The properties of four-wave interaction via the nonlinear quantum vacuum is investigated. The effect of the quantum vacuum is to generate photons with new frequencies and wave vectors, due to elastic photon-photon scattering. An expression for the number of generated photons is derived and using state-of-the-art laser data it is found that the number of photons can reach detectable levels. In particular, the prospect of using the high repetition Astra Gemini system at the Rutherford Appleton Laboratory is discussed. The problem of noise sources is reviewed, and it is found that the noise level can be reduced well below the signal level. Thus, detection of elastic photon-photon scattering may for the first time be achieved. PACS numbers: 12.20.Fv, 42.50.Xa Classically, elastic photon-photon scattering does not take place in vacuum. However, according to quantum electrodynamics (QED), such a process may occur owing to the interaction with virtual electron-positron pairs. Several suggestions to detect photon-photon scattering have been made, for example using harmonic generation in an inhomogeneous magnetic field [1], using resonant interaction between eigenmodes in microwave cavities [2,3], using ultra-intense fields occurring in plasma channels [4], as well as many others, see e.g. Refs. [5,6,7]. Related to, but physically different from, elastic photon-photon scattering is photon splitting [8] (see also [9]) and Delbrück scattering [10], the latter being detectable using high-Z atomic targets [10]. However, no suggestions have yet led to detection of elastic scattering among real photons.In the present Letter we will investigate the possibility to detect photon-photon scattering in vacuum using four-wave mixing, where three colliding laser pulses stimulate emission in a fourth direction with a new frequency. Four-wave mixing has the advantage of not being limited by the low cross section for photon-photon scattering [11]. A similar scheme was first studied by Ref. [12], and further theoretical [13,14,15] and experimental [16] studies of QED four-wave mixing have been performed since then. However, so far the available laser technology has not been sufficiently powerful to allow for successful detection of scattering events. Moreover, even with high laser power, the laser setup and geometry in such an experiment is important. In particular, we argue that a two-dimensional (2D) setup preferred in earlier literature is unlikely to produce a detectable signal. This is in contrast to the 3D setup presented in this Letter. We have calculated the coupling coefficient for four-wave mixing as a function of incident angles and laser polarization. For given data of the laser beams, together with the coupling coefficients, the number of scattered photons can be calculated. The analysis is then used * Currently at: Department of Physics, Stockholm University, SE-106 91, Sweden to suggest a novel concrete experiment, where the parameters are chosen to fit the Astra Gemini (AG) system (operational 2007) located at the Centr...

Studies of charged-particle acceleration processes remain one of the most important areas of research in laboratory, space and astrophysical plasmas. In this paper, we present the underlying physics and the present status of high gradient and high energy plasma accelerators. We will focus on the acceleration of charged particles to relativistic energies by plasma waves that are created by intense laser and particle beams. The generation of relativistic plasma waves by intense lasers or electron beams in plasmas is important in the quest for producing ultra-high acceleration gradients for accelerators. With the development of compact short pulse high brightness lasers and electron positron beams, new areas of studies for laser/particle beam-matter interactions is opening up. A number of methods are being pursued vigorously to achieve ultrahigh acceleration gradients. These include the plasma beat wave accelerator mechanism, which uses conventional long pulse (∼100 ps) modest intensity lasers (I ∼ 10 14 -10 16 W cm −2 ), the laser wakefield accelerator (LWFA), which uses the new breed of compact high brightness lasers (<1 ps) and intensities >10 18 W cm −2 , the self-modulated LWFA concept, which combines elements of stimulated Raman forward scattering, and electron acceleration by nonlinear plasma waves excited by relativistic electron and positron bunches. In the ultra-high intensity regime, laser/particle beam-plasma interactions are highly nonlinear and relativistic, leading to new phenomena such as the plasma wakefield excitation for particle acceleration, relativistic self-focusing and guiding of laser beams, high-harmonic generation, acceleration of electrons, positrons, protons and photons. Fields greater than 1 GV cm −1 have been generated with particles being accelerated to 200 MeV over a distance of millimetre. Plasma wakefields driven by positron beams at the Stanford Linear Accelerator Center facility have accelerated the tail of the positron beam. In the near future, laser plasma accelerators will be producing GeV particles.

Using both fluid and kinetic descriptions, where repulsive forces between near by atoms are included, we discuss the basic oscillations and waves of a cloud of ultra-cold atoms confined in a magneto-optical trap. The existence of a hybrid mode, with properties similar to both plasma and acoustic waves is described in detail. Tonks-Dattner resonances for confined hybrid modes in a spherical cloud are discussed and the prediction of a nonlinear coupling between the dipole resonanc and the hybrid modes is considered. Landau damping processes and quasi-linear diffusion in velocity space are also discussed.

We study theoretically the exchange of angular momentum between electromagnetic and electrostatic waves in a plasma, due to the stimulated Raman and Brillouin backscattering processes. Angular momentum states for plasmon and phonon fields are introduced for the first time. We demonstrate that these states can be excited by nonlinear wave mixing, associated with the scattering processes. This could be relevant for plasma diagnostics, both in laboratory and in space. Nonlinearly coupled paraxial equations and instability growth rates are derived. PACS numbers:It is well known that the angular momentum of electromagnetic radiation contains two distinct parts, one associated with its polarization state, or photon spin, the other being the external or orbital photon angular momentum (OAM). With the advent of laser beams, an increasing interest is being given to the study of photon OAM, and various optical experimental configurations have been considered [1,2,3,4]. It is now well understood that collimated electromagnetic beams, such as laser or radio wave beams, can be described by LaguerreGaussian functions, which provide a natural orthonormal basis for a generic beam representation. Utilization of photon OAM states in the low frequency (≤ GHz) radio wave domain was also recently proposed in Ref. 5, as a new method for studying and characterizing radio sources in astrophysics.The possibility of remote study of space plasma vorticity by measuring the OAM of radio beams interacting with vortical plasmas was pointed out in Ref. 6, and a more detailed theoretical analysis was given recently by studying the electromagnetic wave scattering from the plasma medium, with the associated OAM exchanges between the plasma and probing photon beams [7]. A more speculative work was also recently published where the strong similarities between photon and neutrino dispersion relations were explored, and OAM states of neutrino beams interacting with dense plasmas were considered [8].Here we consider the important problem of stimulated Raman and Brillouin backscattering of collimated electromagnetic beams with finite OAM in a plasma. This also leads us to consider, to our knowledge for the first * Electronic address: titomend@ist.utl.pt † Also at LOIS Space Centre, Växjö University, SE-351 95 Växjö, Sweden time, the possible existence of plasmon and phonon states with finite orbital angular momentum. Raman and Brillouin scattering instabilities are well known in the context of laser fusion [9], as possible sources of anomalous plasma reflectivity. Raman backscattering is now recognized as a dominant process for ultra-intense laser plasma interactions, in the context of inertial fusion research [10]. In all these studies, angular momentum in general, and photon OAM in particular, have been systematically ignored. On the other hand, there seems to be experimental evidence of OAM dependence in Brillouin scattering of radio waves in the ionosphere [11], which awaits for a deeper theoretical understanding. In contrast with the traditional t...

One of the biggest unsolved problems in physics is the unification of quantum mechanics and general relativity. The lack of experimental guidance has made the issue extremely evasive, though various attempts have been made to relate the loss of matter wave coherence to quantum spacetime fluctuations. We present a new approach to the gravitational decoherence near the Planck scale, made possible by recently discovered conformal structure of canonical gravity. This leads to a gravitational analogue of the Brownian motion whose correlation length is given by the Planck length up to a scaling factor. With input from recent matter wave experiments, we show that the minimum value of this factor to be well within the expected range for quantum gravity theories. This suggests that the sensitivities of advanced matter wave interferometers may be approaching the fundamental level due to quantum spacetime fluctuations and that investigating Planck scale physics using matter wave interferometry may become a reality in the near future.Physics on the large scale is based on Einstein's theory of general relativity (GR), which interprets gravity as the curvature of spacetime. Despite its tremendous success as an isolated theory of gravity, GR has proved problematic in integration with physics as a whole, in particular the physics of the very small governed by quantum mechanics. There can be no unification of physics, which does not include them both. Superstring theory [1] and its recent extension to the more general theory of branes is a popular candidate, but the links with experiment are very tenuous. Loop quantum gravity [2,3] attempts to quantize GR without unification, and has so far received no obvious experimental verification.One hundred years ago, when Planck introduced the constant named after him, he also introduced the Planck scales, which combined this constant with the velocity of light c and Newton's gravitational constant G to give the fundamental Planck time T Planck = ( G/c 5 ) 1/2 ≈ 10 −43 s, Planck length L Planck = c T Planck ≈ 10 −35 m and Planck mass M Planck = /(c 2 T Planck ) ≈ 10 −8 kg. Experiments on quantum gravity require access to these scales. To access these scales directly using accelerators would require 10 19 GeV accelerators, well beyond any conceivable experiments.