Quantum Electrodynamics Processes Research Articles

On refractive processes in strong laser field quantum electrodynamics

Refractive processes in strong-field QED are pure quantum processes, which involve only external photons and the background electromagnetic field. We show analytically that such processes occurring in a plane-wave field and involving external real photons are all characterized by a surprisingly modest net exchange of energy and momentum with the laser field, corresponding to a few laser photons, even in the limit of ultra-relativistic laser intensities. We obtain this result by a direct calculation of the transition matrix element of an arbitrary refractive QED process and accounting exactly for the background plane-wave field. A simple physical explanation of this modest net exchange of laser photons is provided, based on the fact that the laser field couples with the external photons only indirectly through virtual electron–positron pairs. For stronger and stronger laser fields, the pairs cover a shorter and shorter distance before they annihilate again, such that the laser can transfer to them an energy corresponding to only a few photons. These results can be relevant for the future experiments aiming to test strong-field QED at present and next-generation facilities.

Spontaneous emission in cavity QED with a terminated waveguide

We investigate the effects of a nanophotonic boundary on the spontaneous emission properties of an excited two-level atom in cavity quantum electrodynamics (QED) geometry. We show that a boundary provides temporally delayed interference, which can be either constructive or destructive. Consequently, the decay of the atomic excitation can be either increased or greatly inhibited. As a concrete example, we investigate the spontaneous emission process in cavity QED with a terminated line-defect waveguide, and show the rich behavior of the atomic response due to the boundary. We also show that the output photonic wave form is strongly influenced by the boundary.

On the phase-space analysis of photon mediated quantum state transfer in nanophotonic waveguidance

A cavity quantum electrodynamics (QED) based approach for transferring quantum state between quantum nodes has been proposed, wherein a rubidium (87Rb) atom trapped inside a two-mode optical cavity forms the quantum node and photons serve as the information carrier between two such nodes. Information is encoded into polarized photon states generated through the application of a system of lasers. The focus is made on the phase-space analysis of the approach, wherein two subspaces of the hyperfine energy levels with magnetic sub-levels of rubidium (87Rb) atom represent the logic states ‘0’ and ‘1’. The system of lasers initiates a cavity assisted Raman process which, in turn, generates a right- or left-circularly polarized photon depending on the logic state of the transmit node. Once the photon is received (at the receive node), the logic state of the transmit node is restored into the receive node through a cavity QED process.

Laser Absorption in Relativistically Underdense Plasmas by Synchrotron Radiation

A novel absorption mechanism for linearly polarized lasers propagating in relativistically underdense solids in the ultrarelativistic (a ~ 100) regime is presented. The mechanism is based on strong synchrotron emission from electrons reinjected into the laser by the space charge field they generate at the front of the laser pulse. This laser absorption, termed reinjected electron synchrotron emission, is due to a coupling of conventional plasma physics processes to quantum electrodynamic processes in low density solids at intensities above 10(22) W/cm(2). Reinjected electron synchrotron emission is identified in 2D QED-particle-in-cell simulations and then explained in terms of 1D QED-particle-in-cell simulations and simple analytical theory. It is found that between 1% (at 10(22) W/cm(2)) and 14% (at 8 × 10(23) W/cm(2)) of the laser energy is converted into gamma ray photons, potentially providing an ultraintense future gamma ray source.

Quasiclassical approach to high-energy QED processes in strong laser and atomic fields

An approach, based on the use of the quasiclassical Greenʼs function, is developed for investigating high-energy quantum electrodynamical processes in combined strong laser and atomic fields. Employing an operator technique, we derive the Greenʼs function of the Dirac equation in an arbitrary plane wave and a localized potential. Then, we calculate the total cross section of high-energy electron–positron photoproduction in an atomic field of arbitrary charge number (Bethe–Heitler process) in the presence of a strong laser field. It is shown that the laser field substantially modifies the cross section at already available incoming photon energies and laser parameters. This makes it feasible in principle to observe with present technology the analogous effect in a laser field of the Landau–Pomeranchuk–Migdal effect for the Bethe–Heitler process.

Nonresonant quantum electrodynamics processes in a pulsed laser field

This review contains theoretical study of nonresonant quantum electrodynamics processes of the first and second orders in the fine-structure constant in a pulsed laser field. The approximation is examined when the pulse width is considerably greater than the characteristic time of wave oscillations. It was demonstrated that for nonrelativistic particle energy the differential cross section of a process in a pulsed light fields may considerably difference from the corresponding cross section in an absence of a laser field. Results obtained may be experimentally verified by the scientific facilities at the SLAC National Accelerator Laboratory and FAIR (Facility for Antiproton and Ion Research, Darmstadt, Germany) project.

Calculation of amplitudes in quantum electrodynamics

A new method of calculation of amplitudes of different processes in quantum electrodynamics is proposed. The method does not use the Feynman technique of trace of product of matrices calculation. The method strongly simplifies calculation of cross sections for different processes. The effectiveness of the method is shown on the cross-section calculation of Coulomb scattering, Compton scattering and electron-positron annihilation.

Quantum electrodynamics resonances in a pulsed laser field

This review contains theoretical study of resonant quantum electrodynamics processes in a pulsed laser field. The approximation is examined when the pulse width is considerably greater than the characteristic time of wave oscillations. The lepton’s interaction with the Coulomb potential of a nucleus and each other is considered in the Born approximation. It is demonstrated that the resonant differential cross section of a process in a pulsed light fields may considerably exceed the corresponding cross section in an absence of a laser field. Results obtained may be experimentally verified by the scientific facilities at the SLAC National Accelerator Laboratory and FAIR (Facility for Antiproton and Ion Research, Darmstadt, Germany) project.

Cavity quantum electrodynamics for photon mediated transfer of quantum states

An enhanced approach for transferring quantum state between quantum nodes is proposed wherein photons serve as the information carrier. Each node consists of a Rubidium (87Rb) atom trapped inside a two-mode optical cavity. The approach is based on cavity quantum electrodynamics (QED) wherein a system of lasers is applied on the atom in order to generate photon through Raman transition. Logic states ‘0’ and ‘1’ are represented by two subspaces of the hyperfine energy levels with magnetic sub-levels of 87Rb atom. A static magnetic field is applied upon the atoms so that the hyperfine states of 87Rb atom are split into the magnetic sub-levels (due to Zeeman effect). Depending on the logic state of the transmit node, a right- or left-circularly polarized photon with designated frequency is produced through a cavity assisted Raman process. When the photon is received at the receive node via an optical fiber, the logic state of the transmit node is restored (through a cavity QED process) into the receive node. A desirable feature of the approach is that, during the transmission of logic state, the transmit node itself should not significantly change its quantum state; this is successfully validated through simulations.

From first principles of QED to an application: hyperfine structure of P states of muonic hydrogenThis paper was presented at the International Conference on Precision Physics of Simple Atomic Systems, held at École de Physique, les Houches, France, 30 May – 4 June, 2010.

The purpose of this article is twofold. First, we attempt to give a brief overview of the different application areas of quantum electrodynamics (QED). These include fundamental physics (prediction of atomic energy levels), where the atom may be exposed to additional external fields (hyperfine splitting and g factor). We also mention QED processes in highly intense laser fields and more applied areas like Casimir and Casimir–Polder interactions. Both the unifying aspects as well as the differences in the the theoretical treatment required by these application areas (such as the treatment of infinities) are highlighted. Second, we discuss an application of the formalism in the fundamentally interesting area of the prediction of energy levels, namely, the hyperfine structure of P states of muonic hydrogen.

QED vacuum effects in intense laser fields

Two quantum electrodynamical processes occurring in the presence of a strong laser field and involving the polarization of quantum vacuum are discussed. First, the process of photon splitting in a laser field is examined and the possibility of its experimental observation is investigated. In particular, it is seen that envisaged progress in the generation of high-flux monochromatic gamma ray beams by employing Compton backscattering by electrons coming from an undulator could lead to an experimental observation of this process. Then, the laser photons merging due to vacuum polarization in the collision of a strong laser field and a proton beam is studied. It is shown that this process allows, in principle, the possibility of observing for the first time non-perturbative refractive vacuum polarization effects primed by a strong laser beam.

Recursive algorithm for arrays of generalized Bessel functions: Numerical access to Dirac-Volkov solutions

In the relativistic and the nonrelativistic theoretical treatment of moderate and high-power laser-matter interaction, the generalized Bessel function occurs naturally when a Schrödinger-Volkov and Dirac-Volkov solution is expanded into plane waves. For the evaluation of cross sections of quantum electrodynamic processes in a linearly polarized laser field, it is often necessary to evaluate large arrays of generalized Bessel functions, of arbitrary index but with fixed arguments. We show that the generalized Bessel function can be evaluated, in a numerically stable way, by utilizing a recurrence relation and a normalization condition only, without having to compute any initial value. We demonstrate the utility of the method by illustrating the quantum-classical correspondence of the Dirac-Volkov solutions via numerical calculations.

Coulomb-field-induced conversion of a high-energy photon into a pair assisted by a counterpropagating laser beam

The laser-induced modification of a fundamental process of quantum electrodynamics, the conversion of a high-energy gamma photon in the Coulomb field of a nucleus into an electron–positron pair, is studied theoretically. Although the employed formalism allows for the general case where the gamma photon and laser photons cross at an arbitrary angle, we here focus on a theoretically interesting and numerically challenging setup, where the laser beam and gamma photon counterpropagate and impinge on a nucleus at rest. For a peak laser field smaller than the critical Schwinger field and gamma photon energy larger than the field-free threshold, the total cross section is verified to be almost unchanged with respect to the field-free case, whereas the differential cross section is drastically modified by the laser field. The modification of the differential cross section is explained by classical arguments. We also find the laser-dependent maximal energy of the produced pair and point out several interesting features of the angular spectrum.

Measurement of collision integral cross-sections of double-photon Compton effect using a single gamma ray detector: A response matrix approach

The collision integral cross-sections of double-photon Compton process are measured experimentally for 662 keV incident gamma photons. The measurements are successfully carried out using a single gamma ray detector, and do not require the complicated slow-fast coincidence technique used till now for observing this higher order quantum electrodynamics (QED) process. The energy spectra of one of the two final photons, originating in this process, in direction of the gamma ray detector are observed as a long tail to the single-photon Compton line on lower side of the full energy peak in the observed spectra. An inverse response matrix converts the observed pulse-height distribution of a NaI(Tl) scintillation detector to a true photon spectrum. This also results in extraction of events originating from double-photon Compton interactions. The present measured values of collision integral cross-section, although of same magnitude, deviate from the corresponding values obtained from the theory. In view of the magnitude of deviations, in addition to small value of probability of occurrence of this process, the agreement of measured values with theory is reasonably acceptable.

Efficiency of a feedback process in cavity quantum electrodynamics

Utilizing the continuous frequency mode quantization scheme, we study from first principles the efficiency of a feedback scheme that can generate maximally entangled states of two atoms in an optical cavity through their interactions with a single input photon. The spectral function of the photon emitted from the cavity, which will be used as the input of the next round in the feedback process, is obtained analytically. We find that the spectral function of the photon is modified in each round and deviates from the original one. The efficiency of the feedback scheme consequently deteriorates gradually after several rounds of operation.

Matching perturbative and parton shower corrections to Bhabha process at flavour factories

We report on a high-precision calculation of the Bhabha process in quantum electrodynamics, of interest for precise luminosity determination of electron–positron colliders involved in R measurements in the region of hadronic resonances. The calculation is based on the matching of exact next-to-leading order corrections with a parton shower algorithm. The accuracy of the approach is demonstrated in comparison with existing independent calculations and through a detailed analysis of the main components of theoretical uncertainty, including two-loop corrections, hadronic vacuum polarization and light pair contributions. The calculation is implemented in an improved version of the event generator BABAYAGA with a theoretical accuracy of the order of 0.1%. The generator is now available for high-precision simulations of the Bhabha process at flavour factories.

Electron-positron pair creation by powerful laser-ion impact

In view of recently achieved extreme laser powers for which the ponderomotive energy ${U}_{p}\ensuremath{\gg}2m{c}^{2}$, it has become of interest to reconsider fundamental processes of quantum electrodynamics in such powerful laser fields. In the present work, we evaluate the rates of producing electron-positron pairs by the impact of laser pulses of extreme power on highly charged ions at rest. Contrary to previous investigations, we shall consider a linearly polarized laser pulse, since at extreme laser powers the contribution of the ${A}^{2}$ part of the electromagnetic interaction becomes significant. As long as the duration of the laser pulses considered is in the femtosecond domain, the description of the laser beam by a powerful, monochromatic electromagnetic plane wave will still be justified, which simplifies considerably the calculations required.

Ultra low momentum neutron catalyzed nuclear reactions on metallic hydride surfaces

Ultra low momentum neutron catalyzed nuclear reactions in metallic hydride system surfaces are discussed. Weak interaction catalysis initially occurs when neutrons (along with neutrinos) are produced from the protons which capture ``heavy'' electrons. Surface electron masses are shifted upwards by localized condensed matter electromagnetic fields. Condensed matter quantum electrodynamic processes may also shift the densities of final states allowing an appreciable production of extremely low momentum neutrons which are thereby efficiently absorbed by nearby nuclei. No Coulomb barriers exist for the weak interaction neutron production or other resulting catalytic processes.

Electron-positron pair creation by powerful laser-ion interaction

Presently available high-power laser pulses of ponderomotive energy U p ≫ 2mc 2 should permit the fundamental processes of quantum electrodynamics in such fields, in particular, the formation of electron-positron pairs in impacts of laser pulses with highly charged ions, to be observed. We evaluate the highly nonlinear production rates of this process and investigate the most favorable conditions of pair production, in particular, either along the direction of linear polarization or in the propagation direction of the laser pulse. For femtosecond radiation pulses, it is possible to represent the laser beam by a monochromatic and linearly polarized electromagnetic plane wave. This approximation considerably simplifies the calculations required.

Radiation reaction on charged particles in three-dimensional motion in classical and quantum electrodynamics

We extend our previous work (see arXiv:quant-ph/0501026), which compared the predictions of quantum electrodynamics concerning radiation reaction with those of the Abraham-Lorentz-Dirac theory for a charged particle in linear motion. Specifically, we calculate the predictions for the change in position of a charged scalar particle, moving in three-dimensional space, due to the effect of radiation reaction in the one-photon-emission process in quantum electrodynamics. The scalar particle is assumed to be accelerated for a finite period of time by a three-dimensional electromagnetic potential dependent only on one of the spacetime coordinates. We perform this calculation in the $\hbar\to 0$ limit and show that the change in position agrees with that obtained in classical electrodynamics with the Lorentz-Dirac force treated as a perturbation. We also show for a time-dependent but space-independent electromagnetic potential that the forward-scattering amplitude at order $e^2$ does not contribute to the position change in the $\hbar \to 0$ limit after the mass renormalization is taken into account.