured count rate. For the purpose of gaining a better understanding of these effects, the photon detections were separated from the dark counts via a software-induced gating mechanism. The model was verified by experimental data for mean photon numbers covering three orders of magnitude as well as for laser repetition frequencies below and above the inverse dead time. Consequently, our model would be of interest for predicting the detector response not only in the field of quantum communications, but also in any other quantum physics experiment where high detection rates are needed.

quant-ph↗
arXiv2020

Coherent phase transfer for real-world twin-field quantum key distribution

Quantum mechanics allows the distribution of intrinsically secure encryption keys by optical means. Twin-field quantum key distribution is the most promising technique for its implementation on long-distance fibers, but requires stabilizing the optical length of the communication channels between parties. In proof-of-principle experiments based on spooled fibers, this was achieved by interleaving the quantum communication with periodical adjustment frames. In this approach, longer duty cycles for the key streaming come at the cost of a looser control of channel length, and a successful key-transfer using this technique in a real world remains a significant challenge. Using interferometry techniques derived from frequency metrology, we developed a solution for the simultaneous key streaming and channel length control, and demonstrate it on a 206 km field-deployed fiber with 65 dB loss. Our technique reduces the quantum-bit-error-rate contributed by channel length variations to <1%, representing an effective solution for real-world quantum communications.

quant-ph↗
arXiv2020

A quantum model for rf-SQUIDs based metamaterials enabling 3WM and 4WM Travelling Wave Parametric Amplification

A quantum model for Josephson-based metamaterials working in the Three-Wave Mixing (3WM) and Four-Wave Mixing (4WM) regimes at the single-photon level is presented. The transmission line taken into account, namely Traveling Wave Josephson Parametric Amplifier (TWJPA), is a bipole composed by a chain of rf-SQUIDs which can be biased by a DC current or a magnetic field in order to activate the 3WM or 4WM nonlinearities. The model exploits a Hamiltonian approach to analytically determine the time evolution of the system both in the Heisenberg and interaction pictures. The former returns the analytic form of the gain of the amplifier, while the latter allows recovering the probability distributions vs. time of the photonic populations, for multimodal Fock and coherent input states. The dependence of the metamaterial's nonlinearities is presented in terms of circuit parameters in a lumped model framework while evaluating the effects of the experimental conditions on the model validity.

cond-mat.supr-con↗
arXiv2019

Quantum differential ghost microscopy

Quantum correlations become formidable tools for beating classical capacities of measurement. Preserving these advantages in practical systems, where experimental imperfections are unavoidable, is a challenge of the utmost importance. Here we propose and realize a quantum ghost imaging protocol able to compensate for the detrimental effect of detection noise and losses. This represents an important improvement as quantum correlations allow low brightness imaging, desirable for reducing the absorption dose. In particular, we develop a comprehensive model starting from a ghost imaging scheme elaborated for bright thermal light, known as differential ghost imaging and particularly suitable in the relevant case of faint or sparse objects. We perform the experiment using SPDC light in microscopic configuration. The image is reconstructed exploiting non-classical intensity correlation rather than photon pairs detection coincidences. On one side we validate the theoretical model and on the other we show the applicability of this technique by reconstructing a biological object with 5 micrometers resolution.

quant-ph↗
arXiv2017

Unbiased estimation of an optical loss at the ultimate quantum limit with twin-beams

Loss measurements are at the base of spectroscopy and imaging, thus perme- ating all the branches of science, from chemistry and biology to physics and material science. However, quantum mechanics laws set the ultimate limit to the sensitivity, constrained by the probe mean energy. This can be the main source of uncertainty, for example when dealing with delicate system such as biological samples or photosensitive chemicals. It turns out that ordinary (clas- sical) probe beams, namely with Poissonian photon number distribution, are fundamentally inadequate to measure small losses with the highest sensitivity. Conversely, we demonstrate that a quantum-correlated pair of beams, known as twin-beam state, allows reaching the ultimate sensitivity for all energy regimes (even less than one photon per mode) with the simplest measurement strategy. One beam of the pair addresses the sample, while the second one is used as a reference to compensate both for classical drifts and for uctuation at the most fundamental quantum level. This scheme is also absolute and accurate, since it self-compensates for unavoidable instability of the sources and detectors, which could otherwise lead to strongly biased results. Moreover, we report the best sensitivity per photon ever achieved in loss estimation experiments.

quant-ph↗
arXiv2016

Photon number correlation for quantum enhanced imaging and sensing

In this review we present the potentialities and the achievements of the use of non-classical photon number correlations in twin beams (TWB) states for many applications, ranging from imaging to metrology. Photon number correlations in the quantum regime are easy to be produced and are rather robust against unavoidable experimental losses, and noise in some cases, if compared to the entanglement, where loosing one photon can completely compromise the state and its exploitable advantage. Here, we will focus on quantum enhanced protocols in which only phase-insensitive intensity measurements (photon number counting) are performed, which allow probing transmission/absorption properties of a system, leading for example to innovative target detection schemes in a strong background. In this framework, one of the advantages is that the sources experimentally available emit a wide number of pairwise correlated modes, which can be intercepted and exploited separately, for example by many pixels of a camera, providing a parallelism, essential in several applications, like wide field sub-shot-noise imaging and quantum enhanced ghost imaging. Finally, non-classical correlation enables new possibilities in quantum radiometry, e.g. the possibility of absolute calibration of a spatial resolving detector from the on-off- single photon regime to the linear regime, in the same setup.

quant-ph↗
arXiv2016

Realisation of the first sub shot noise wide field microscope

In the last years several proof of principle experiments have demonstrated the advantages of quantum technologies respect to classical schemes. The present challenge is to overpass the limits of proof of principle demonstrations to approach real applications. This letter presents such an achievement in the field of quantum enhanced imaging. In particular, we describe the realization of a sub-shot noise wide field microscope based on spatially multi-mode non-classical photon number correlations in twin beams. The microscope produces real time images of 8000 pixels at full resolution, for (500micrometers)2 field-of-view, with noise reduced to the 80% of the shot noise level (for each pixel), suitable for absorption imaging of complex structures. By fast post-elaboration, specifically applying a quantum enhanced median filter, the noise can be further reduced (less than 30% of the shot noise level) by setting a trade-off with the resolution, demonstrating the best sensitivity per incident photon ever achieved in absorption microscopy.

quant-ph↗
arXiv2010

Experimental Quantum Imaging exploiting multi-mode spatial correlation of twin beams

Properties of quantum states have disclosed new and revolutionary technologies, ranging from quantum information to quantum imaging. This last field is addressed to overcome limits of classical imaging by exploiting specific properties of quantum states of light. One of the most interesting proposed scheme exploits spatial quantum correlations between twin beams for realizing sub-shot-noise imaging of the weak absorbing objects, leading ideally to a noise-free imaging. Here we discuss in detail the experimental realization of this scheme, showing its capability to reach a larger signal to noise ratio with respect to classical imaging methods and, therefore, its interest for future practical applications.

quant-ph↗
arXiv2009

Experimental estimation of entanglement at the quantum limit

Entanglement is the central resource of quantum information processing and the precise characterization of entangled states is a crucial issue for the development of quantum technologies. This leads to the necessity of a precise, experimental feasible measure of entanglement. Nevertheless, such measurements are limited both from experimental uncertainties and intrinsic quantum bounds. Here we present an experiment where the amount of entanglement of a family of two-qubit mixed photon states is estimated with the ultimate precision allowed by quantum mechanics.

quant-ph↗