The vacuum structure in QCD and hadron-hadron scattering

Standard ideas on the structure of the vacuum in QCD suggest it to be full of fluctuating color fields. We investigate the possible influence of the chromomagnetic vacuum fields on high energy hadron-hadron reactions. We suggest that high energy quarks traversing these fields will produce soft gluon and photon radiation analogous to synchrotron radiation from electrons and positrons in a storage ring. We argue that this radiation will lead to polarization phenomena for quarks in spin- and colorspace which in turn can explain theK-factor in the Drell-Yan reaction. We point out that jet production offers another way to study these polarization phenomena. We present then a calculation of the number of “synchrotron” photons which should be emitted inp?p collisions at high energies. Thus, we predict a sizable signal of prompt photons of nnergy less than a few hundred MeV with a characteristic frequency distribution. Observation of such photons would give strong support to our naive picture. Finally we point out a number of other phenomena like charmed particle decays where our “synchrotron” effect may be of importance.     

Formation of radiation spectrum at the planar channeling of a positron bunch with allowance for the medium inhomogeneity

The radiation of a positron in a planar channel of a crystal is studied, considering the dependence of the medium polarization on the transverse coordinate. The expression for the spectral distribution of the number of radiated photons is obtained for a single positron. It is shown that because of the medium polarization in the soft and hard regions not all positrons make a contribution to the radiation formation. An analytical formula for the total spectral distribution of the number of photons is obtained.     

Bremsstrahlung of nonrelativistic electrons in metals with allowance made for the polarization channel

We theoretically investigate the bremsstrahlung that appears when nonrelativistic electrons are scattered in a metal target with allowance made for the polarization contribution. We take into account the interference of ordinary and polarization bremsstrahlung, the absorption of radiation in the target material, the energy losses and elastic scattering of an electron by atoms of the medium, and the coherent effects when the radiating electron interacts with the target. We analyze the influence of the target thickness on the process and the contribution of polarization bremsstrahlung to the total yield of bremsstrahlung photons as a function of the problem parameters.     

Hybrid optical pumping of optically dense alkali-metal vapor without quenching gas

Optical pumping of an optically thick atomic vapor typically requires a quenching buffer gas, such as N2, to prevent radiation trapping of unpolarized photons which would depolarize the atoms. We show that optical pumping of a trace contamination of Rb present in K metal results in a 4.5 times higher polarization of K than direct optical pumping of K in the absence of N2. Such spin-exchange polarization transfer from optically thin species is useful in a variety of areas, including spin-polarized nuclear scattering targets and electron beams, quantum-nondemolition spin measurements, and ultrasensitive magnetometry.     

A relativistic description of the polarization mechanism of elastic bremsstrahlung

A completely relativistic mechanism for describing polarization bremsstrahlung caused by an elastic collision of a charged particle with a many-electron target was suggested. Multipole expansions for the amplitude and cross section of the process taking into account radiation lag effects were obtained. Including higher order multipoles was shown to result in substantial asymmetry of the angular distribution of emitted photons compared with the dipole case and in a noticeable change in the spectral characteristics of polarization radiation. The cross section of polarization bremsstrahlung was found to increase logarithmically as the energy of incident particles grew.     

New methods for determining the polarization state of vacuum ultraviolet radiation

Two new methods are proposed for determining the polarization of vacuum ultraviolet radiation which permit the determination of an arbitrary polarization mode for photons with energies of 10–100 eV. The essence of these methods is to create and detect a nonequilibrium population of the magnetic sublevels of atoms and molecules excited by the original VUV radiation and then determine the polarization of this radiation based on these measurements in accordance with known formulas. Zh. Tekh. Fiz. 69, 115–122 (September 1999)     

Generation of narrow-band hyperentangled nondegenerate paired photons

We report the generation of nondegenerate narrow-bandwidth paired photons with time-frequency and polarization entanglements from laser cooled atoms. We observe the two-photon interference caused by Rabi splitting with a coherence time of about 30 ns and a visibility of 81.8% which verifies the time-frequency entanglement of the paired photons. The polarization entanglement is confirmed by polarization correlation measurements which exhibit a visibility of 89.5% and characterized by quantum-state tomography with a fidelity of 90.8%. Taking into account the transmission losses and duty cycle, we estimate that the system generates hyperentangled paired photons into opposing single-mode fibers at a rate of 320 pairs per second.     

Importance of shadowing and multiple reflections in emission polarization

Polarization characteristics of thermal radiation emitted from surfaces are investigated within the geometrical optics approximation. Analytical results are presented for photons emitted without subsequent reflection from surfaces having sawtooth corrugations with different slope distributions. Analytical results are used to validate a Monte Carlo simulation designed to determine and quantify the effects of multiple reflection of emitted photons from surface structures and, in addition, to treat two-dimensional surfaces. Results are shown that illustrate the dependence of the degree of polarization on the relative orientation of the viewing angle with respect to the corrugations. Simulations of emission from structured and random two-dimensional surfaces show that, whilst the total emission can saturate, the degree of polarization decreases with increasing roughness of the surface morphology. The prospect for manipulating surfaces to have specific polarization signatures is discussed.     

Non-classicality of photon added coherent and thermal radiations

Production and analysis of non-Gaussian radiation fields has evinced a lot of attention recently. Simplest way of generating such non-Gaussians is through adding (subtracting) photons to Gaussian fields. Interestingly, when photons are added to classical Gaussian fields, the resulting states exhibit non-classicality. Two important classical Gaussian radiation fields are coherent and thermal states. Here, we study the non-classical features of such states when photons are added to them. Non-classicality of these states shows up in the negativity of the Wigner function. We also work out the entanglement potential, a recently proposed measure of non-classicality for these states. Our analysis reveals that photon added coherent states are non-classical for all seed beam intensities; their non-classicality increases with the addition of more number of photons. Thermal state exhibits non-classicality at all temperatures, when a photon is added; lower the temperature, higher is their non-classicality.     

Enhancement of laser interaction with vacuum for a large angular aperture

We study the nonlinear interaction of laser light with vacuum for a large angular aperture at electromagnetic field strengths far below the Schwinger limit. The polarization and magnetization in vacuum irradiated by a focused laser beam clearly differ from those in matter. This is due to the dependence on the Lorentz invariant, which results in a ring-shaped radiation distribution in vacuum. The number of the radiated photons increases nonlinearly with increasing angular aperture.     

Photon polarization as a probe for quark–gluon plasma dynamics

Prospects of measuring polarized photons emitted from a quark–gluon plasma (QGP) are discussed. In particular, the detection of a possible quark spin polarization in a QGP using circularly polarized photons emitted from the plasma is studied. Photons leave the QGP without further interaction and thus provide a primary probe for quark polarization within the QGP. We find that photon polarization cannot solely arise due to a possible QGP momentum space anisotropy, but may be enhanced due to it. In particular, for oblate momentum distributions and high photon energies, quark polarization is efficiently transfered to photon polarization. The role of competing sources of polarized photons in heavy-ion collisions is discussed.     

Forward circularly polarized photon emission in longitudinally polarized e+e− collisions

We discuss the emission of circularly polarized forward photons in high energy e⁺e⁻ collisions with longitudinally polarized beams. We find substantial asymmetry under inversion of longitudinal electron polarization, thus supporting the proposal that measurement of circular polarization of forward emitted photons may allow for monitoring of longitudinal beam polarization.     

Compton-Polarimeter für 2 MeV-Bremsstrahlung

A polarimeter for bremsstrahlung must simultaneously perform two functions: it must select photons within a small energy interval out of the continuous bremsstrahlung spectrum and it must measure the polarization of these photons. The polarimeter depends on the polarization sensitivity of the Compton process with a 5 mm Si(Li-drift) detector as scatterer and a plastic scintillator as photon detector. To improve the energy resolution of the polarimeter a second 0,5 mm Si(Li-drift) detector behind the scatterer has been placed in anticoincidence with the scatterer.     

Entanglement generation by Fock-state filtration

We demonstrate a Fock-state filter which is capable of preferentially blocking single photons over photon pairs. The large conditional nonlinearities are based on higher-order quantum interference, using linear optics, an ancilla photon, and measurement. We demonstrate that the filter acts coherently by using it to convert unentangled photon pairs to a path-entangled state. We quantify the degree of entanglement by transforming the path information to polarization information; applying quantum state tomography we measure a tangle of T=(20+/-9)%.     

High-Capacity Three-Party Quantum Secret Sharing With Hyperentanglement

We presents a novel scheme for high-capacity three-party quantum secret sharing (QSS) with the hyperentanglement in both the polarization and the spatial-mode degrees of freedom of photon pairs. The boss Alice need only prepare a sequence of photon pairs and some decoy photons. Her two agents measure their photons received from the boss Alice with two bases by choosing two unsymmetrical probabilities. The present QSS scheme has a high capacity as each pair can carry 2 bits of information, several times as other QSS schemes. Moreover, our setups with linear optical elements show that our QSS scheme does not increase the difficulty of its implementation in experiment and it is feasible with current techniques.     

Measurement of geometric phase for mixed states using single photon interferometry

Geometric phase may enable inherently fault-tolerant quantum computation. However, due to potential decoherence effects, it is important to understand how such phases arise for mixed input states. We report the first experiment to measure mixed-state geometric phases in optics, using a Mach-Zehnder interferometer, and polarization mixed states that are produced in two different ways: decohering pure states with birefringent elements; and producing a nonmaximally entangled state of two photons and tracing over one of them, a form of remote state preparation.     

Analysis of the uniform magnetic field free-electron laser with scalarized photons in the microwave-spectral region

The recently developed concept of scalarized photons (formally photons of any polarization) is used to analyze the spontaneous and stimulated emission in the uniform magnetic field free-electron laser in the microwave spectral region. In fact, this free-electron laser is the simplest of many other, wiggler and wiggler-free free-electron lasers whose analyses could be done with scalarized photons in the small signal regime and whose physical parameters can be conveniently chosen for radiation to be generated in the microwave spectral region. As to the uniform magnetic field free-electron laser, which is treated here in some detail, with the electron beam energy of up to 10 MeV and the uniform magnetic field of up to 4 Tesla, the radiation (occurring with the fundamental and higher harmonic frequencies) can cover easily a 10 to 10,000 GHz spectral region.     

Probing vacuum birefringence by phase-contrast Fourier imaging under fields of high-intensity lasers

In vacuum high-intensity lasers can cause photon–photon interaction via the process of virtual vacuum polarization which may be measured by the phase velocity shift of photons across intense fields. In the optical frequency domain, the photon–photon interaction is polarization-mediated described by the Euler–Heisenberg effective action. This theory predicts the vacuum birefringence or polarization dependence of the phase velocity shift arising from nonlinear properties in quantum electrodynamics (QED). We suggest a method to measure the vacuum birefringence under intense optical laser fields based on the absolute phase velocity shift by phase-contrast Fourier imaging. The method may serve for observing effects even beyond the QED vacuum polarization.     

Polarization and angular correlation studies of X-rays emitted in relativistic ion-atom collisions

Particle and photon polarization phenomena occurring in collisions of relativistic ions with matter have recently attracted particular interest. Investigations of the emitted characteristic x-ray and radiative electron capture radiation has been found to be a versatile tool for probing our present understanding of the dynamics of particles in extreme electromagnetic fields. Owing to the progress in x-ray detector technology, in addition, accurate measurements of the linear polarization for hard x-ray photons as well as the determination of the polarization plane became possible. This new diagnostic tool enables one today to derive information about the polarization of the ion beams from the photon polarization features of the radiative electron capture process.