Non-linear pair production in scattering of photons on ultra-short laser pulses at high energy

We consider scattering of a photon on a short intense laser pulse at high energy. We argue that for ultra-short laser pulses the interaction is coherent over the entire length of the pulse. At low pulse intensity I the total cross section for electron–positron pair production is proportional to I  . However, at pulse intensities higher than the characteristic value Is$I_s$, the total cross section saturates – it becomes proportional to the logarithm of intensity. This non-linear effect is due to multi-photon interactions. We derive the total cross section for pair production at high energies by resuming the multi-photon amplitudes to all orders in intensity. We calculate the saturation intensity Is$I_s$ and show that it is significantly lower than the Schwinger's critical value. We discuss possible experimental tests.     

Electron–positron pair production from vacuum in the field of high-intensity laser radiation

The works dealing with the theory of e⁺e^– pair production from vacuum under the action of highintensity laser radiation are reviewed. The following problems are discussed: pair production in a constant electric field E and time-variable homogeneous field E(t); the dependence of the number of produced pairs \({N_{{e^ + }{e^ - }}}\) on the shape of a laser pulse (dynamic Schwinger effect); and a realistic three-dimensional model of a focused laser pulse, which is based on exact solution of Maxwell’s equations and contains parameters such as focal spot radius R, diffraction length L, focusing parameter Δ, pulse duration τ, and pulse shape. This model is used to calculate \({N_{{e^ + }{e^ - }}}\) for both a single laser pulse (n = 1) and several (n ≥ 2) coherent pulses with a fixed total energy that simultaneously “collide” in a laser focus. It is shown that, at n ? 1, the number of pairs increases by several orders of magnitude as compared to the case of a single pulse. The screening of a laser field by the vapors that are generated in vacuum, its “depletion,” and the limiting fields to be achieved in laser experiments are considered. The relation between pair production, the problem of a quantum frequency-variable oscillator, and the theory of groups SU(1, 1) and SU(2) is discussed. The relativistic version of the imaginary time method is used in calculations. In terms of this version, a relativistic theory of tunneling is developed and the Keldysh theory is generalized to the case of ionization of relativistic bound systems, namely, atoms and ions. The ionization rate of a hydrogen-like ion with a charge 1 ≤ Z ≤ 92 is calculated as a function of laser radiation intensity (F and ellipticity ρ.     

Quantum Theory of Self‐Phase Modulation of Ultrashort Light Pulses in a Medium with Inertial Electronic Nonlinearity

We develop a systematic quantum theory of formation of ultrashort light pulses in a squeezed state at selfphase modulation. The response time of the electronic Kerr nonlinearity of the medium is accounted for and the dispersion of linear properties of the medium is described in first approximation. The theory uses the approach based on the momentum operator for the electric field. The response function of the nonlinearity is contained in the interaction operator. The results obtained are valid when the pulse duration is far greater than the nonlinear response time and the carrier pulse frequency is offresonance. It is established that the instantaneous spectral distribution depends quasistatically on the pulse form. In the general case, the spectral distribution of the quadraturesqueezed component is defined by both the response time of the nonlinearity and the magnitude of the nonlinear phase addition. It is also shown that the frequency at which suppression of quantum fluctuations is greatest can be controlled by adjusting the phase of the initial coherent light pulse. It is found that, by registering the quadraturesqueezed pulse using a balanced homodyne detector, the squeezing effect can be efficiently measured if the timedependent phase of the heterodyne pulse obeys a specific dependence defined by the pulse form. The spectral photonnumber distribution of the quadraturesqueezed pulse is studied, and the photon antibunching effect is found for the photon number in a limited spectral band.     

Quantum effects with an x-ray free-electron laser

A quantum kinetic equation coupled with Maxwell's equation is used to estimate the laser power required at an x-ray free-electron laser (XFEL) facility to expose intrinsically quantum effects in the process of QED vacuum decay via spontaneous pair production. A 9 -TW-peak XFEL laser with photon energy of 8.3 keV could be sufficient to initiate particle accumulation and the consequent formation of a plasma of spontaneously produced pairs. The evolution of the particle number in the plasma will exhibit non-Markovian aspects of the strong-field pair production process, and the plasma's internal currents will generate an electric field whose interference with that of the laser leads to plasma oscillations.     

Entangled trajectories during ionization of an H atom driven by n-cycle laser pulse

We study the ionization of an H atom in a linearly polarized laser field with different optical cycle numbers by calculating the entangled trajectories in phase space. The results indicate that, for a two-cycle laser pulse, the entangled trajectory is simple owing to the simple laser electric distribution. As the number of optical cycles increases, the complexity of the laser electric field distribution, and subsequently, that of the entangled trajectories increase. From these entangled trajectories, re-scattering ionization can be observed. Further, we investigate the effect of quantum force on trajectories by comparing them with classical trajectories. We find that for few-cycle laser pulses ($n_p=2$ and 4), the effect of the quantum force is more distinct than classical behavior, whereas for longer laser pulses ($n_p=6$ and 8), it is quite similar to the classical behavior. Because of the quantum force, the final kinetic energy is slightly higher than that obtained by classical calculation. The different initial positions have some influence on the individual trajectories, but the individual trajectories still keep similar configuration to the mean trajectories.     

Photon and axion splitting in an inhomogeneous magnetic field

The axion–photon system in an external magnetic field, when the direction of propagation of axions and photons is orthogonal to the direction of the external magnetic field, displays a continuous axion–photon duality symmetry in the limit the axion mass is neglected. The conservation law that follow in this effective (2+1)$(2+1)$-dimensional theory from this symmetry is obtained. The magnetic field interaction is seen to be equivalent to first order to the interaction of a complex charged field with an external electric potential, where this fictitious “electric potential” is proportional to the external magnetic field. This allows one to solve for the scattering amplitudes using already known scalar QED results. From the scalar QED analog the axion and the photon are symmetric and antisymmetric combinations of particle and antiparticle. If one considers therefore scattering experiments in which the two spatial dimensions of the effective theory are involved nontrivially, one observes that both particle and antiparticle components of photons and axions are preferentially scattered in different directions, thus producing the splitting or decomposition of the photon and axion into their particle and antiparticle components in an inhomogeneous magnetic field. This observable in principle effect is of first order in the axion–photon coupling, unlike the “light shining through a wall phenomena”, which is second order.     

Experimental and theoretical radiative properties of odd-parity highly excited levels in Mo II

Investigations are made for the collective spontaneous emission of a Bose-Einstein condensate consisting of N two-level atoms when s atoms are initially excited in a multi-photon q-deformed Dicke model. The model is based on the generalized deformed oscillator algebra in which the field radiation operators are deformed by an operator-valued function $f\left( {\hat n} \right)$ of the photon number operator $\hat n$ . The time evolution of the expectation value of the atomic inversion is calculated for s = 1, s = 2 and s = 3. When s = 3 its spectra are characterized by nonequidistant eigenvalues, and the phenomenon of the quantum collapse and revival is demonstrated. In particular, the influences of photon multiplicity and q-deformation on the spontaneous emission of the system are discussed. The results show that the nonlinearities due to the photon multiplicity and q-deformation may lead to the inhibition of collective spontaneous emission.     

On <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> pair production by colliding electromagnetic pulses

Electron-positron pair production from vacuum in an electromagnetic field created by two counterpropagating focused laser pulses interacting with each other is analyzed. The dependence of the number of produced pairs on the intensity of a laser pulse and the focusing parameter is studied with a realistic three-dimensional model of the electromagnetic field of the focused wave, which is an exact solution of the Maxwell equations. It has been shown that e⁺e^? pair production can be experimentally observed when the intensity of each beam is I～10²⁶ W/cm², which is two orders of magnitude lower than that for a single pulse.     

多模简并多光子Tavis-Cummings模型中的量子纠缠特性

利用冯·纽曼约化熵理论研究了多模相干态光场与两等同二能级原子简并多光子相互作用系统量子纠缠演化特性,得到了多模光场量子纠缠的解析表达式,并给出了双模光场与两原子相互作用时量子纠缠的数值计算结果.结果表明:量子纠缠随着光子简并度的增大而增强;随着初始平均光子数的增加,量子纠缠演化的周期性变得越来越明显;当场与原子远离共振时,量子纠缠随着频率失谐量的增大而减弱;当失谐量足够大时,场与原子几乎总是处于纠缠状态.这些结论对于纠缠态或纯态的制备及获取光学系统中的量子信息研究中有一定参考价值.     

Polarization operator of a photon in a magnetic field

The polarization operator of a photon in a static uniform magnetic field has been studied at photon energies both above and below the threshold of electron–positron pair production by a photon. In the first order of the fine-structure constant α, expressions for the refractive index of a photon with a certain polarization in both low and high fields as compared to the critical field H₀ = 4.41 × 10¹³ G have been obtained. Both the purely quantum range of photon energies, where the particles of a pair are produced at the lowest Landau levels, and the region of applicability of the semiclassical approximation in the case of the population of high energy levels have been considered. A general spectral integral formula has been obtained with divergent threshold terms separated in an explicit form.     

Adjusting the properties of the photon generated via an optical parametric oscillator by using a pulse pumped laser

The cavity-enhanced spontaneous parametric down-conversion far below threshold can be used to generate a narrow-band photon pair efficiently. Previous experiments on the cavity-enhanced spontaneous parametric down-conversion almost always utilize continuous wave pump light, but the pulse pumped case is rarely reported. One disadvantage of the continuous wave case is that the photon pair is produced randomly within the coherence time of the pump, which limits its application in the quantum information realm. However, a pulse pump can help to solve this problem. In this paper, we theoretically analyze pulse pumped cavity-enhanced spontaneous parametric down-conversion in detail and show how the pump pulse affects the multi-photon interference visibility, two-photon waveform, joint spectrum and spectral brightness.     

Study of the photon’s pole structure in the noncommutative Schwinger model

The photon magnetic moment for radiation propagating in magnetized vacuum is defined as a pseudotensor quantity, proportional to the external electromagnetic field tensor. After expanding the eigenvalues of the polarization operator in powers of \(k^2\) , we obtain approximate dispersion equations (cubic in \(k^2\) ), and analytic solutions for the photon magnetic moment, valid for low momentum and/or large magnetic field. The paramagnetic photon experiences a redshift, with opposite sign to the gravitational one, which differs for parallel and perpendicular polarizations. It is due to the drain of photon transverse momentum and energy by the external field. By defining an effective transverse momentum, the constancy of the speed of light orthogonal to the field is guaranteed. We conclude that the propagation of the photon non-parallel to the magnetic direction behaves as if there is a quantum compression of the vacuum or a warp of space-time in an amount depending on its angle with regard to the field.     

强场下真空中粒子对产生的研究进展

随着激光技术的飞快发展,激光强度不断提高,超强外场下真空中正负电子对产生的过程,即能量向质量转化过程,已经成为一个研究热点。主要综述了近几年量子Vlasov方程方法和计算量子场论(数值求解Dirac方程)方法在研究强场下真空中正负电子对产生方面的进展,分别介绍了空间均匀场和空间不均匀场下的粒子对产生的情况。第一种情况主要介绍双脉冲结构振荡电场中电子-正电子对的产生、强双频振荡电场中非微扰电子-正电子对的产生、频率调制的激光场中电子-正电子对的产生和Dirac真空对啁啾外场的快速分辨。第二种情况主要介绍优化空间局域电场提高粒子对的产生率、多个势阱-垒结构的振荡场对粒子对产生的增强、振荡 Sauter 电势中正负电子对产生的问题、操纵Dirac真空以控制其在场诱导下的衰变、作为信息传输介质的Dirac真空还有正负电子对产生中的相干和非相干啁啾机制的转变。     

Electron-Positron Pair Production in a Strong Laser Field Enhanced by an Assisted High Frequency Weak Field

Electron-positron pair production in a strong laser field enhanced by an assisted high frequency weak field is investigated by solving the quantum Vlasov equation.The average and residual pair number densities are obtained for sinusoid electric field and it is found that the high frequency assisted weak field will enhance pair production significantly.There exists an optimal frequency of assisted field that makes the pair production number density get a maximum one,which is a few orders of higher than that without assisted field.We also discuss the other possible assisted fields.     

1.55 μm InGaAs-InP Quantum Wire Optical Modulators: Optimization of Wire Width to Maximize Absorption and Index of Refraction Changes Due to Excitonic Transitions

Quantum wire/dot modulators offer superior performance over their quantum counterpart due to enhanced excitonic binding energy. This paper presents simulations on InGaAs-InP quantum wire Stark effect optical modulators showing a novel trend. While the excitonic binding energies and absorption coefficients increase as the width of the wire is decreased, the refractive index change n is maximized at a wire width depending on the magnitude of the applied electric field. For example, n is maximized at a width of about 100Å for an external electric field of 120kV/cm in an InGaAs quantum wire. This behavior is explained by considering the opposing effects of the wire width on binding energy and changes in the electron-hole overlap function in the presence of an external electric field. Practical InGaAs-InP modulators using V-groove structures are also presented.     

Photoproduction of Triplets on Free Electrons and the Search for the Dark Photon

We have investigated the photoproduction of triplets on free electrons, γe^– → e⁺e^–e^–, in which dark photon A′ can be formed as an intermediate state with subsequent decay into an e⁺e^– pair. This effect appears as a result of the so-called kinetic mixing and is characterized by the small parameter describing the intensity of the interaction of the dark photon with charged Standard Model (SM) particles in terms of electric charge e. The search for a manifestation of A′ in this process is advantageous since the background to the A′ signal is purely electrodynamic and, hence, can be calculated with the required accuracy. We calculate this background taking into account the identity of final electrons. As regards A′, its contribution is taken into account only in Compton-type diagrams (of virtual Compton scattering) in which a virtual dark photon is a time-like particle and its propagator has the form of a Breit–Wigner resonance. It is only in the vicinity of resonance that A′ can be manifested. We calculate the invariant mass distribution of both e⁺e^– pairs formed in the process and analyze the kinematic region in which relatively small squares of momenta transferred from the target electron to the formed electrons are excluded. In these conditions, the contribution to the differential cross section from Compton-type diagrams is not suppressed relative to the contribution of the remaining (Borsellino) diagrams. A number of limitations on parameter depending on the dark photon mass and the statistics (number) of events are obtained for a special method of gathering events in which the invariant mass of one e⁺e^– pair remains fixed and of the other pair is scanned.     

Combined effects of electric and magnetic fields on confined donor states in a diluted magnetic semiconductor parabolic quantum dot

A variational formalism for the calculation of the binding energies of hydrogenic donors in a parabolic diluted magnetic semiconductor quantum dot is discussed. Results are obtained for Cd Mn Te/Cd Mn Te structures as a function of the dot radius in the presence of external magnetic and electric fields applied along the growth axis. The donor binding energies are computed for different field strengths and for different dot radii. While the variation of impurity binding energy with dot radii and electric field are as expected, the polarizability values enhance in a magnetic field. However, for certain values of dot radii and in intense magnetic fields the polarizability variation is anomalous. This variation of polarizability is different from non- magnetic quantum well structures. Spin polaronic shifts are estimated using a mean field theory. The results show that the spin polaronic shift increases with magnetic field and decreases as the electric field and dot radius increase.