Synchronization of a WDM Packet-Switched Slotted Ring

In this paper, we present two different strategies of slot synchronization in wavelength-division-multiplexing (WDM) packet-switched slotted-ring networks. Emphasis is given to the architecture behind the WDM Optical Network Demonstrator over Rings (WONDER) project, which is based on tunable transmitters and fixed receivers. The WONDER experimental prototype is currently being developed at the laboratories of Politecnico di Torino. In the former strategy, a slotsynchronization signal is transmitted by the master station on a dedicated control wavelength; in the latter, slave nodes achieve slot synchronization aligning on data packets that are received from the master. The performance of both synchronization strategies, particularly in terms of packet-collision probability, was evaluated by simulation. The technique based on transmitting a timing signal on a dedicated control wavelength achieves better performance, although it is more expensive due to the need for an additional wavelength. However, the technique based on aligning data packets that are received from the master, despite attaining lower timing stability, still deserves further study, particularly if limiting the number of wavelengths and receivers is a major requirement. Some experimental results, which were measured on the WONDER prototype, are also shown. Measurement results, together with theoretical findings, demonstrate the good synchronization performance of the prototype

A system for online beam emittance measurements and proton beam characterization

A system for online measurement of the transverse beam emittance was developed. It is named $^{4}$PrOB$\varepsilon$aM (4-Profiler Online Beam Emittance Measurement) and was conceived to measure the emittance in a fast and efficient way using the multiple beam profiler method. The core of the system is constituted by four consecutive UniBEaM profilers, which are based on silica fibers passing across the beam. The $^{4}$PrOB$\varepsilon$aM system was deployed for characterization studies of the 18~MeV proton beam produced by the IBA Cyclone 18 MeV cyclotron at Bern University Hospital (Inselspital). The machine serves daily radioisotope production and multi-disciplinary research, which is carried out with a specifically conceived Beam Transport Line (BTL). The transverse RMS beam emittance of the cyclotron was measured as a function of several machine parameters, such as the magnetic field, RF peak voltage, and azimuthal angle of the stripper. The beam emittance was also measured using the method based on the quadrupole strength variation. The results obtained with both techniques were compared and a good agreement was found. In order to characterize the longitudinal dynamics, the proton energy distribution was measured. For this purpose, a method was developed based on aluminum absorbers of different thicknesses, a UniBEaM detector, and a Faraday cup. The results were an input for a simulation of the BTL developed in the MAD-X software. This tool allows machine parameters to be tuned online and the beam characteristics to be optimized for specific applications.Comment: published in Journal of Instrumentatio

Orbital angular momentum multiplication in plasmonic vortex cavities

Orbital angular momentum of light is a core feature in photonics. Its confinement to surfaces using plasmonics has unlocked many phenomena and potential applications. Here we introduce the reflection from structural boundaries as a new degree of freedom to generate and control plasmonic orbital angular momentum. We experimentally demonstrate plasmonic vortex cavities, generating a succession of vortex pulses with increasing topological charge as a function of time. We track the spatio-temporal dynamics of these angularly decelerating plasmon pulse train within the cavities for over 300 femtoseconds using time-resolved Photoemission Electron Microscopy, showing that the angular momentum grows by multiples of the chiral order of the cavity. The introduction of this degree of freedom to tame orbital angular momentum delivered by plasmonic vortices, could miniaturize pump-probe-like quantum initialization schemes, increase the torque exerted by plasmonic tweezers and potentially achieve vortex lattice cavities with dynamically evolving topology

A Theoretical Framework for Target Propagation

The success of deep learning, a brain-inspired form of AI, has sparked interest in understanding how the brain could similarly learn across multiple layers of neurons. However, the majority of biologically-plausible learning algorithms have not yet reached the performance of backpropagation (BP), nor are they built on strong theoretical foundations. Here, we analyze target propagation (TP), a popular but not yet fully understood alternative to BP, from the standpoint of mathematical optimization. Our theory shows that TP is closely related to Gauss-Newton optimization and thus substantially differs from BP. Furthermore, our analysis reveals a fundamental limitation of difference target propagation (DTP), a well-known variant of TP, in the realistic scenario of non-invertible neural networks. We provide a first solution to this problem through a novel reconstruction loss that improves feedback weight training, while simultaneously introducing architectural flexibility by allowing for direct feedback connections from the output to each hidden layer. Our theory is corroborated by experimental results that show significant improvements in performance and in the alignment of forward weight updates with loss gradients, compared to DTP.Comment: 13 pages and 4 figures in main manuscript; 41 pages and 8 figures in supplementary materia

3D correlative light and electron microscopy of cultured cells using serial blockface scanning electron microscopy

The processes of life take place in multiple dimensions, but imaging these processes in even three dimensions is challenging. Here, we describe a workflow for 3D correlative light and electron microscopy (CLEM) of cell monolayers using fluorescence microscopy to identify and follow biological events, combined with serial blockface scanning electron microscopy to analyse the underlying ultrastructure. The workflow encompasses all steps from cell culture to sample processing, imaging strategy, and 3D image processing and analysis. We demonstrate successful application of the workflow to three studies, each aiming to better understand complex and dynamic biological processes, including bacterial and viral infections of cultured cells and formation of entotic cell-in-cell structures commonly observed in tumours. Our workflow revealed new insight into the replicative niche of Mycobacterium tuberculosis in primary human lymphatic endothelial cells, HIV-1 in human monocytederived macrophages, and the composition of the entotic vacuole. The broad application of this 3D CLEM technique will make it a useful addition to the correlative imaging toolbox for biomedical research

Active Coordination in Ad Hoc Networks

Abstract. The increasing ubiquity of communicating mobile devices and vastly different mobile application needs have led to the emergence of middleware models for ad hoc networks that simplify application pro-gramming. One such system, EgoSpaces, addresses specific needs of indi-vidual applications, allowing them to define what data is included in their operating context using declarative specifications constraining properties of data, agents that own the data, hosts on which those agents are run-ning, and attributes of the ad hoc network. In the resulting coordination model, application agents interact with a dynamically changing environ-ment through a set of views, or custom defined projections of the set of data present in the surrounding ad hoc network. This paper builds on EgoSpaces by allowing agents to assign behaviors to their personal-ized views. Behaviors consist of actions that are automatically performed in response to specified changes in a view. Behaviors discussed in this paper encompass reactive programming, transparent data migration, au-tomatic data duplication, and event capture. Formal semantic definitions and programming examples are given for each behavior.

A facility for radiation hardness studies based on a medical cyclotron

The development of instrumentation for operation in high-radiation environments represents a challenge in various research fields, particularly in particle physics experiments and space missions, and drives an ever-increasing demand for irradiation facilities dedicated to radiation hardness studies. Depending on the application, different needs arise in terms of particle type, energy and dose rate. In this article, we present a versatile installation based on a medical cyclotron located at the Bern University Hospital (Inselspital), which is used as a controlled 18-MeV proton source. This accelerator is used for daily production of medical radioisotopes, as well as for multidisciplinary research, thanks to a 6.5-meter long beam transfer line that terminates in an independent bunker, dedicated only to scientific activities. The facility offers a wide range of proton fluxes, due to an adjustable beam current from approximately 10 pA to the micro-ampere range, together with a series of steering and focusing magnets along the beamline that allow for the beam spot to be focused down to a few mm^2. The beamline can be instrumented with a variety of beam monitoring detectors, collimators, and beam current measurement devices to precisely control the irradiation conditions. The facility also hosts a well equipped laboratory dedicated to the characterisation of samples after irradiation. An experimental validation of the irradiation setup, with proton fluxes ranging from 5×10^9 cm^-2s^-1 to 4×10^11 cm^-2s^-1, is reported