Showing 1–11 of 11 results for author: Amaral, C

Demonstration of an LLO CV-QKD system over 12 km of optical fiber

Authors: Christiano M. S. Nascimento, Artur A. Matoso, Gustavo C. Amaral, Guilherme P. Temporão

Abstract: Continuous-variable quantum key distribution (CV-QKD) promises high rates and seamless integration with classical beams within a single optical fiber. Over the years, implementations have been performed by transmitting a local oscillator reference along with the quantum channel, opening security loopholes for eavesdroppers and limiting potential applications. Here, we report on a Gaussian CV-QKD i… ▽ More Continuous-variable quantum key distribution (CV-QKD) promises high rates and seamless integration with classical beams within a single optical fiber. Over the years, implementations have been performed by transmitting a local oscillator reference along with the quantum channel, opening security loopholes for eavesdroppers and limiting potential applications. Here, we report on a Gaussian CV-QKD implementation using fully independent transmitter and receiver lasers (local-oscillator sources) over a 12 km fiber spool. The system was experimentally evaluated using logical frames containing approximately $10^7$ coherent states, each composed of ten independently processed subframes of approximately $10^6$ states, and security was assessed in both asymptotic and finite-size regimes under a trusted-device model. The full-fledged classical post-processing is capable of recovering the channel parameters and extracting secret key rates of 5.11 Mbit/s in the asymptotic regime and 4.67 Mbit/s in the finite-size regime, showing good agreement with theoretical predictions. This work establishes the foundation for metropolitan fiber deployment of CV-QKD under strict security constraints. △ Less

Submitted 7 August, 2026; originally announced August 2026.

The meaning of closeness to women and/or LGBTQ+ physicists: a social network analysis

Authors: Chase Hatcher, Adrienne Traxler, Lily Donis, Camila Amaral, Justin Gutzwa, Charles Henderson, Ramón Barthelemy

Abstract: 'Closeness' is well-defined as a quantitative measure of centrality in social network analysis (SNA), but it is not as well defined qualitatively as a description of social relationships. This paper presents a qualitative analysis of 'closeness' as it is defined both implicitly and explicitly in interviews with 100 women and/or LGBTQ+ PhD physicists. The interviews include a social network constru… ▽ More 'Closeness' is well-defined as a quantitative measure of centrality in social network analysis (SNA), but it is not as well defined qualitatively as a description of social relationships. This paper presents a qualitative analysis of 'closeness' as it is defined both implicitly and explicitly in interviews with 100 women and/or LGBTQ+ PhD physicists. The interviews include a social network construction component, and we define a quantitative network parameter that serves as a proxy for closeness, which we examine in relation to attributes of network members. We find that physicists in this sample see trust, relaxed boundaries, reliance, and support as concepts that most directly define closeness in their relationships. Consistent interaction, positive affect, and commonalities are also often present in (and in some cases, defining of) these relationships. From the quantitative analysis, we find that these physicists tend to view family and partners, friends, and professional friends as comprising their closest relationships. These results are consistent with other studies which have sought to define closeness qualitatively, but the prevalence of trust in this dataset in particular suggests that these physicists see the ability to confide as uniquely important in their relationships. We make recommendations to institutions supporting and employing physicists on how to better support gender and sexual minority (GSM) physicists going forward, and we believe these results help us gain a better understanding of how GSM physicists find success in physics despite the barriers they face due to their identities. △ Less

Submitted 5 February, 2026; v1 submitted 15 January, 2026; originally announced January 2026.

Comments: This article was withdrawn so that all authors could approve the final draft and provide feedback for significant revisions

COMBUST: Gridded combustible mass estimates of the built environment in the conterminous United States (1975-2020)

Authors: Johannes H. Uhl, Maxwell C. Cook, Cibele Amaral, Stefan Leyk, Jennifer K. Balch, Alan Robock, Owen B. Toon

Abstract: The increasing occurrence of natural hazards such as wildfires and drought, along with urban expansion and land consumption, causes increasing levels of fire risk to populations and human settlements. Moreover, increasing geopolitical instability in many regions of the world requires evaluation of scenarios related to potential hazards caused by military operations. Quantitative knowledge on burna… ▽ More The increasing occurrence of natural hazards such as wildfires and drought, along with urban expansion and land consumption, causes increasing levels of fire risk to populations and human settlements. Moreover, increasing geopolitical instability in many regions of the world requires evaluation of scenarios related to potential hazards caused by military operations. Quantitative knowledge on burnable fuels and their spatio-temporal distribution across landscapes is crucial for risk and potential damage assessments. While there is good understanding of the distributions of biomass fuels based on remote sensing observations, the combustible mass of the built environment has rarely been quantified in a spatially explicit manner. Therefore, we developed fine-grained estimates of urban fuels for the conterminous United States, estimating the combustible mass of building materials, building contents, and personal vehicles at 250 m spatial resolution. The resulting dataset is called COMBUST (Combustible mass of the built environment in the conterminous United States) and includes different backcasting scenarios from 1975 to 2020. COMBUST is based on the integration of a variety of geospatial data sources such as Earth-observation derived data, real estate data, statistical estimates and volunteered geographic information. COMBUST is accompanied by COMBUST PLUS, a set of consistently enumerated gridded datasets facilitating combustion exposure modelling of buildings and population. These datasets constitute a rich resource for ecological and social science applications, as well as for disaster risk management and planning-related decision making for U.S. settlements. COMBUST is available at https://doi.org/10.5281/zenodo.15611963. △ Less

Submitted 11 November, 2025; originally announced November 2025.

Quantum Key Distribution in the Iberian Peninsula

Authors: Vicky Domínguez Tubío, Mario Badás Aldecocea, David L. Bakker, Gustavo C. Amaral, Diego López, Johannes Borregaard

Abstract: A promising use of quantum networking is quantum key distribution (QKD), which can provide information-theoretic security unattainable by classical means. While optical fiber-based QKD networks suffer from exponential loss, satellite-assisted quantum communication offers a scalable solution for long-distance secure key exchange. In this work, we propose and evaluate a satellite-based QKD setup cov… ▽ More A promising use of quantum networking is quantum key distribution (QKD), which can provide information-theoretic security unattainable by classical means. While optical fiber-based QKD networks suffer from exponential loss, satellite-assisted quantum communication offers a scalable solution for long-distance secure key exchange. In this work, we propose and evaluate a satellite-based QKD setup covering the Iberian Peninsula, linking Madrid with Barcelona, Bilbao, and Lisbon. Our proposed setup uses a Low-Earth-Orbit (LEO) state-of-the-art satellite equipped with a spontaneous parametric down-conversion (SPDC) source to distribute entangled photon pairs to ground stations. Considering vibrations in the satellite, we optimize the beam waist to enhance the transmission probability and improve the secret key rate (SKR). Our results show that key rates sufficient for real-world applications, such as secure communication between hospitals, using hybrid classical-quantum protocols are feasible with existing protocols. Our results highlight the viability of near-term satellite-based QKD networks for national-scale secure communications. △ Less

Submitted 14 October, 2025; originally announced October 2025.

Efficient single-precision simulations of nematohydrodynamics

Authors: Guilherme N. C. Amaral, Mahmoud Sedahmed, Margarida M. Telo da Gama, Rodrigo C. V. Coelho

Abstract: Simulations of nematohydrodynamics on graphics processing units (GPUs) are typically performed using double precision, which ensures accuracy but significantly increases computational cost. However, consumer-grade GPUs are optimized for single-precision calculations, making double-precision simulations inefficient on widely available hardware. In this work, we demonstrate that single-precision sim… ▽ More Simulations of nematohydrodynamics on graphics processing units (GPUs) are typically performed using double precision, which ensures accuracy but significantly increases computational cost. However, consumer-grade GPUs are optimized for single-precision calculations, making double-precision simulations inefficient on widely available hardware. In this work, we demonstrate that single-precision simulations can achieve the same accuracy as double-precision methods while delivering a 27-fold increase in computational speed. To achieve this, we introduce two key improvements: (i) the shifted distribution function in the lattice Boltzmann method, which mitigates precision loss at low velocities, and (ii) the use of larger time steps in the finite-difference solver, which reduces numerical errors and improves overall accuracy. We find that, unlike in double precision, accuracy in single-precision simulations follows a non-monotonic trend with respect to the finite-difference time step, revealing an optimal regime for precise computations. To illustrate the effectiveness of our approach, we simulate the dynamics of single and multiple skyrmionic tubes in Poiseuille flow. Our results confirm that optimized single-precision simulations enable fast and accurate modeling of complex nematohydrodynamic systems, making large-scale simulations feasible on standard gaming GPUs. △ Less

Submitted 9 March, 2025; originally announced March 2025.

Journal ref: Physics of Fluids 37, 053103 (2025)

Person-centered and qualitative approaches to network analysis in physics education research

Authors: Adrienne L. Traxler, Camila Mani Dias do Amaral, Charles Henderson, Evan LaForge, Chase Hatcher, Madison Swirtz, Ramón Barthelemy

Abstract: Network analysis has become a well-recognized methodology in physics education research (PER), with study topics including student performance and persistence, faculty change, and the structure of conceptual networks. The social network analysis side of this work has focused on quantitative analysis of whole-network cases, such as the structure of networks in single classrooms. Egocentric or perso… ▽ More Network analysis has become a well-recognized methodology in physics education research (PER), with study topics including student performance and persistence, faculty change, and the structure of conceptual networks. The social network analysis side of this work has focused on quantitative analysis of whole-network cases, such as the structure of networks in single classrooms. Egocentric or personal network approaches are largely unexplored, and qualitative methods are underdeveloped. In this paper, we outline theoretical and practical differences between two major network paradigms--whole-network and egocentric--and introduce theoretical frameworks and methodological considerations for egocentric studies. We also describe qualitative and mixed-methods approaches that are currently missing from the PER literature. We identify areas where these additional network methods may be of particular interest to physics education researchers, and end by discussing example cases and implications for new PER studies. △ Less

Submitted 5 February, 2024; originally announced February 2024.

Comments: 25 pages, 3 figures, 1 table, submitted to Phys. Rev. Physics Education Research

Towards a spectrally multiplexed quantum repeater

Authors: Tanmoy Chakraborty, Antariksha Das, Hedser van Brug, Oriol Pietx-Casas, Peng-Cheng Wang, Gustavo Castro do Amaral, Anna L. Tchebotareva, Wolfgang Tittel

Abstract: Extended quantum networks are based on quantum repeaters that often rely on the distribution of entanglement in an efficient and heralded fashion over multiple network nodes. Many repeater architectures require multiplexed sources of entangled photon pairs, multiplexed quantum memories, and photon detection that distinguishes between the multiplexed modes. Here we demonstrate the concurrent employ… ▽ More Extended quantum networks are based on quantum repeaters that often rely on the distribution of entanglement in an efficient and heralded fashion over multiple network nodes. Many repeater architectures require multiplexed sources of entangled photon pairs, multiplexed quantum memories, and photon detection that distinguishes between the multiplexed modes. Here we demonstrate the concurrent employment of (1) spectrally multiplexed cavity-enhanced spontaneous parametric down-conversion in a nonlinear crystal; (2) a virtually-imaged phased array that enables mapping of spectral modes onto distinct spatial modes for frequency-selective detection; and (3) a cryogenically cooled Tm3+:LiNbO3 crystal that allows spectral filtering in an approach that anticipates its use as a spectrally-multiplexed quantum memory. Through coincidence measurements, we demonstrate quantum correlations between energy-correlated photon pairs and a strong reduction of the correlation strength between all other photons. This constitutes an important step towards a frequency multiplexed quantum repeater. △ Less

Submitted 7 December, 2023; v1 submitted 20 May, 2022; originally announced May 2022.

Comments: 20 pages, 18 figures (including supplementary information)

A long-lived solid-state optical quantum memory for high-rate quantum repeaters

Authors: Mohsen Falamarzi Askarani, Antariksha Das, Jacob H. Davidson, Gustavo C. Amaral, Neil Sinclair, Joshua A. Slater, Sara Marzban, Charles W. Thiel, Rufus L. Cone, Daniel Oblak, Wolfgang Tittel

Abstract: We argue that long optical storage times are required to establish entanglement at high rates over large distances using memory-based quantum repeaters. Triggered by this conclusion, we investigate the $^3$H$_6$ $\leftrightarrow$ $^3$H$_4$ transition at 795.325 nm of Tm:Y$_3$Ga$_5$O$_{12}$ (Tm:YGG). Most importantly, we show that the optical coherence time can reach 1.1 ms, and, using laser pulses… ▽ More We argue that long optical storage times are required to establish entanglement at high rates over large distances using memory-based quantum repeaters. Triggered by this conclusion, we investigate the $^3$H$_6$ $\leftrightarrow$ $^3$H$_4$ transition at 795.325 nm of Tm:Y$_3$Ga$_5$O$_{12}$ (Tm:YGG). Most importantly, we show that the optical coherence time can reach 1.1 ms, and, using laser pulses, we demonstrate optical storage based on the atomic frequency comb protocol up to 100 $μ$s as well as a memory decay time T$_M$ of 13.1 $μ$s. Possibilities of how to narrow the gap between the measured value of T$_m$ and its maximum of 275 $μ$s are discussed. In addition, we demonstrate quantum state storage using members of non-classical photon pairs. Our results show the potential of Tm:YGG for creating quantum memories with long optical storage times, and open the path to building extended quantum networks. △ Less

Submitted 4 June, 2021; originally announced June 2021.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 127, 220502 (2021)

Alignment-free characterization of polarizing beamsplitters

Authors: Felipe Calliari, Pedro Tovar, Christiano Nascimento, Breno Perlingeiro, Gustavo Amaral, Guilherme Temporão

Abstract: Traditional methods for measurement of Polarizing Beamsplitter (PBS) parameters, especially the extinction ratio, require highly polarized light sources, alignment procedures and/or experimental parameters that change over time, such as polarization rotations. In this work, a new method is presented, which employs unpolarized light and a Faraday Mirror. It is shown that precise extinction ratio an… ▽ More Traditional methods for measurement of Polarizing Beamsplitter (PBS) parameters, especially the extinction ratio, require highly polarized light sources, alignment procedures and/or experimental parameters that change over time, such as polarization rotations. In this work, a new method is presented, which employs unpolarized light and a Faraday Mirror. It is shown that precise extinction ratio and insertion loss values can be achieved in three single-sweep measurements, without any alignment requirements or time-varying signals of any kind. △ Less

Submitted 5 June, 2019; v1 submitted 17 March, 2019; originally announced March 2019.

Comments: 6 pages, 3 figures

Journal ref: Appl. Opt. 58, 4395-4399 (2019)

Full Optical Fiber Link Characterization with the BSS-Lasso

Authors: Raphael Saavedra, Pedro Tovar, Gustavo C. Amaral, Bruno Fanzeres

Abstract: Manipulation of the detected backscattered Rayleigh signal inside the bandwidth of a frequency-swept optical sub-carrier propagating into an optical fiber permits an efficient localization of faults through a Fourier operator. When the bandwidth is restricted, analysis in the frequency domain can overcome the spatial resolution limitation while also inducing a high-dimensional problem. Introducing… ▽ More Manipulation of the detected backscattered Rayleigh signal inside the bandwidth of a frequency-swept optical sub-carrier propagating into an optical fiber permits an efficient localization of faults through a Fourier operator. When the bandwidth is restricted, analysis in the frequency domain can overcome the spatial resolution limitation while also inducing a high-dimensional problem. Introducing the Lasso as a signal processing technique paired with the Baseband Subcarrier Sweep (BSS) framework allows for a methodology to consistently evaluate fiber defects. In this work, a novel technique for optical fiber monitoring within the BSS framework, hereinafter called the BSS-Lasso, is proposed and tested in simulated and real-world environments, taking into account both reflective and non-reflective events. The results show that, for fiber links ranging from 2 to 15 km with up to 3 faults, over 80% of faults are detected within a 50 m range, and indicate that the proposed methodology significantly outperforms current state-of-the-art BSS-based supervision techniques. Finally, the BSS-Lasso allows for precise, low-cost, transmitter-embedded full characterization of optical fiber links. △ Less

Submitted 29 December, 2018; v1 submitted 8 June, 2018; originally announced June 2018.

Comments: IEEE Transactions on Instrumentation and Measurement, 21 Dec. 2018

A Low-Frequency Tone Sweep Method for in-Service Fault Location in Sub-Carrier Multiplexed Optical Fiber Networks

Authors: Gustavo C. Amaral, Diego C. Villafani, Andrea Baldivieso, Joaquim Dias Garcia, Renata G. Leibel, Luis E. Y. Herrera, Patryk J. Urban, Jean Pierre von der Weid

Abstract: We demonstrate an optical fiber fault location method based on the frequency response of the modulated fiber optical backscattered signal in a steady state low-frequency step regime. Careful calibration and measurement allows for the reconstruction of the fiber transfer function, which, associated to its mathematical model, is capable of extracting the fiber characteristics. The technique is capab… ▽ More We demonstrate an optical fiber fault location method based on the frequency response of the modulated fiber optical backscattered signal in a steady state low-frequency step regime. Careful calibration and measurement allows for the reconstruction of the fiber transfer function, which, associated to its mathematical model, is capable of extracting the fiber characteristics. The technique is capable of identifying non-reflective fault events in an optical fiber link and is perfectly compatible with previous methods that focus on the reflective events. The fact that the recuperation of the complex signal is performed in the frequency domain and not via a Fourier Transform enables the measurements to overcome the spatial resolution limitation of Fourier Transform incoherent-OFDR measurements even with frequency sweep ranges down to 100-100000 Hz. This result is backed up by a less than 10 meters difference in fault location when compared to standard OTDR measurements. △ Less

Submitted 6 September, 2016; originally announced September 2016.

Comments: 11 pages, 5 figures, submitted to Journal of Lightwave Technology on September 4th 2016