Virtual Photon Exchange Research Articles

Dipole‐dipole coupling in the presence of dispersing and absorbing bodies

AbstractAn effective Hamiltonian and equations of motion for treating both the resonant dipole‐dipole interaction between two‐level atoms and the resonant atom‐field interaction are derived, which can suitably be used for studying the influence of arbitrary dispersing and absorbing material surroundings on these interactions. It is shown that the dipole‐dipole interaction and the atom‐field interaction, respectively, are closely related to the real part and the imaginary part of the (classical) Green tensor. The theory is applied to the study of the transient behavior of two atoms that initially share a single excitation, with special emphasis on the role of the two competing processes of virtual and real photon exchange in the energy transfer between the atoms. To illustrate the powerfulness of the theory, specific results for the case of the atoms being near a dispersing and absorbing microsphere are presented. In particular, the regimes of weak and strong atom‐field coupling are addressed.

Forward-backward charge asymmetry ine+e−annihilation into hadrons at high energies

The forward-backward asymmetry in ${e}^{+}{e}^{\ensuremath{-}}$ annihilation into a quark-antiquark pair is considered in the double-logarithmic approximation at energies much higher than the masses of the weak bosons. It is shown that after accounting for the exchange of virtual photons and $W,Z$ bosons to all orders one is led to the following effect (asymmetry): quarks with positive electric charge (e.g., u, $\overline{d})$ tend to move in the ${e}^{+}$ direction whereas quarks with negative charges (e.g., d, $\ifmmode \bar{u}\else \={u}\fi{})$ tend to move in the ${e}^{\ensuremath{-}}$ direction. The value of the asymmetry grows with increasing energy when the produced quarks are within a cone with an opening angle, in the center of mass frame, ${\ensuremath{\theta}}_{0}\ensuremath{\sim}{2M}_{Z}/\sqrt{s}$ around the ${e}^{+}{e}^{\ensuremath{-}}$ beam. Outside this cone, at ${\ensuremath{\theta}}_{0}\ensuremath{\ll}\ensuremath{\theta}\ensuremath{\ll}1,$ the asymmetry is inversely proportional to $\ensuremath{\theta}.$

Effect of Virtual Photon Exchange on the Interaction of Light Field with N Atoms**The project supported in part by National Natural Science Foundation of China (No. 20176035)

Under no rotating wave approximation and including the effect of dipole-dipole interaction between the atoms, exact squeezing properties for the output optical field and the atomic field are obtained. It is shown that an atom laser keeping squeezed all the time can be generated, though the input optical field is at the coherent state or vacuum state.

Resonant dipole-dipole interaction in the presence of dispersing and absorbing surroundings

Within the framework of quantization of the macroscopic electromagnetic field, equations of motion and an effective Hamiltonian for treating both the resonant dipole-dipole interaction between two-level atoms and the resonant atom-field interaction are derived, which can suitably be used for studying the influence of arbitrary dispersing and absorbing material surroundings on these interactions. The theory is applied to the study of the transient behavior of two atoms that initially share a single excitation, with special emphasis on the role of the two competing processes of virtual and real photon exchange in the energy transfer between the atoms. In particular, it is shown that for weak atom-field interaction there is a time window, where the energy transfer follows a rate regime of the type obtained by ordinary second-order perturbation theory. Finally, the resonant dipole-dipole interaction is shown to give rise to a doublet spectrum of the emitted light for weak atom-field interaction and a triplet spectrum for strong atom-field interaction.

Coherence retention by resonance dynamic dipole–dipole interaction among quantum dot ensemble for quantum computing

The coherence retention of the resonance dynamic dipole–dipole interaction (RDDDI) in an ensemble of two level systems (quantum dots) is discussed, in terms of parity inheritance and exchange of virtual photons, together with the energetic stability. This coherence is expected to be useful for quantum computing. Furthermore, inter-ensemble (inter-block) RDDDI is considered, and a qubit swap ( Π4) gate and quantum permutation circuits that can transfer a qubit state to any position in the quantum circuit are proposed.

Bremsstrahlung radiation as a coherent state in thermal QED

Based on fully finite temperature field theory we investigate the radiation probability in the bremsstrahlung process in thermal QED. It turns out that the infrared divergences resulting from the emission and absorption of the real photons are cancelled by the virtual photon exchange processes at finite temperature. The full quantum calculation results for soft photon radiation coincide completely with those obtained in the semiclassical approximation. In the framework of thermofield dynamics it is shown that the bremsstrahlung radiation in thermal QED is a coherent state and its quasi-classical behaviour leads to the above coincidence.

Measurement of the vector analyzing power in elastic electron-proton scattering as a probe of the double virtual Compton amplitude

We report the first measurement of the vector analyzing power in inclusive transversely polarized elastic electron-proton scattering at Q^2 = 0.1 (GeV/c)^2 and large scattering angles. This quantity should vanish in the single virtual photon exchange, plane wave impulse approximation for this reaction, and can therefore provide information on double photon exchange amplitudes for electromagnetic interactions with hadronic systems. We find a non-zero value of A=-15.4+/-5.4 ppm. No calculations of this observable for nuclei other than spin 0 have been carried out in these kinematics, and the calculation using the spin orbit interaction from a charged point nucleus of spin 0 cannot describe these data.

A quantum electrodynamic theory of two-centre two-photon circular dichroism

Expressions for the rate of two-photon circular dichroism occurring at two identical oriented or completely randomly oriented centres are calculated using perturbation theory and the Fermi golden rule within the framework of non-relativistic quantum electrodynamics. Circular differential absorption in the two-group model arises from the relative dissymmetric juxtaposition of two inherently achiral functional groups. Inter-chromophore coupling is included through the exchange of virtual photons. Absorption of two circularly polarized photons takes place at each centre, and the interference between the two amplitudes depends on the chromophore separation and on the handedness of the incident radiation. In the near zone, the two-centre two-photon circular dichroism rate is found to depend linearly upon the inter-group separation distance.

The quantum origins of the free induction decay signal and spin noise.

Experiments are described that elucidate the quantum mechanical origins of the free induction decay voltage and of spin noise. It is shown that the experimentally measured FID voltage induced in a Hertzian loop receiving coil following a 90 degrees pulse is typically two orders of magnitude too large to be accounted for by the current quantum theory of signal reception-coherent spontaneous emission. An experiment is then presented in which spin noise is easily observed in a circuit with a Q-factor of order unity, thereby undermining a popular hypothesis that such noise is due to spontaneous emission and is only observable because of the enhancement in the density of the radiation field in a high Q-factor tuned circuit, the NMR probe. Both the free induction decay and the spin noise are shown to be accurately predicted by near-field Faraday induction, which is described in the theory of quantum electrodynamics by an exchange of virtual photons. A heuristic approach to understanding the nature and role of virtual photons in the signal reception process is then given. Thus current popular statements that observation of the magnetic resonance phenomenon relies on the absorption and emission of radio waves are shown to be wrong.

Deep-inelastic electron-photon scattering at high Q2 : Neutral and charged current reactions

We present the results of a calculation of deep inelastic electron-photon scattering at a linear collider for very high virtuality of the intermediate gauge boson up to NLO in perturbative QCD. The real photon is produced unpolarized via the Compton back scattering of laser light of the incoming beam. For Q 2 values close to the masses squared of the Z and W gauge bosons, the deep inelastic electron-photon scattering process receives important contributions not only from virtual photon exchange but also from the exchange of a Z-boson and a W-boson. We find that the total cross section for center of mass energies above 500GeV is at least of O ( pb) and has an important charged current contribution.

Deep inelastic eγ scattering at high Q2

The electromagnetic and weak structure functions of the photon can be studied in the deep inelastic electron-photon processes eγ→ e+ X and eγ→ ν+ X. While at low energies only virtual photon exchange is operative in the neutral-current process, additional Z-exchange becomes relevant for high Q 2 of order M Z 2 at e + e − linear colliders. Likewise the charged-current process can be observed at these high energy colliders. By measuring the electroweak neutral- and charged-current structure functions, the up- and down-type quark content of the photon can be determined separately.

Full length article

The coherence retention of the resonance dynamic dipole–dipole interaction (RDDDI) in an ensemble of two level systems (quantum dots) is discussed, in terms of parity inheritance and exchange of virtual photons, together with the energetic stability. This coherence is expected to be useful for quantum computing. Furthermore, inter-ensemble (inter-block) RDDDI is considered, and a qubit swap ( Π4) gate and quantum permutation circuits that can transfer a qubit state to any position in the quantum circuit are proposed.

The problem of two electrons in an external field and the method of integral equations in optics

This paper solves the problem of the interaction, via the field of virtual photon field with the emission or absorption of a real photon, of two atomic electrons located at arbitrary distances from one another. The interaction is interpreted as a third-order QED effect in the coordinate representation. The role of intermediate states with positive and negative frequencies is studied. A general expression is derived for the matrix elements of the operator of the effective electron-electron interaction energy for different types of quantum transitions. The expression makes it possible to calculate the probabilities of the corresponding transitions and to examine various patterns of induction of polarizing fields by one atom at the point occupied by the other atom. The exchange of virtual photons between the atoms located at arbitrary distances from one another is shown to lead to additional terms in the operators of spin-orbit and spin-spin coupling of the atomic electrons, over and above those in the corresponding Breit operators. It is shown that there is an important difference between the induction of polarizing fields and the transfer of optical photons. In particular, it is found that when polarizing fields are induced, a situation may arise in which the disappearance (production) of a photon takes place at the point occupied by one atom, while absorption (emission) of the same photon occurs at the place occupied by the other atom. A block diagram of an experimental device that could be used to study this property of polarizing fields is presented. Finally, a method of deriving integral field equations is proposed. The method is based on allowing for polarizing fields, and its effectiveness is demonstrated by the example of electric dipole and spin transitions in the spectrum of interacting atomic electrons.

NMR signal reception: Virtual photons and coherent spontaneous emission

In portions of the magnetic resonance community, there is a misunderstanding of the process of nuclear magnetic resonance (NMR) signal generation and reception, and even in accepted texts, it is frequently described in terms of absorption and emission of radio waves, or radiation, by a two-level quantum system. While this explanation can be refuted, for those who do understand that the NMR free induction decay signal is easily explained by Faraday's law of induction, reconciling the presence of an induced electromotive force with an apparent absence of transitions between nuclear energy levels causes conceptual problems. This difficulty is examined, and an explanation of the signal given whereby Faraday's law is explained simply in terms of an exchange of virtual photons. The article thus attempts to reconcile the standard engineering approach to signal reception with a quantum mechanical description of the NMR phenomenon. Radiation damping and its relations to detection of the induced signal and to signal-to-noise ratio are then scrutinized, and the misleading nature of the appellation noted. In the process, it is shown that while damping is inherently necessary for signal detection and the transfer of energy that it entails, the degree of such damping depends on the efficiency of the detector—in other words, of the preamplifier in the NMR receiver—and can therefore be minimized. Mathematics in the article is kept to a minimum; proofs of the Principle of Reciprocity description of Faraday's law for reception of both signal and noise from a conducting sample are given in an appendix. ©1997 John Wiley & Sons, Inc. Concepts Magn Reson 9: 277–297, 1997

Angular correlations of electron-positron pairs in internal electric monopole conversion of aligned heavy nuclei

We consider the influence of the nuclear alignment which, e.g., may be caused by Coulomb excitation on the spatial distribution of the electron-positron pairs resulting from electric monopole conversion processes. E0 conversion is a pure penetration effect, i.e., there is no virtual photon exchange. Approximating the nucleus by a homogeneously charged sphere we find that the angular distribution of electrons and positrons is independent on the alignment axis of the excited nucleus and is determined solely by the opening angle of the emitted lepton pairs.

Generalized dicke model:δm=±2 transitions

The theory of two-atom quantum transitions with frequency doubling of the emitted or absorbed photons in coherent optical processes is constructed. Third-order quantum electrodynamics (QED) effects, including virtual photon exchange and emission (absorption) of a double frequency real photon, are considered. In these effects, the two parts of the interaction accounting for intermediate states with positive or negative frequencies in the atomic spectrum are separated. Correspondingly, the electron and positron polarizations of the medium leading to the appearance of two mechanisms of frequency doubling with different physical properties are distinguished. In this paper, a generalized Dicke model for a system of N two-level atoms is used to describe coherent processes in the electric dipole approximation, allowing quantum transitions with ΔM=±1, ±2, where M is the half-difference of the resonance level populations in the atoms. In the framework of this model, a formula for the intensity of quantum transitions with ΔM=±2 is obtained.

Evaluation of beta-decay Part V. The Z-independent outer radiative corrections for allowed decay

Before interpreting nuclear beta-decay in terms of nucleonic wave functions and weak coupling constants allowance must be made for the influence of virtual photon exchange and real photon emission together with Z-boson exchange—the radiative corrections. This paper discusses in detail the lowest order ‘outer’ radiative correction of Sirlin, energy-dependent but weak-interaction-mechanism-independent, and presents graphical and other material to enable its accurate evaluation for allowed decay for all beta-decay transition energies up to 20 MeV and for 0 ≤ | Z| ≤ 100. The need to use electron spectra due to a nuclear charge distribution of finite spatial extension with appropriate convolution of the leptonic and nucleonic wave functions throughout the nuclear volume is emphasized; the variation of these effects between the neutron and proton drip lines is explored. Exponentiation and higher-order Z-independent effects are also considered.

Two- and three-body dispersion forces with one excited molecule

Abstract The interaction between molecules and the Maxwell electromagnetic field is used to determine two- and three-body dispersion forces, taking into account retardation effects fully. The case in which one of the molecules is in an electronically excited state is contrasted with that in which all molecules are in their ground states. The very long-range, retarded, potentials involving excited molecules are dominated by the contributions arising from real photon exchange. For pair interaction energies the lead term behaves as R −2 , for the three-body energies a modulated potential falling off as 1 abc is predicted ( a , b , c are the distances between pairs of molecules). However for small separations the lead terms arise from both real and virtual photon exchange. The resulting potentials are identical with those obtained from elementary perturbation theory with electrostatic couplings. Two approaches have been used to obtain these potentials. The first is based on a form of response theory involving the use of induced dipole moments and zero-point energy differences. In the second, the energy shifts are calculated directly using fourth-order perturbation theory taking into account the contribution of the poles which arise for systems with excited molecules.

Inelastic fast-electron-hydrogen-atom collision in a laser field.

The laser-assisted electron-impact excitation of 2s and 2p dressed states of hydrogen is studied. The interaction of a linearly polarized laser field with the colliding system is treated by the nonperturbative Floquet theory, while the interaction of the fast incident electron with the target atom is treated in the first Born approximation. The nonresonant collision in a low-intensity, low-frequency laser field is dominated by the process with no exchanged photon with the field, while at larger intensities the collision accompanied by the exchange of real or virtual photons is much more important. For the stimulated bremsstrahlung, the 1s\ensuremath{\rightarrow}2s transition is strongly influenced by the resonant coupling between the final 2s and the intermediate 3p states. In the resonant cases, where laser frequency matches a transition frequency between the initial or final and an intermediate state, the collisions with the largest cross section are those in which the projectile induces a dipole transition in the atom.

Dispersion forces between molecules with one or both molecules excited.

Dispersion energies between molecules involving one in an electronically excited state are calculated using fourth-order perturbation theory within the framework of the multipolar form of quantum electrodynamics. There are significant differences between the energies for these cases and those where both molecules are in their ground states. The calculations are performed within the electric-dipole approximation for the interaction of the molecules with the electromagnetic field. The energies found are valid for all separations beyond the electronic overlap region. The results of previous investigations are shown to be incomplete and the origin of the incompleteness is traced to the neglect of certain real-photon contributions. The energies obtained in this paper are in agreement with our earlier calculations based on a form of response theory. They are made up of two types: one resulting from virtual-photon exchange and the other from real photons. The virtual-photon term has the same structure as the Casimir-Polder potential for ground-state molecules. The real-photon term is a polynomial in the inverse intermolecular separation ${\mathit{R}}^{\mathrm{\ensuremath{-}}1}$, in contrast to the modulated contributions in the previous incomplete investigations. The asymptotic forms for the total energy are discussed. The far-zone behavior is dominated by the real-photon term and shows an ${\mathit{R}}^{\mathrm{\ensuremath{-}}2}$ dependence. The near-zone behavior shows an ${\mathit{R}}^{\mathrm{\ensuremath{-}}6}$ dependence, arising in the multipolar formalism from both virtual- and real-photon exchange.