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– Every physical theory involving quantum fields does require a model of the quantum vacuum. The vacuum associated to quantum gravity must incorporate the prescriptions from both the theory of relativity and quantum physics. In this work, starting from the hypothesis of nucleation of sub-Planckian bubbles from a de Sitter vacuum, we study the necessary conditions to obtain baby universes, black holes and particles. The de Sitter-Planck background is described by an “infinite” Quantum Boltzmann statistics that generates fermions and bosons, and manifests itself as a deformation of the geometry that leads to a generalized uncertainty principle and a unified expression for the generalized Compton wavelength and event horizon size, drawing a connection between quantum black holes and elementary particles, the latter seen as a collective organization of the bubbles of the vacuum described by the generalized Compton wavelength. The quantum thermodynamics of black holes is then outlined and the physical history of each bubble is found to depend on the cosmological constant described in terms of thermodynamic pressure. Finally, a suggestive treatment of the Casimir effect is provided in the de Sitter-Planck background with Quantum-Boltzmann statistics and wormholes are explored as bubble coalescence processes.

In this paper we want to analyze the Dark Matter problem and the distribution of matter through two different approaches, which are linked by the possibility that the solution of these astronomical puzzles should be sought in the quantum imprinting of the Universe. The first approach is based on a cosmological model formulated and developed in the last ten years by the first and third authors of this paper; the so-called ”Archaic Universe”. The second approach was formulated by Rosen in 1993 by considering the Friedman–Einstein equations as a simple one-dimensional dynamical system reducing the cosmological equations to a sort of Schrodinger equation. The quantum memory in cosmological dynamics could explain, for example, the apparently periodic structures of the Universe while Archaic Universe shows how the quantum phase concerns not only an ancient era of the Universe, but quantum facets permeating the entire Universe today.

We apply the analogy between gravitational fields and optical media in the general relativistic geometric optics framework to describe how light can acquire orbital angular momentum (OAM) when it traverses the gravitational field of a massive rotating compact object and the interplay between OAM and polarization. Kerr spacetimes are known not only to impose a gravitational Faraday rotation on the polarization of a light beam, but also to set a characteristic fingerprint in the orbital angular momentum distribution of the radiation passing nearby a rotating black hole (BH). Kerr spacetime behaves like an inhomogeneous and anisotropic medium, in which light can acquire orbital angular momentum and spin-to-orbital angular momentum conversion can occur, acting as a polarization and phase changing medium for the gravitationally lensed light, as confirmed by the data analysis of M87* black hole., 9 pages, 1 figure

The speed of light in vacuum, $c$, is a fundamental constant of nature. Photons belonging to a structured beam of finite transverse size have been observed to travel with a group velocity ${v}_{g}$, smaller than $c$, also when propagating in vacuum. The paradox of slow photons in vacuum could, in a first approach, be explained as a simple projection effect that occurs when one measures the speed of light in a diverging vortex beam along the direction of the beam propagation. This effect, instead, reveals fundamental properties of structured light beams: they form a class of quasiparticle states characterized by the geometric properties of the beams such as the beam divergence, spin angular momentum, orbital angular momentum (OAM), and the group velocity. These anomalous propagation modes with group velocities different than that of $c$ represent a class of Majorana quasiparticles with bosonic spectrum for integer OAM modes, cast in an infinite tower of quasiparticle states similar to those expected with photons propagating in a turbulent plasma.

We propose a new thought experiment, based on present-day Quantum Information Technologies, to measure quantum gravitational effects through the Bose-Marletto-Vedral (BMV) effect by revealing the gravitational $t^3$ phase term, its expected relationships with low-energy quantum gravity phenomena and test the equivalence principle of general relativity. The technique here proposed promise to reveal gravitational field fluctuations from the analysis of the stochastic noise associated to an ideal output of a measurement process of a quantum system. To improve the sensitivity we propose to cumulate the effects of the gravitational field fluctuations in time on the outputs of a series of independent measurements acted on entangled states of particles, like in the building of a quantum cryptographic key, and extract from the associated time series the effect of the expected gravitational field fluctuations. In fact, an ideal quantum cryptographic key, built with the sharing of maximally entangled states of particles, is represented by a random sequence of uncorrelated symbols mathematically described by a perfect white noise, a stochastic process with zero mean and without correlation between its values taken at different times. Gravitational field perturbations, including quantum gravity fluctuations and gravitational waves, introduce additional phase terms that decohere the entangled pairs used to build the quantum cryptographic key, with the result of coloring the white noise. We find that this setup, built with massive mesoscopic particles, can potentially reveal the $t^3$ gravitational phase term and thus, the BMV effect., Comment: 9 pages 1 figure. arXiv admin note: substantial text overlap with arXiv:0910.2322

Following Rosen's quantization rules, two of the Authors (CC and FF) recently described the Schwarzschild black hole (BH) formed after the gravitational collapse of a pressureless "star of dust" in terms of a "gravitational hydrogen atom". Here we generalize this approach to the gravitational collapse of a charged object, namely, to the geometry of a Reissner-Nordstrom BH (RNBH) and calculate the gravitational potential, the Schr\"odinger equation and the exact solutions of the energy levels of the gravitational collapse. By using the concept of BH effective state, previously introduced by one of us (CC), we describe the quantum gravitational potential, the mass spectrum and the energy spectrum for the extremal RNBH. The area spectrum derived from the mass spectrum finds agreement with a previous result by Bekenstein. The stability of these solutions, described with the Majorana approach to the Archaic Universe scenario, show the existence of oscillatory regimes or exponential damping for the evolution of a small perturbation from a stable state., Comment: 7 pages, to appear in Europhysics Letters

We discuss, in the relativistic limits, Heisenberg's uncertainty principle for the electromagnetic orbital angular momentum (OAM) of monochromatic fields. The Landau and Peierls relativistic approach applied to the Heisenberg indetermination between the azimuthal angle Delta phi and the OAM state Delta l, when a limit speed is present, exposes the conceptual limits of a possible formulation of the uncertainty principle for OAM states as it leads to the Ehrenfest's paradox of the rotation of a rigid disk in special relativity (SR). To face this problem, in a way similar to Einstein's discussion of a rotating rigid disk, one has to adopt a general relativistic approach and include the limits from smallest and largest scales present in nature, such as the Planck scale and the Hubble horizon.

We present the first observational evidence that light propagating near a rotating black hole is twisted in phase and carries orbital angular momentum (OAM). This physical observable allows a direct measurement of the rotation of the black hole. We extracted the OAM spectra from the radio intensity data collected by the Event Horizon Telescope from around the black hole M87* by using wavefront reconstruction and phase recovery techniques and from the visibility amplitude and phase maps. This method is robust and complementary to black-hole shadow circularity analyses. It shows that the M87* rotates clockwise with an estimated rotation parameter $a=0.90\pm0.05$ with $\sim 95\%$ confidence level (c.l.) and inclination $i=17^\circ \pm2^\circ$, equivalent to a magnetic arrested disk with inclination $i=163^\circ\pm2^\circ$. From our analysis we conclude, within a 6 $\sigma$ c.l., that the M87* is rotating., Comment: Small addition on coherence. 5 pages, 2 figures Accepted for publication in MNRAS Letters

The Riemann Hypothesis states that the Riemann zeta function $\zeta(z)$ admits a set of ``non-trivial'' zeros that are complex numbers supposed to have real part $1/2$. Their distribution on the complex plane is thought to be the key to determine the number of prime numbers before a given number. Hilbert and P\'olya suggested that the Riemann Hypothesis could be solved through the mathematical tools of physics, finding a suitable Hermitian or unitary operator that describe classical or quantum systems, whose eigenvalues distribute like the zeros of $\zeta(z)$. A different approach is that of finding a correspondence between the distribution of the $\zeta(z)$ zeros and the poles of the scattering matrix $S$ of a physical system. Our contribution is articulated in two parts: in the first we apply the infinite-components Majorana equation in a Rindler spacetime and compare the results with those obtained with a Dirac particle following the Hilbert-P\'olya approach showing that the Majorana solution has a behavior similar to that of massless Dirac particles and finding a relationship between the zeros of zeta end the energy states. Then, we focus on the $S$-matrix approach describing the bosonic open string scattering for tachyonic states with the Majorana equation. Here we find that, thanks to the relationship between the angular momentum and energy/mass eigenvalues of the Majorana solution, one can explain the still unclear point for which the poles and zeros of the $S$-matrix of an ideal system that can satisfy the Riemann Hypothesis, exist always in pairs and are related via complex conjugation. As claimed in the literature, if this occurs and the claim is correct, then the Riemann Hypothesis could be in principle satisfied, tracing a route to a proof., Comment: 25 pages, 1 figure

Einstein&rsquo, s equations of general relativity (GR) can describe the connection between events within a given hypervolume of size L larger than the Planck length L P in terms of wormhole connections where metric fluctuations give rise to an indetermination relationship that involves the Riemann curvature tensor. At low energies (when L ≫ L P ), these connections behave like an exchange of a virtual graviton with wavelength &lambda, G = L as if gravitation were an emergent physical property. Down to Planck scales, wormholes avoid the gravitational collapse and any superposition of events or space&ndash, times become indistinguishable. These properties of Einstein&rsquo, s equations can find connections with the novel picture of quantum gravity (QG) known as the &ldquo, Einstein&ndash, Rosen (ER)=Einstein&ndash, Podolski&ndash, Rosen (EPR)&rdquo, (ER = EPR) conjecture proposed by Susskind and Maldacena in Anti-de-Sitter (AdS) space&ndash, times in their equivalence with conformal field theories (CFTs). In this scenario, non-traversable wormhole connections of two or more distant events in space&ndash, time through Einstein&ndash, Rosen (ER) wormholes that are solutions of the equations of GR, are supposed to be equivalent to events connected with non-local Einstein&ndash, Rosen (EPR) entangled states that instead belong to the language of quantum mechanics. Our findings suggest that if the ER = EPR conjecture is valid, it can be extended to other different types of space&ndash, times and that gravity and space&ndash, time could be emergent physical quantities if the exchange of a virtual graviton between events can be considered connected by ER wormholes equivalent to entanglement connections.

A novel technique for generating high-order vortex beams in the radio-frequency domain is reported. Vortex beams with non-null radial index are generated by means of a dielectric mask, mounted upon either a twisted parabolic reflector or a conventional one. The far-field intensity pattern is compared to analytical models and numerical simulations, thus illustrating how diffraction and illumination can affect the generated fields.

Radio beams that carry nonzero orbital angular momentum (OAM) are analyzed from the viewpoint of a multiple-input-multiple-output (MIMO) communication system. Often, the natural OAM-beam orthogonality cannot be fully exploited because of spatial constraints on the receiving antenna size. Therefore, we investigate how far OAM-induced phase variations can be exploited in spatial demultiplexing based on conventional (linear momentum) receivers. Performances are investigated versus position and size of the transmitting and receiving devices. The use of OAM-mode coherent superpositions is also considered, in view of recent work by Edfors et al. Our final goal is to assess the merits of an OAM-based MIMO system, in comparison with a conventional one.

In this paper we report the results from outdoor experiments showing that it is possible to increase the data transmission capacity using at least three coherent, orthogonal beams on the same frequency, 17.128 GHz, each in a unique orbital angular momentum state. Each beam was encoded with the digital modulations used in present-day telecommunications. We achieved an error-free throughput of 3 × 11 Mbit/s with four-Quadrature Amplitude Modulation over a 7 MHz bandwidth over 100 m and 150 m long links.

The Hawking-Perry-Strominger (HPS) work [1] states a new controversial idea about the black hole (BH) information paradox [2-5] where BHs maximally entropize and encode information in their event horizon area [6,7], with no "hair" were thought to reveal information outside but angular momentum, mass and electric charge only [8,9] in a unique quantum gravity (QG) vacuum state. This new idea invokes new conservation laws involving gravitation and electromagnetism [10,11], to generate different QG vacua and preserve more information in hair implants. In the context of black holes and the HPS proposal we find that BH photon hair implants can be spatially shaped ad hoc and encode structured and densely organized information on the event horizon involving novel aspect in the discussion a particular aspect of EM fields, namely the spatial information of the field associated to its orbital angular momentum. BHs can have "curly", twisted, soft-hair implants with vorticity where structured information is holographically encoded in the event horizon in an organized way., Comment: 6 pages 2 figures

We propose a new energy extraction mechanism from the rotational energy of a Kerr-Newman black hole by a gravitating massive photon field generated by electromagnetic and gravitational field coupling effects. Numerical studies show that this mechanism that depends on the black hole rotation parameter, $a$, shows a clear dependence on the black hole mass, $M$, and charge, $Q$, and can extract energies up to $1{0}^{54}\text{ }\text{ }\text{erg}$ for a black hole of the solar mass size. With this mechanism we can set a lower bound on the coupling $\ensuremath{\xi}\ensuremath{\sim}{10}^{\ensuremath{-}38}$ between electromagnetic and gravitational fields that might be used to explain the hypothetical extremely high energy release, $g{10}^{53}\text{ }\text{ }\text{erg}$, suggested by the observations of some gamma-ray bursts in the controversial ``energy crisis'' problem if and when gamma-ray bursts seem not to show evidence for collimated emission.

The paradox of a free falling radiating charged particle in a gravitational field, is a well-known fascinating conceptual challenge that involves classical electrodynamics and general relativity. We discuss this paradox considering the emission of radiation as a consequence of an explicit space/time symmetry breaking involving the electric field within the trajectory of the particle seen from an external observer. This occurs in certain particular cases when the relative motion of the charged particle does not follow a geodesic of the motion dictated by the explicit Lagrangian formulation of the problem and thus from the metric of spacetime. The problem is equivalent to the breaking of symmetry within the spatial configuration of a radiating system like an antenna: when the current is not conserved at a certain instant of time within a closed region then emission of radiation occurs [1]. Radiation from a system of charges is possible only when there is explicit breaking of symmetry in the electric field in space and time., Comment: 5 pages

The propagation and divergence properties of beams carrying orbital angular momentum (OAM) play a crucial role in many applications. Here we present a general study on the divergence of optical beams with OAM. We show that the mean absolute value of the OAM imposes a lower bound on the value of the beam divergence. We discuss our results for two different definitions of the divergence, the so called rms or encircled-energy. The bound on the rms divergence can be expressed as a generalized uncertainty principle, with applications in long-range communication, microscopy and 2D quantum systems., Comment: RevTex, published version

Exploiting orbital angular momentum (OAM) to implement spatially multiplexed links is an active research topic, both at radio and optical frequencies. We focus on using, in the microwave range, superpositions of modes having opposite OAM values. As known, this approach simplifies dramatically the spatial field distributions, especially in phase. We stress the consequent implementation advantages, and test them experimentally, on an ad-hoc link, about 100 m long, at the unlicensed frequency of 17.2 GHz, in an urban environment. Results are compared with numerical simulations. Performances are discussed vs. those of a conventional MIMO system.

International audience; Context: A new extremely high speed photon-counting photometer, Iqueye, has been installed and tested at the New Technology Telescope, in La Silla. Aims: This instrument is the second prototype of a ``quantum'' photometer being developed for future Extremely Large Telescopes of 30-50 m aperture. Methods: Iqueye divides the telescope aperture into four portions, each feeding a single photon avalanche diode. The counts from the four channels are collected by a time-to-digital converter board, where each photon is appropriately time-tagged. Owing to a rubidium oscillator and a GPS receiver, an absolute rms timing accuracy better than 0.5 ns during one-hour observations is achieved. The system can sustain a count rate of up to 8 MHz uninterruptedly for an entire night of observation. Results: During five nights of observations, the system performed smoothly, and the observations of optical pulsar calibration targets provided excellent results.

In this work, a novel three-dimensional (3D) fabrication process was used to create a series of particular phase masks for astronomical applications. These phase masks, also known as spiral phase plates (SPP), find their application in astronomy for optical vortex coronagraphy in the search of extra solar planets. The 3D profile was directly written on 2.7mm thick polymethylmethacrylate (PMMA) slide by electron beam lithography (EBL). The effect of electron scattering on topography profile was investigated and the relationship among spatial distribution of electron beam irradiation, spot size and dosage level of irradiation were experimentally characterized. The quality of the fabricated profiles was evaluated using atomic force microscopy (AFM) technique. SPPs were then optically tested by injecting a Gaussian beam and an airy diffraction pattern obtained from a laser source comparing the optical vortices so far obtained with those drawn with numerical simulations.

This paper describes the results obtained so far with AquEYE, a single photon counting, fixed aperture photometer for the Asiago 182 cm telescope. AquEYE has been conceived as a prototype of a truly 'quantum' photometer for future Extremely Large Telescopes of 30-50 m aperture. This prototype is characterized by four independent channels equipped with single photon avalanche diodes (SPADs) as detectors. The counts from the four channels are acquired by a TDC board which has a nominal 25 ps time tagging capability. Taking into account the 35 ps jitter in the SPAD itself, the overall precision of the time tags is of the order of 50 ps. The internal oscillator is locked to an external rubidium clock; a GPS pulse per second is collected by the TDC itself to obtain a UTC reference. The maximum photon count rate which the present system can sustain is 12 MHz.

Aims. In this paper we present our first observations at the Asiago 122 cm telescope of � = 1 optical vortices generated with starlight beams. Methods. We used a fork-hologram blazed at the first diffraction order as a phase modifying device. The multiple system Rasalgethi (α Herculis) in white light and the single star Arcturus (α Bootis) through a 300 A bandpass were observed using a fast CCD camera. In the first case we could adopt the Lucky Imaging approach to partially correct for seeing effects. Results. For both stars, the optical vortices could be clearly detected above the smearing caused by the mediocre seeing conditions. The profiles of the optical vortices produced by the beams of the two main components of the α Her system are consistent with numerically simulated on-axis and off-axis optical vortices. The optical vortices produced by α Boo can also be reproduced by numerical simulations. Our experiments confirm that the ratio between the intensity peaks of an optical vortex can be extremely sensitive to off-axis displacements of the beam. Conclusions. Our results give insights for future astronomical applications of optical vortices both for space telescopes and groundbased telescopes with good seeing conditions and adaptive optics devices. The properties of optical vortices can be used to perform high precision astrometry and tip/tilt correction of the isoplanatic field. We are now designing a � = 2 optical vortex coronagraph around a continuous spiral phase plate. We also point out that optical vortices could find extremely interesting applications also in the infrared and radio wavelengths.

New 2003-2006 time-resolved photometric observations of the old nova RW UMi (1956) confirm the QPO nature of the light modulations observed. The analysis of all our historical runs show that QPOs tend to cluster around frequencies 4.96 d -1 and 12.4 d -1 while the historical superhump period remains undetected in our data. Although the two main frequencies observed might appear to be aliases of the superhump frequency 17 d -1 , they should be produced by two independent mechanisms. These rather irregular QPOs might be explained by vertical oscillations of the inner regions of the accretion disk and/or by the cyclical generation in the disk of travelling density waves at the co-rotation radius of an inclined magnetic rotator. We find that the post-nova luminosity is decaying at a rate of ∼0.03 mag y -1 . If RW UMi is an intermediate polar system, during the decline the magnetospheric radius of the accreting white dwarf primary should increase and produce important changes in the periods of QPOs.

Wireless communications, radio astronomy and other radio science applications of today are pre-dominantly implemented with techniques built on top of the electromagnetic linear momentum (Poynting vector) physical layer. As a supplement and/or alternative to this conventional approach, techniques rooted in the electromagnetic angular momentum (moment of momentum) physical layer have been advocated, and promising results from proof-of-concept laboratory as well as real-world radio communication experiments using angular momentum were recently reported.

Present-day state-of-the-art radio science and wireless radio communication techniques are almost exclusively based on the simple physical mechanism by which radiation is generated and detected by a one-dimensional (scalar) oscillating current modulated with information. This approach has the unavoidable fundamental physical limitation that it only utilizes the translational degrees of freedom of the EM field. The physical observables that describe these translational degrees of freedom are the mechanical linear momentum of the charged particles (also known as the charge degree of freedom), pmech, and the associated EM linear momentum (the volume integrated Poynting vector), pfield. The inherently single-state vectorial observable pfield can propagate in free space to a remotely located receiving antenna where the reverse linear momentum exchange process takes place, allowing the information encoded onto pfield to be extracted from the induced mechanical momentum of the charges, i.e. the receiving antenna current.

From 1999-2002 photometric observations of the old nova RW UMi (1956), we systematically detect a dominant modulation with a period P0 peaking in the range 1.7-2.4 hr. This period, on average, is about 45% longer than the previously suggested orbital period of ~1.42 hr. This could be due to the occasional presence of low-frequency quasi-periodic oscillations (QPOs) that can eventually dominate the light curve. Rather irregular QPOs, on the order of the orbital period, can be produced by vertical oscillations of the inner-disk region excited by the rotating magnetosphere of the white dwarf. Thus, RW UMi could be an intermediate polar system. Since we find that the luminosity of the postnova is decaying at a rate of ~0.02 mag yr-1, we should observe some important changes in its photometric behavior in the future.

The search for dark matter (DM) is one of the most active and challenging areas of current research. Possible DM candidates are ultralight fields such as axions and weak interacting massive particles (WIMPs). Axions piled up in the center of stars are supposed to generate matter/DM configurations with oscillating geometries at a very rapid frequency, which is a multiple of the axion mass m B (Brito et al (2015); Brito et al (2016)). Borra and Trottier (2016) recently found peculiar ultrafast periodic spectral modulations in 236 main sequence stars in the sample of 2.5 million spectra of galactic halo stars of the Sloan Digital Sky Survey (~1% of main sequence stars in the F–K spectral range) that were interpreted as optical signals from extraterrestrial civilizations, suggesting them as possible candidates for the search for extraterrestrial intelligence (SETI) program. We argue, instead, that this could be the first indirect evidence of bosonic axion-like DM fields inside main sequence stars, with a stable radiative nucleus, where a stable DM core can be hosted. These oscillations were not observed in earlier stellar spectral classes probably because of the impossibility of starting a stable oscillatory regime due to the presence of chaotic motions in their convective nuclei. The axion mass values, , obtained from the frequency range observed by Borra and Trottier, ) THz, agree with the recent theoretical results from high-temperature lattice quantum chromodynamics (Borsanyi et al (2016); Borsanyi et al (2016b)).

Wireless communication amounts to encoding information onto physical observables carried by electromagnetic (EM) fields, radiating them into surrounding space, and detecting them remotely by an appropriate sensor connected to an informationdecoding receiver. Each observable is second order in the fields and fulfills a conservation law. In present-day radio only the EM linear momentum observable is fully exploited. A fundamental physical limitation of this observable, which represents the translational degrees of freedom of the charges (typically an oscillating current along a linear antenna) and the fields, is that it is single-mode. This means that a linear-momentum radio communication link comprising one transmitting and one receiving antenna, known as a single-input-single-output (SISO) link, can provide only one transmission channel per frequency (and polarization). In contrast, angular momentum, which represents the rotational degrees of freedom, is multi-mode, allowing an angular-momentum SISO link to accommodate an arbitrary number of independent transmission channels on one and the same frequency (and polarization). We describe the physical properties of EM angular momentum and how they can be exploited, discuss real-world experiments, and outline how the capacity of angular momentum links may be further enhanced by employing multi-port techniques, i.e., the angular momentum counterpart of linear-momentum multiple-input-multiple-output (MIMO).

The Large Observatory For x-ray Timing (LOFT) was studied within ESA M3 Cosmic Vision framework and participated in the final down-selection for a launch slot in 2022-2024. Thanks to the unprecedented combination of effective area and spectral resolution of its main instrument, LOFT will study the behaviour of matter under extreme conditions, such as the strong gravitational field in the innermost regions of accretion flows close to black holes and neutron stars, and the supra-nuclear densities in the interior of neutron stars. The science payload is based on a Large Area Detector (LAD, 10 m 2 effective area, 2-30 keV, 240 eV spectral resolution, 1 deg collimated field of view) and a WideField Monitor (WFM, 2-50 keV, 4 steradian field of view, 1 arcmin source location accuracy, 300 eV spectral resolution). The WFM is equipped with an on-board system for bright events (e.g. GRB) localization. The trigger time and position of these events are broadcast to the ground within 30 s from discovery. In this paper we present the status of the mission at the end of its Phase A study., Proc. SPIE 9144, Space Telescopes and Instrumentation 2014: Ultraviolet to Gamma Ray, 91442T

Recently it has become clear that the resonant amplification of quantum field fluctuations at preheating must be accompanied by resonant amplification of scalar metric perturbations, since the two are united by Einstein's equations. Furthermore, this "metric preheating" enhances particle production and leads to gravitational rescattering effects even at linear order. In multi-field models with strong preheating (q \gg 1), metric perturbations are driven nonlinear, with the strongest amplification typically on super-Hubble scales (k \to 0). This amplification is causal, being due to the super- Hubble coherence of the inflaton condensate, and is accompanied by resonant growth of entropy perturbations. The amplification invalidates the use of the linearized Einstein field equations, irrespective of the amount of fine-tuning of the initial conditions. This has serious implications at all scales - from the large-angle cosmic microwave background (CMB) anisotropies to primordial black holes. We investigate the (q,k) parameter space in a two-field model, and introduce the time to nonlinearity, t_{nl}, as the timescale for the breakdown of the linearized Einstein equations. Backreaction effects are expected to shut down the linear resonances, but cannot remove the existing amplification, which threatens the viability of strong preheating when confronted with the CMB. We discuss ways to escape the above conclusions, including secondary phases of inflation and preheating solely to fermions. Finally we rank known classes of inflation from strongest (chaotic and strongly coupled hybrid inflation) to weakest (hidden sector, warm inflation) in terms of the distortion of the primordial spectrum due to these resonances in preheating., 31 pages, 16 figures, Revtex. Final version. Nuclear Physics B (in press)

We introduce a new optical vortex coronagraph (OVC) method to determine the angular distance between two sources when the separation is sub-Rayleigh. We have found a direct relationship between the position of the minima and the source angular separation. A priori knowledge about the location of the two sources is not required. The superresolution capabilities of an OVC, equipped with an ℓ = 2 N-step spiral phase plate in its optical path, were investigated numerically. The results of these investigations show that a fraction of the light, increasing with N, from the secondary source can be detected with a sub-Rayleigh resolution of at least 0.1 λ/D.

We have shown experimentally that it is possible to propagate and use the properties of twisted non-monochromatic incoherent radio waves to simultaneously transmit to infinity more radio channels on the same frequency band by encoding them in different orbital angular momentum states. This novel radio technique allows the implementation of, at least in principle, an infinite number of channels on one and the same frequency, even without using polarization or dense coding techniques. An optimal combination of all these physical properties and techniques represents a solution for the problem of radio band congestion. Our experimental findings show that the vorticity of each twisted electromagnetic wave is preserved after the propagation, paving the way for entirely new paradigms in radio communication protocols., 17 pages, 6 figures, with a public experiment, three audio files in mp3 format

We design and built a novel optical terminal specifically designed for free-space communication operating at light levels at the quantum limit, such as in quantum communication. Our system is particularly well suited for this task, as it is based on an all-spheric catadioptric design, which allows for large and un-obstructed apertures. This design offers an easier and cheaper approach to building high-quality optical terminals with large apertures than schemes based on offaxis parabolic mirrors. We utilized an off-axis version of the original Schwarzschild concentric design, and correct the spherical aberration by substituting the original on-axis secondary spherical mirror with an off-axis catadioptric secondary mirror. A prototype of the optical terminal was realized and tested and it meet the expected performance.

Kerr black holes are among the most intriguing predictions of Einstein's general relativity theory. These rotating massive astrophysical objects drag and intermix their surrounding space and time, deflecting and phase-modifying light emitted nearby them. We have found that this leads to a new relativistic effect that imposes orbital angular momentum onto such light. Numerical experiments, based on the integration of the null geodesic equations of light from orbiting point-like sources in the Kerr black hole equatorial plane to an asymptotic observer, indeed identify the phase change and wavefront warping and predict the associated light-beam orbital angular momentum spectra. Setting up the best existing telescopes properly, it should be possible to detect and measure this twisted light, thus allowing a direct observational demonstration of the existence of rotating black holes. Since non-rotating objects are more an exception than a rule in the Universe, our findings are of fundamental importance., Article: 18 pages (11 pages in form of an Appendix). Total number of figures: 5

The optical vortex coronagraph (OVC) is an innovative instrument that can be applied to both space and groundbased telescopes for direct imaging of planets around bright stars. OVC rejects the light of the on-axis star without altering that of off-axis sources, means of a spiral phase plate (SPP) used as a phase modyfing device (PMD). We present the fabrication process made by lithographic nanofabrication and tests for the characterization of two different high-quality SPPs.

We analyse the Anderson-Higgs mechanism of photon mass acquisition in a plasma and study the contribution to the mass from the orbital angular momentum acquired by a beam of photons when it crosses a spatially structured charge distribution. To this end we apply Proca-Maxwell equations in a static plasma with a particular spatial distribution of free charges, notably a plasma vortex, that is able to impose orbital angular momentum (OAM) onto light. In addition to the mass acquisition of the conventional Anderson-Higgs mechanism, we find that the photon acquires an additional mass from the OAM and that this mass reduces the Proca photon mass., Four pages, no figures. Error corrections, improved notation, refined derivations

In this paper we present the fabrication process and tests of two different types of l = 2 spiral phase plates (SPPs), designed for an Optical Vortex Coronagraph (OVC) in the visible wavelength regime. Each phase mask is realized dividing the spirals area in sectors respectively of 8 and 512 of levels using lithographic nanofabrication approach. The SPPs produces different optical vortices (OVs) with topological charge l that depends on the number of steps and on the wavelength. We found that the residual light in the central dark region of the OV tends to zero as the number of steps increases.

Recently, wavelets and R/S analysis have been used as statistical tools to characterize the optical flickering of cataclysmic variables. Here we present the first comprehensive study of the statistical properties of X-ray flickering of cataclysmic variables in order to link them with physical parameters. We analyzed a sample of 97 X-ray light curves of 75 objects of all classes observed with the XMM-Newton space telescope. By using the wavelets analysis, each light curve has been characterized by two parameters, alpha and Sigma, that describe the energy distribution of flickering on different timescales and the strength at a given timescale, respectively. We also used the R/S analysis to determine the Hurst exponent of each light curve and define their degree of stochastic memory in time. The X-ray flickering is typically composed of long time scale events (1.5 < alpha < 3), with very similar strengths in all the subtypes of cataclysmic variables (-3 < Sigma < -1.5). The X-ray data are distributed in a much smaller area of the alpha-Sigma parameter space with respect to those obtained with optical light curves. The tendency of the optical flickering in magnetic systems to show higher Sigma values than the non-magnetic systems is not encountered in the X-rays. The Hurst exponents estimated for all light curves of the sample are larger than those found in the visible, with a peak at 0.82. In particular, we do not obtain values lower than 0.5. The X-ray flickering presents a persistent memory in time, which seems to be stronger in objects containing magnetic white dwarf primaries. The similarity of the X-ray flickering in objects of different classes together with the predominance of a persistent stochastic behavior can be explained it terms of magnetically-driven accretion processes acting in a considerable fraction of the analyzed objects., 10 pages, 3 figures, 2 tables. Language revision. Accepted for publication in A&A

Aims. We apply Floquet analysis to study the dynamical stability of two-dimensional axisymmetric Keplerian flows distorted by an m = 2 epicyclic periodic forcing term, representing a tidal perturbation in a thin accretion disk (AD) of a cataclysmic variable (CV). This tidal mode is responsible for the disk truncation mechanism. Methods. Floquet analysis reveals, with a positive Floquet exponent, exponentially growing modes of the local instabilities that can cause the disruption of the AD. The equations of the inviscid fluid used to model the AD describe an oversimplified two-dimensional tidally-distorted accretion disk. Viscosity and the other properties expected in a CV AD are taken from the Shakura-Sunyaev α-disk solution and introduced in the model through the thermodynamical quantities present in the equations. In particular, we address this investigation in search of parametric resonances of the radial Fourier components of the velocity field in the multi-dimensional parameter space of the solutions describing a fluid stream in a CV AD: the mass of the primary M1, the orbital period p of the system, the orbital fiducial radius r of the fluid stream and the normalized Fourier mode of the radial motion, k. Results. We find that, in region of the r3:2 orbital resonance radius, where superhumps (SH) form, the m = 2 tidal epicyclic modes alone would not introduce any perturbation in this region of the AD, and therefore the Floquet exponent of the m = 2 tidal force is found to be either negative or zero. However, only by introducing the 3:1 tidal perturbation, will the dynamics of the fluid streams at the SH radius become unstable indicating the onset of SH. Finally, Floquet analysis of the Fourier decomposition of the derivative along the radial x-direction shows the important role of the low Fourier modes of the radial velocity perturbations in the disk truncation mechanism.

We study the properties of the Fraunhofer diffraction patterns produced by Gaussian beams crossing spiral phase plates. We show, both analytically and numerically, that off-axis displacements of the input beam produce asymmetric diffraction patterns. The intensity profile along the direction of maximum asymmetry shows two different peaks. We find that the intensity ratio between these two peaks decreases exponentially with the off-axis displacement of the incident beam, the decay being steeper for higher strengths of the optical singularity of the spiral phase plate. We analyze how this intensity ratio can be used to measure small misalignments of the input beam with a very high precision., 8 pages, 4 figures. Accepted for publication in PRA

The properties of optical vortices produced with spiral phase plates have recently found interesting applications in astronomical coronagraphy. Here we review the characteristics of the optical vortex coronagraph. Our simulations show that the intensity of an on-axis star can be fainted by 10 orders of magnitude, thus allowing the detection of close faint sources like extrasolar planets. We also discuss the expected coronagraphic performances achievable with a stepped spiral phase plate.