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The ground-state quantum geometry is at the root of several static and adiabatic properties, while genuinely dynamic properties are routinely addressed via Kubo formulae, whose essential entries are the excited states. It is shown here that the ground-state metric-curvature tensor evolves in time by means of a causal unitary operator, which by construction elucidates the geometrical effect of the excited states in compact form. In the condensed-matter case the generalized tensor encompasses the whole conductivity tensor at arbitrary frequencies in both insulators and metals, with the exception of the Drude term in the metallic case; the latter is shown to be eminently nongeometrical., Comment: 4 pages + 1 page Supplemental. Minor typos corrected

In quantum-degenerate metallic carbon nanotubes (CNTs), the conduction electron gas forms a solid-state plasma environment where collective dynamics are dominant. This dense plasma is capable of supporting the excitation of surface plasmon on the CNT surface. In this study, we demonstrate a new class of microscale relativistic surface plasmons (RSPs) that, independent of laser polarisation and matching medium, can be directly excited by paraxial propagation of a high-intensity optical laser pulse through a cylindrical vacuum channel nanostructured in a CNT forest. The leaky field of the resonant RSP mode can support a non-evanescent longitudinal component with amplitude up to several TV/m-levels and a phase-matched focusing field. This field enables the sustained acceleration of relativistic electrons and positrons over long distances. We also investigate two possible mechanisms for electron injection, including edge injection and self-injection, both of which are feasible with current laser facilities, paving the way for experimental realisation. As well as highlighting a promising avenue for the development of ultra-compact, high-energy solid-state plasma particle accelerators, this work also opens up the potential for high-energy plasmonics.

Solid-state materials, such as carbon nanotubes (CNTs), have the potential to support ultra-high accelerating fields in the TV/m range for charged particle acceleration. In this study, we explore the feasibility of using nanostructured CNTs forest to develop plasma-based accelerators at the 100s TeV-level, driven by high-density, ultra-relativistic electron beams, using fully three-dimensional particle-in-cell simulations. Two different acceleration mechanisms are proposed and investigated: the surface plasmon leakage field and the bubble wakefield. The leakage field, driven by a relatively low-density beam, can achieve an acceleration field up to TV/m, capable of accelerating both electron and positron beams. In particular, due to the direct acceleration by the driving beam, the positron acceleration is highly efficient with an average acceleration gradient of 2.3 TeV/m. In contrast, the bubble wakefield mechanism allows significantly higher acceleration fields, e.g. beyond 400 TV/m, with a much higher energy transfer efficiency of 66.7%. In principle, electrons can be accelerated to PeV energies over distances of several meters. If the beam density is sufficiently high, the CNT target will be completely blown out, where no accelerating field is generated. Its threshold has been estimated. Two major challenges in these schemes are recognised and investigated. Leveraging the ultra-high energy and charge pumping rate of the driving beam, the nanostructured CNTs also offer significant potential for a wide range of advanced applications. This work represents a promising avenue for the development of ultra-compact, high-energy particle accelerators. We also outline conceptual experiments using currently available facilities, demonstrating that this approach is experimentally accessible.

We describe Evalita-LLM, a new benchmark designed to evaluate Large Language Models (LLMs) on Italian tasks. The distinguishing and innovative features of Evalita-LLM are the following: (i) all tasks are native Italian, avoiding issues of translating from Italian and potential cultural biases; (ii) in addition to well established multiple-choice tasks, the benchmark includes generative tasks, enabling more natural interaction with LLMs; (iii) all tasks are evaluated against multiple prompts, this way mitigating the model sensitivity to specific prompts and allowing a fairer and objective evaluation. We propose an iterative methodology, where candidate tasks and candidate prompts are validated against a set of LLMs used for development. We report experimental results from the benchmark's development phase, and provide performance statistics for several state-of-the-art LLMs., Comment: 42 pages, 1 figure, 32 tables

Wakefield wavelengths associated with solid-state plasmas greatly limit the accelerating length. An alternative approach employs 2D carbon-based nanomaterials, like graphene or carbon nanotubes (CNTs), configured into structured targets. These nanostructures are designed with voids or low-density regions to effectively reduce the overall plasma density. This reduction enables the use of longer-wavelength lasers and also extends the plasma wavelength and the acceleration length. In this study, we present, to our knowledge, the first numerical demonstration of electron acceleration via self-injection into a wakefield bubble driven by an infrared laser pulse in structured CNT targets, similar to the behavior observed in gaseous plasmas for LWFA in the nonlinear (or bubble) regime. Using the PIConGPU code, bundles of CNTs are modeled in a 3D geometry as 25 nm-thick carbon tubes with an initial density of $10^{22}$ cm$^{-3}$. The carbon plasma is ionized by a three-cycle, 800 nm wavelength laser pulse with a peak intensity of $10^{21}$ W cm$^{-2}$, achieving an effective plasma density of $10^{20}$ cm$^{-3}$. The same laser also drives the wakefield bubble, responsible for the electron self-injection and acceleration. Simulation results indicate that fs-long electron bunches with hundreds of pC charge can be self-injected and accelerated at gradients exceeding 1~TeV$/$m. Both charge and accelerating gradient figures are unprecedented when compared with LWFA in gaseous plasma., Comment: 11 pages, 8 figures

The interaction of fast charged particles with graphene layers can generate electromagnetic modes. This wake effect has been recently proposed for short-wavelength, high-gradient particle acceleration and for obtaining brilliant radiation sources. In this study, the excitation of wakefields produced by a point-like charged particle moving parallel to a multilayer graphene array (which may be supported by an insulated substrate) is studied using the linearized hydrodynamic theory. General expressions for the excited longitudinal and transverse wakefields have been derived. The dependencies of the wakefields on the positions of the layers and the substrate, the velocity and the surface density have been extensively analyzed. This study provides a deeper understanding of the physical phenomena underlying plasmonic excitations in graphene layers, paving the way for potential applications of these structures in particle acceleration, nanotechnology and materials science.

While laser wakefield acceleration (LWFA) in the bubble regime demands ultra-short, high-peak-power laser pulses, operation in the self-modulated regime (SM-LWFA) works with more relaxed pulse conditions, albeit at the cost of lower beam quality. Modern laser systems can deliver pulses with durations of a few tens of femtoseconds and peak powers on the order of a few terawatts, at kHz repetition rates. These systems are well-suited for developing SM-LWFA applications where high average energy and charge are prioritized over beam quality. Such beams could be used to generate high-energy bremsstrahlung photons, capable of triggering photonuclear reactions to produce radioisotopes like molybdenum-99. This isotope decays into technetium-99m, the most widely used medical radioisotope, with over 30 million applications worldwide per year. This work explores the use of Bayesian optimization to maximize the energy and charge of electron beams accelerated via SM-LWFA. Particle-in-cell (PIC) simulations model a 5 TW, 15 fs-long Gaussian laser pulse, propagating through tailored hydrogen gas-density profiles. In these simulations, over multiple iterations, the algorithm optimizes a set of input parameters characterizing the gas-density profile and the laser focal position. Three distinct profiles, with total lengths ranging from 200 to 400 micrometers and combining ramps and plateaus, were investigated. Optimal configurations were found to produce electron beams with median energies ranging from 14 to 17 MeV and charges of 600 to 1300 pC, considering electrons with energies above 8 MeV. Preliminary estimates of the molybdenum-99 yields for the optimal beams were obtained by employing their phase spaces, retrieved from PIC simulations, as radiation source inputs in Monte Carlo simulations irradiating a combined tantalum and molybdenum target., Comment: 19 pages, 19 figures

Since their inception, superconducting nanowire single-photon detectors have been enabling quantum optical applications and the rise of the photonic quantum industry. The evolution in the detector design and read-out strategies has led to the introduction of devices with a plurality of independent pixels, which have been able to operate with high system detection efficiency at high speed while also supporting photon number resolution capabilities. However, this comes at the cost of a complex readout that requires one coaxial cable for each pixel of the array. Here, we report a 28-pixel SNSPD with a dedicated parallel architecture that, while maintaining a simple readout with a single coaxial line, enables the detector to operate at high speed with low-performance degradation. The device shows a maximum single-photon efficiency of 88% and is able to maintain its efficiency above 50%, coupled with a timing jitter lower than 80 ps, up to a detection rate of 200 million counts per second. The detector also provides state-of-the-art photon-number-resolving performances with a 2-photon efficiency of 75% and a 3-photon efficiency of 62%.

Gallium phosphide (GaP) has recently received considerable attention as a suitable material for building photonic integrated circuits due to its remarkable optical and piezoelectric properties. Usually, GaP is grown epitaxially on III-V substrates to keep its crystallinity and later transferred to silicon wafers for further processing. Here, an alternative promising route for the fabrication of optomechanical (OM) cavities on GaP epitaxially grown on nominally (001)-oriented Si is introduced by using a two-step process consisting of a low-temperature etching of GaP followed by selective etching of the underneath silicon. The low-temperature (-30 $^o$C) during the dry-etching of GaP hinders the lateral etching rate, preserving the pattern with a deviation between the design and the pattern in the GaP layer lower than 5 %, avoiding the complex process of transferring and bonding a GaP wafer to a silicon-on-insulator wafer. To demonstrate the quality and feasibility of the proposed fabrication route, suspended OM cavities are fabricated and experimentally characterized. The cavities show optical quality factors between 10$^3$ and 10$^4$, and localized mechanical resonances at frequencies around 3.1 GHz. Both optical and mechanical resonances are close to those previously reported on crystalline GaP structures. These results suggest a simple and low-cost way to build GaP-based photonic devices directly integrated on industry-standard Si(001) photonic wafers., Comment: 11 pages, 5 figures

Modern Neural Machine Translation systems exhibit strong performance in several different languages and are constantly improving. Their ability to learn continuously is, however, still severely limited by the catastrophic forgetting issue. In this work, we leverage a key property of encoder-decoder Transformers, i.e. their generative ability, to propose a novel approach to continually learning Neural Machine Translation systems. We show how this can effectively learn on a stream of experiences comprising different languages, by leveraging a replay memory populated by using the model itself as a generator of parallel sentences. We empirically demonstrate that our approach can counteract catastrophic forgetting without requiring explicit memorization of training data. Code will be publicly available upon publication. Code: https://github.com/m-resta/sg-rep, Comment: Accepted at NAACL 2024

Autonomous systems, including generative AI, have been adopted faster than previous digital innovations. Their impact on society might as well be more profound, with a radical restructuring of the economy of knowledge and dramatic consequences for social and institutional balances. Different attitudes to control these systems have emerged rooted in the classical pillars of legal systems, proprietary rights, and social responsibility. We show how an illusion of control might be guiding governments and regulators, while autonomous systems might be driving us to inescapable delusion., Comment: 11 pages, preliminary version, currently under submission to a conference

The electric field of light induces--in a non centrosymmetric insulator--a dc current, quadratic in the field magnitude, and called "shift current". When addressed from a many-electron viewpoint, the shift current has a simple explanation and a simple formulation as well, deeply rooted in quantum geometry. The basic formula is then specialized to the independent-electron case, first for a disordered system in a supercell formulation, and then for a crystalline system. In the latter case the known shift-current formula is retrieved in a very transparent way., Comment: 4 pages

The ``comma sequence'' starts with 1 and is defined by the property that if k and k' are consecutive terms, the two-digit number formed from the last digit of k and the first digit of k' is equal to the difference k'-k. If there is more than one such k', choose the smallest, but if there is no such k' the sequence terminates. The sequence begins 1, 12, 35, 94, 135, ... and, surprisingly, ends at term 2137453, which is 99999945. The paper analyzes the sequence and its generalizations to other starting values and other bases. A slight change in the rules allows infinitely long comma sequences to exist., Comment: 19 pages, 4 figures, 1 table. Corrected a formula, expanded discussion of an asymptotic estimate, other small changes

The interactions of charged particles moving paraxially in multi-walled carbon nanotubes (MWCNTs) may excite electromagnetic modes. This wake effect has recently been proposed as a potential novel method of short-wavelength high-gradient particle acceleration. In this work, the excitation of wakefields in double-walled carbon nanotubes (DWCNTs) is studied by means of the linearized hydrodynamic theory. General expressions have been derived for the excited longitudinal and transverse wakefields and related to the resonant wavenumbers which can be obtained from the dispersion relation. In the absence of friction, the stopping power of the wakefield driver, modelled here as a charged macroparticle, can be written solely as a function of these resonant wavenumbers. The dependencies of the wakefields on the radii of the DWCNT and the driving velocity have been studied. DWCNTs with inter-wall distances much smaller than the internal radius may be a potential option to obtain higher wakefields for particle acceleration compared to single-walled carbon nanotubes (SWCNTs)., Comment: arXiv admin note: text overlap with arXiv:2308.08399

The nonlocal potential contributes an extra term to the velocity operator; I show here that such term affects the formal expression of the Drude weight in a nontrivial way. Notably, the present main result fixes a disturbing discrepancy in the Dreyer-Coh-Stengel sum rule [Phys. Rev. Lett. {\bf 128}, 095901 (2022)]., Comment: 2 pages. Transparent derivation of the Drude-weight formula

A sharp definition of what "adiabatic" means is given; it is then shown that the time-dependent expectation value of a quantum-mechanical observable in the adiabatic limit can be expressed -- in many cases -- by means of the appropriate Berry curvature. Condensed-matter observables belonging to this class include: Born effective charges in insulators and in metals, quantized Faraday charges in electrolytes, and linear dc conductivities (longitudinal and transverse). Remarkably, the adiabatic limit is well defined even in metals, despite the absence of a spectral gap therein. For all of the above observables the explicit Berry-curvature expressions are derived in a general many-body setting, which also allows for compact and very transparent notations and formulas. Their conversion into band-structure formulas in the independent-electron crystalline case is straightforward., Comment: Comprehensive & pedagogical presentation in 10 pages

The interactions of charged particles with carbon nanotubes may excite electromagnetic modes in the electron gas produced in the cylindrical graphene shell constituting the nanotube wall. This wake effect has recently been proposed as a potential novel method of short-wavelength high-gradient particle acceleration. In this work, the excitation of these wakefields is studied by means of the linearized hydrodynamic model. In this model, the electronic excitations on the nanotube surface are described treating the electron gas as a 2D plasma with additional contributions to the fluid momentum equation from specific solid-state properties of the gas. General expressions are derived for the excited longitudinal and transverse wakefields. Numerical results are obtained for a charged particle moving within a carbon nanotube, paraxially to its axis, showing how the wakefield is affected by parameters such as the particle velocity and its radial position, the nanotube radius, and a friction factor, which can be used as a phenomenological parameter to describe effects from the ionic lattice. Assuming a particle driver propagating on axis at a given velocity, optimal parameters were obtained to maximize the longitudinal wakefield amplitude.

Superconducting-nanowire single-photon detectors (SNSPDs) have enabled the realization of several quantum optics technologies thanks to their high detection efficiency, low dark-counts, and fast recovery time. However, the widespread use of technologies such as linear optical quantum computing (LOQC), quasi-deterministic single photon sources and quantum repeaters requires faster detectors that can distinguish between different photon number states. Here, we report the fabrication of an SNSPD array composed of 14 independent pixels, achieving a system detection efficiency (SDE) of 90% in the telecom band. By reading each pixel of the array independently we show that the detector can detect telecom photons at 1.5 GHz with 45% absolute SDE. We exploit the dynamic PNR of the array to demonstrate accurate state reconstruction for different photon-number statistics for a wide range of light inputs, including operation with long-duration light pulses, as commonly obtained with some cavity-based sources. We show 2-photon and 3-photon fidelities of 74% and 57% respectively, which represent state-of-the-art results for fiber-coupled SNSPDs.

Adiabatic transport in a many-electron system is expressed in terms of the appropriate Berry curvature, owing to the Niu-Thouless theory [J. Phys A {\bf 17}, 2453 (1984)]; the main equation is very compact and very general. I address here three paradigmatic adiabatic response tensors -- -the atomic polar tensor, the atomic axial tensor, and the rotational $g$ factor -- and I show that, for all of them, the known formulas do not need an independent proof. They are just case studies of the general expression, for different choices the curvature's two arguments., Comment: 3 1/2 pages, no figures

Neuroscience is moving towards a more integrative discipline, where understanding brain function requires consolidating the accumulated evidence seen across experiments, species, and measurement techniques. A remaining challenge on that path is integrating such heterogeneous data into analysis workflows such that consistent and comparable conclusions can be distilled as an experimental basis for models and theories. Here, we propose a solution in the context of slow wave activity ($<1$ Hz), which occurs during unconscious brain states like sleep and general anesthesia, and is observed across diverse experimental approaches. We address the issue of integrating and comparing heterogeneous data by conceptualizing a general pipeline design that is adaptable to a variety of inputs and applications. Furthermore, we present the Collaborative Brain Wave Analysis Pipeline (Cobrawap) as a concrete, reusable software implementation to perform broad, detailed, and rigorous comparisons of slow wave characteristics across multiple, openly available ECoG and calcium imaging datasets.

Quantum Key Distribution has made continuous progress over the last 20 years and is now commercially available. However, the secret key rates (SKR) are still limited to a few Mbps. Here, we present a custom multipixel superconducting nanowire single-photon detectors and fast acquisition and real-time key distillation electronics, removing two roadblocks and allowing an increase of the SKR of more than an order of magnitude. In combination with a simple 2.5 GHz clocked time-bin quantum key distribution system, we can generate secret keys at a rate of 64 Mbps over a distance of 10.0 km and at a rate of 3.0 Mbps over a distance of 102.4 km with real-time key distillation., Comment: 5 pages, 5 figures, submitted to Nature Photonics

Charged particle acceleration using solid-state nanostructures has attracted attention in recent years as a method of achieving ultra-high-gradient acceleration in the TV/m domain. More concretely, metallic hollow nanostructures could be suitable for particle acceleration through the excitation of wakefields by a laser or a high-intensity charged particle beam in a high-density solid-state plasma. For instance, due to their special channelling properties as well as optoelectronic and thermo-mechanical properties, carbon nanotubes could be an excellent medium for this purpose. This article investigates the feasibility of generating ultra-high gradient acceleration using carbon nanotube arrays, modelled as solid-state plasmas in conventional particle-in-cell simulations performed in a two-dimensional axisymmetric quasi}-3D geometry. The generation of beam-driven plasma wakefields depending on different parameters of the solid structure is discussed in detail. Furthermore, by adopting an effective plasma-density approach, existing analytical expressions, originally derived for homogeneous plasmas, can be used to describe wakefields driven in periodic non-uniform plasmas., Comment: 15 pages, 13 figures