arXiv subjects

Simone Donadello

Publications and source records attributed to Simone Donadello.

17 recordsLinked to original sources

arXiv2026

International Optical Clock Comparison Using the European Optical Fiber Network

Optical clocks have achieved remarkable estimated fractional frequency uncertainties reaching the $10^{-18}$ level and below, enabling applications in fundamental physics, general relativity, and geodesy. However, the challenge of verifying the international consistency of optical clocks remains critical as efforts intensify toward redefining the SI second based on an optical transition or transitions. We report on a two-month international clock comparison campaign involving seven optical clocks in four national metrology institutes (INRIM, LNE-OP, NPL, and PTB) connected via the optical fiber network established in Europe. The campaign resulted in optical frequency ratios with uncertainties ranging from $7.7\times10^{-18}$ to $6.1\times10^{-17}$. Among the results, the $^{171}$Yb$^+$(E3) clocks at NPL and PTB demonstrated agreement within an uncertainty of $7.7\times10^{-18}$, marking the first international verification of two independently developed optical clocks below one part in $10^{17}$. The operation of the $^{199}$Hg clock at LNE-OP (formerly LNE-SYRTE) resulted in frequency ratios with improved uncertainties with $^{171}$Yb$^+$(E3), $^{171}$Yb, and $^{87}$Sr optical clocks. These results provide input for the redefinition of the second and underscore how fiber-linked clock networks can advance metrology and scientific applications.

physics.atom-ph↗

arXiv2026

A High-frequency Geodetic VLBI Experiment for Optical Clock Comparison

An intercontinental metrological clock comparison between Italy and the Republic of Korea was performed by means of geodetic K-band VLBI observations. The comparison involved the hydrogen masers (H-masers) used at Medicina and Sejong radio telescopes. The same clocks were simultaneously compared by a satellite link and by high-precision optical clocks maintained at the National Metrology Institutes, KRISS in Korea and INRIM in Italy, and delivered to VLBI antennas via optical fiber. The H-masers frequency difference was estimated by extrapolating the clock rate from VLBI data using two geodetic VLBI software. This was subsequently compared with clock differences derived by satellite link and by local optical clocks. Results obtained with different approaches were in agreement at the level of $10^{-15}$ s/s. This pilot study demonstrates that standard high-frequency (K-band) geodetic VLBI campaigns could be a viable approach to conduct intercontinental clock comparisons, now only possible via satellite links. This uncertainty can be reduced thanks to the planned installation of new-generation, broadband, high-frequency receivers on the involved telescopes. K/Q/W-band geodetic observations will allow an improvement of the accuracy of the resulting group delays through broad bandwidth synthesis from 20 to 100 GHz. Furthermore, the Frequency Phase Transfer (FPT) method will also be explored together with the use of PCAL systems installed at the radio telescopes to improve phase stability and thus allow a better estimation of the station clock parameters.

astro-ph.IM↗

arXiv2025

Coordinated international comparisons between optical clocks connected via fiber and satellite links

Optical clocks provide ultra-precise frequency references that are vital for international metrology as well as for tests of fundamental physics. To investigate the level of agreement between different clocks, we simultaneously measured the frequency ratios between ten optical clocks in six different countries, using fiber and satellite links. This is the largest coordinated comparison to date, from which we present a subset of 38 optical frequency ratios and an evaluation of the correlations between them. Four ratios were measured directly for the first time, while others had significantly lower uncertainties than previously achieved, supporting the advance towards a redefinition of the second and the use of optical standards for international time scales.

physics.atom-ph↗

arXiv2024

Hybrid optical sensor for combined thermal and dimensional monitoring in laser processing

Optical measurements enable non-contact and high-speed monitoring of physical processes, offering a non-invasive and versatile approach across a wide range of fields, from scientific research to industrial applications. This work presents an optical sensor capable of simultaneously measuring both the distance and thermal emission from surfaces, based on a simple laser diode probe. The principle of operation integrates triangulation with pyrometry in a single device, leveraging the monitor photodiode embedded within the probe module. By alternating the probe laser emission, the system can rapidly switch between dimensional and thermal measurements, resulting in combined data acquisition. The proposed method is compact, easy to integrate, and cost-effective. The hybrid sensor is demonstrated in a laser processing setup, where a metallic target is heated and melted by a high-power laser beam. Its inline operation allows for real-time dynamic measurements of melt pool distance and radiance, in a coaxial and self-aligning configuration. This innovative approach can be applied to various fields, such as remote environmental sensing and closed-loop control systems for stabilizing high-temperature processes, including laser welding and additive manufacturing.

physics.optics↗

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

Seismic monitoring using the telecom fiber network

Laser interferometry enables to remotely measure microscopical length changes of deployed telecommunication cables originating from earthquakes. Long range and compatibility with data traffic make it unique to the exploration of remote regions, as well as highly-populated areas where optical networks are pervasive, and its large-scale implementation is attractive for both Earth scientists and telecom operators. However, validation and modeling of its response and sensitivity are still at an early stage and suffer from lack of statistically-significant event catalogs and limited availability of co-located seismometers. We implemented laser interferometry on a land-based telecommunication cable and analyzed 1.5 years of continuous acquisition, with successful detections of events in a broad range of magnitudes, including very weak ones. By comparing fiber and seismometer recordings we determined relations between a cable's detection probability and the magnitude and distance of events, and showed that spectral analysis of recorded data allows considerations on the earthquake dynamics. Our results reveal that quantitative analysis is possible for this sensing technique and support the interpretation of data from the growing amount of interferometric deployments. We anticipate the high integration and scalability of laser interferometry into existing telecommunication grids to be useful for the daily seismicity monitoring, in perspective exploitable for civilian protection use.

physics.geo-ph↗

arXiv2023

Embedded digital phase noise analyzer for optical frequency metrology

Digital signal processing (DSP) is supporting novel in-field applications of optical interferometry, such as in laser ranging and distributed acoustic sensing. While the highest performances are achieved with field-programmable gated arrays (FPGAs), their complexity and cost are often too high for many tasks. Here, we describe an alternative solution for processing optical signals in real-time, based on a dual-core 32-bit microcontroller. We implemented in-phase and quadrature (IQ) demodulation of optical beat-notes resulting from the interference of independent laser sources, phase noise analysis of deployed optical fibers covering intercity distances, and synchronization of remote acquisitions via optical trigger signals. The embedded architecture can efficiently accomplish a large variety of tasks in the context of optical signal processing, being also easily configurable, compact, and upgradable. These features make it attractive for applications that require long-term, remotely operated, and field-deployed instrumentation.

eess.SP↗

arXiv2021

Interplay between powder catchment efficiency and layer height in self-stabilized laser metal deposition

In laser metal deposition (LMD) the height of the deposited track can vary within and between layers, causing significant deviations during the process evolution. Previous works have shown that in certain conditions a self-stabilizing mechanism occurs, maintaining a regular height growth and a constant standoff distance between the part and the deposition nozzle. Here we analyze the link between the powder catchment efficiency and the deposition height stability. To this purpose, a monitoring system was developed to study the deposition in different process conditions, using inline measurements of the specimen weight in combination with the layer height information obtained with coaxial optical triangulation. An analytical model was used to estimate the deposition efficiency in real-time from the height monitoring and the process parameters, which was verified by the direct mass measurements. The results show that the track height stabilization is associated to a reduction of the powder catchment efficiency, which is governed by the melt pool relative position with respect to the powder cone and the laser beam. For a given set of parameters, the standoff distance can be estimated to achieve the highest powder catchment efficiency and a regular height through the build direction.

physics.app-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

Time-resolved quantification of plasma accumulation induced by multi-pulse laser ablation using self-mixing interferometry

In this work a method based on self-mixing interferometry (SMI) is presented for probing the concentration of plasma plumes induced by multi-pulse laser ablation. An analytical model is developed to interpret the single-arm interferometric signal in terms of plasma electron number density. Its time dependence follows a power-law scaling which is determined by concurrent effects of plume accumulation and propagation. The model has been applied for the experimental study of plume formation at variable laser pulse frequencies on different materials. The plume expansion dynamics has been observed with high-speed imaging, and the SMI measurements allowed for a time-resolved estimation of the electron number density. The intrinsic advantages of the SMI technique in terms of robustness and low intrusiveness would allow for its usage as a fast diagnostic tool for the dynamical scaling of laser-induced plumes. Moreover it can be easily applied in laser-based manufacturing technologies where plasma concentration monitoring and control is important.

physics.plasm-ph↗

arXiv2018

Monitoring of laser metal deposition height by means of coaxial laser triangulation

Laser metal deposition (LMD) is an additive manufacturing technique, whose performances can be influenced by several factors and parameters. Monitoring their evolution allows for a better comprehension and control of the process, hence enhancing the deposition quality. In particular, the deposition height is an important variable that, if it does not match the process growth, can bring to defects and geometrical inaccuracies in the deposited structures. The current work presents a system based on optical triangulation for the height monitoring, implemented on a LMD setup composed of a fiber laser, a deposition head, and an anthropomorphic robot. Its coaxial and non-intrusive configuration allows for flexibility in the deposition strategy and direction. A measurement laser beam is launched through the powder nozzle and hits the melt pool. A coaxial camera acquires the probe spot, whose position is converted to relative height. The device has been demonstrated for monitoring the deposition of a stainless steel cylinder. The measurements allowed to reconstruct a spatial map of the height variation, highlighting a transient in the deposition growth which can be explained in terms of a self-regulating mechanism for the layer thickness.

physics.ins-det↗

arXiv2018

Probing multipulse laser ablation by means of self-mixing interferometry

In this work, self-mixing interferometry (SMI) is implemented inline to a laser microdrilling system to monitor the machining process by probing the ablation-induced plume. An analytical model based on the Sedov-Taylor blast wave equation is developed for the expansion of the process plume under multiple-pulse laser percussion drilling conditions. Signals were acquired during laser microdrilling of blind holes on stainless steel, copper alloy, pure titanium, and titanium nitride ceramic coating. The maximum optical path difference was measured from the signals to estimate the refractive index changes. An amplitude coefficient was derived by fitting the analytical model to the measured optical path differences. The morphology of the drilled holes was investigated in terms of maximum hole depth and dross height. The results indicate that the SMI signal rises when the ablation process is dominated by vaporization, changing the refractive index of the processing zone significantly. Such ablation conditions correspond to limited formation of dross. The results imply that SMI can be used as a nonintrusive tool in laser micromachining applications for monitoring the process quality in an indirect way.

physics.app-ph↗

arXiv2016

Creation and counting of defects in a temperature quenched Bose-Einstein Condensate

We study the spontaneous formation of defects in the order parameter of a trapped ultracold bosonic gas while crossing the critical temperature for Bose-Einstein Condensation (BEC) at different rates. The system has the shape of an elongated ellipsoid, whose transverse width can be varied to explore dimensionality effects. For slow enough temperature quenches we find a power-law scaling of the average defect number with the quench rate, as predicted by the Kibble-Zurek mechanism. A breakdown of such a scaling is found for fast quenches, leading to a saturation of the average defect number. We suggest an explanation for this saturation in terms of the mutual interactions among defects.

cond-mat.quant-gas↗

arXiv2015

Dynamics and interaction of vortex lines in an elongated Bose-Einstein condensate

We study the real-time dynamics of vortex lines in a large elongated Bose-Einstein condensate (BEC) of sodium atoms using a stroboscopic technique. Vortices are spontaneously produced via the Kibble-Zurek mechanism in a quench across the BEC transition and then they slowly precess keeping their orientation perpendicular to the long axis of the trap as expected for solitonic vortices in a highly anisotropic condensate. Good agreement with theoretical predictions is found for the precession period as a function of the orbit amplitude and the number of condensed atoms. In configurations with two or more vortex lines, we see signatures of vortex-vortex interaction in the shape and visibility of the orbits. In addition, when more than two vortices are present, their decay is faster than the thermal decay observed for one or two vortices. The possible role of vortex reconnection processes is discussed.

cond-mat.quant-gas↗

arXiv2014

Solitonic Vortices in Bose-Einstein Condensates

We analyse, theoretically and experimentally, the nature of solitonic vortices (SV) in an elongated Bose-Einstein condensate. In the experiment, such defects are created via the Kibble-Zurek mechanism, when the temperature of a gas of sodium atoms is quenched across the BEC transition, and are imaged after a free expansion of the condensate. By using the Gross-Pitaevskii equation, we calculate the in-trap density and phase distributions characterizing a SV in the crossover from an elongate quasi-1D to a bulk 3D regime. The simulations show that the free expansion strongly amplifies the key features of a SV and produces a remarkable twist of the solitonic plane due to the quantized vorticity associated with the defect. Good agreement is found between simulations and experiments.

cond-mat.quant-gas↗

arXiv2014

Observation of Solitonic Vortices in Bose-Einstein Condensates

We observe solitonic vortices in an atomic Bose-Einstein condensate after free expansion. Clear signatures of the nature of such defects are the twisted planar density depletion around the vortex line, observed in absorption images, and the double dislocation in the interference pattern obtained through homodyne techniques. Both methods allow us to determine the sign of the quantized circulation. Experimental observations agree with numerical simulations. These solitonic vortices are the decay product of phase defects of the BEC order parameter spontaneously created after a rapid quench across the BEC transition in a cigar-shaped harmonic trap and are shown to have a very long lifetime.

cond-mat.quant-gas↗

arXiv2013

Spontaneous creation of Kibble-Zurek solitons in a Bose-Einstein condensate

When a system crosses a second-order phase transition on a finite timescale, spontaneous symmetry breaking can cause the development of domains with independent order parameters, which then grow and approach each other creating boundary defects. This is known as Kibble-Zurek mechanism. Originally introduced in cosmology, it applies both to classical and quantum phase transitions, in a wide variety of physical systems. Here we report on the spontaneous creation of solitons in Bose-Einstein condensates via the Kibble-Zurek mechanism. We measure the power-law dependence of defects number with the quench time, and provide a check of the Kibble-Zurek scaling with the sonic horizon. These results provide a promising test bed for the determination of critical exponents in Bose-Einstein condensates.

cond-mat.quant-gas↗