激光脉冲和电子束对撞准确度的测量方法

超强激光脉冲与相对论电子束相互对撞是当前主要的强场量子电动力学（QED）实验手段。如何测量超强激光脉冲和电子束对撞的准确度,进而实现微米精度的准确对撞,是目前限制实验发展的重要因素。利用蒙特卡罗数值模拟方法,系统研究了超强激光脉冲和相对论电子束相互对撞过程,重点关注了电子和辐射光子动力学信息与激光脉冲和电子束对撞偏移量之间的对应关系。研究发现:辐射光子的空间分布信息,可以有效反映出激光脉冲和电子束的对撞偏移量。基于该研究结果,实验中可利用光子空间分布的信息,实现对激光脉冲和电子束对撞准确度的调节,从而有望促进强场QED实验技术的发展。     

Lightfront DHW Functions and Strong Field QED

We investigate strong-field effects in QED using a phase space approach based on the Dirac–Heisenberg–Wiger function. We calculate the electron–positron DHW function in a null (laser) field background exactly, using lightfront field theory. The DHW function exhibits the distinct features of strong-field QED: in particular we explicitly identify the effective fermion mass for arbitrary temporal pulse profiles.     

The stimulated Breit-Wheeler process as a source of background <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> pairs at the international linear collider

Passage of beamstrahlung photons through the bunch fields at the interaction point of the ILC determines background pair production. The number of background pairs per bunch crossing due to the Breit-Wheeler, Bethe-Heitler and Landau-Lifshitz processes is well-known. However, the Breit-Wheeler process also takes place in and is modified by the bunch fields. A full QED calculation of this stimulated Breit-Wheeler process reveals cross-section resonances due to the virtual particle reaching the mass shell. The one-loop electron self-energy in the bunch field is also calculated and included as a radiative correction. The bunch field is considered to be a constant crossed electromagnetic field with associated bunch field photons. Resonance is found to occur whenever the energy of contributed bunch field photons is equal to the beamstrahlung photon energy. The stimulated Breit-Wheeler cross-section exceeds the ordinary Breit-Wheeler cross-section by several orders of magnitude and a significantly different pair background may result.      

Strong laser fields as a probe for fundamental physics

Upcoming high-intensity laser systems will be able to probe the quantum-induced nonlinear regime of electrodynamics. So far unobserved QED phenomena such as the discovery of a nonlinear response of the quantum vacuum to macroscopic electromagnetic fields can become accessible. In addition, such laser systems provide for a flexible tool for investigating fundamental physics. Primary goals consist in verifying so far unobserved QED phenomena. Moreover, strong-field experiments can search for new light but weakly interacting degrees of freedom and are thus complementary to accelerator-driven experiments. I review recent developments in this field, focusing on photon experiments in strong electromagnetic fields. The interaction of particle-physics candidates with photons and external fields can be parameterized by low-energy effective actions and typically predict characteristic optical signatures. I perform first estimates of the accessible new-physics parameter space of high-intensity laser facilities such as POLARIS and ELI.     

激光参数对非理想真空激光光强极限的影响

人类在实验室可实现的激光强度极限是强场量子电动力学（QED）的重要问题。在非理想真空条件下,极端超强激光与残留的电子相互作用触发伽马光子辐射与正负电子对产生的QED级联效应,从而显著消耗激光能量,大幅降低可实现的激光峰值强度。考虑到QED级联效应与激光偏振、焦斑尺寸、脉宽长度有着密切的关系,基于囊括QED过程的粒子网格模拟方法（Particle-in-cell, PIC）对上述参数的效应进行分析,同时构建了激光场演化的自洽方程来进行解释,二者结果基本保持一致,获得的强度极限在考虑的参数范围内为1026～1027 W/cm?2。结果表明,同等情形下,圆偏振激光可激发更强的QED级联,使得激光强度上限略低于线偏振。此外,紧聚焦激光由于QED级联发生的时空间尺度更小,从而激光的吸收效应被显著抑制,进而可以实现更强的聚焦强度。对于更长脉宽的激光,由于正负电子对吸收的能量区域更加弥散,使得可实现的激光强度上限阈值有所提升。但对于超短脉宽情形（如单周期）,由于QED级联的种子源电子束不能很好地被约束在激光区域,理论分析耗散的激光能量偏高。此外,在高真空度的情形下,残余电子的随机性也会对可实现激光强度产生一定的影响。研究结果可为后续开展极端强场QED实验和数100 PW级超强超短激光装置建设提供指导。     

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.     

Trident pair production in strong laser pulses

We calculate the trident pair production amplitude in a strong laser background. We allow for finite pulse durations, while still treating the laser fields nonperturbatively in strong-field QED. Our approach reveals explicitly the individual contributions of the one-step and two-step processes. We also expose the role gauge invariance plays in the amplitudes and discuss the relation between our results and the optical theorem.     

超强激光与固体靶作用驱动量子电动力学级联和稠密正电子产生的研究进展

极端超短超强激光脉冲的诞生将光与物质的相互作用推进到由辐射阻尼效应和量子电动力学（QED）效应占主导的高度非线性物理范畴。强场QED效应蕴含了丰富的物理过程包括辐射阻尼、高能伽马辐射、正负电子对产生、QED级联、真空极化等,是高能量密度物理和强场物理研究领域的前沿热点。QED级联是解释致密天体辐射和伽马射线暴形成的重要物理机制,其产生的稠密正电子源在高能物理、材料无损探测、癌症诊断等领域亦有重要的应用前景。介绍了QED级联过程及其理论模型,讨论了固体靶中的QED级联发展及其诱导的非线性物理效应,并回顾了固体靶中稠密正电子产生的主要研究成果。     

Finite size effects in stimulated laser pair production

We consider stimulated pair production employing strong-field QED in a high-intensity laser background. In an infinite plane wave, we show that light-cone quasi-momentum can only be transferred to the created pair as a multiple of the laser frequency, i.e. by a higher harmonic. This translates into discrete resonance conditions providing the support of the pair creation probability which becomes a delta-comb. These findings corroborate the usual interpretation of multi-photon production of pairs with an effective mass. In a pulse, the momentum transfer is continuous, leading to broadening of the resonances and sub-threshold behaviour. The peaks remain visible as long as the number of cycles per pulse exceeds unity. The resonance patterns in pulses are analogous to those of a diffraction process based on interference of the produced pairs. We finally comment on the dependence of the peak positions, and in turn the effective mass, on the pulse shape.     

Investigating the QED vacuum with ultra-intense laser fields

In view of the increasingly stronger available laser fields it is becoming feasible to employ them to probe the nonlinear dielectric properties of the vacuum as predicted by quantum electrodynamics (QED) and to test QED in the presence of intense laser beams. First, we discuss vacuum-polarization effects that arise in the collision of a high-energy proton beam with a strong laser field. In addition, we investigate the process of light-by-light diffraction mediated by the virtual electron-positrons of the vacuum. A strong laser beam “diffracts” a probe laser field due to vacuum polarization effects, and changes its polarization. This change of the polarization is shown to be in principle measurable. Also, the possibility of generating harmonics by exploiting vacuum-polarization effects in the collision in vacuum of two ultra-strong laser beams is discussed. Moreover, when two strong parallel laser beams collide with a probe electromagnetic field, each photon of the probe may interact through the “polarized” quantum vacuum with the photons of the other two fields. Analogously to “ordinary” double-slit set-ups involving matter, the vacuum-scattered probe photons produce a diffraction pattern, which is the envisaged observable to measure the quantum interaction between the probe and strong field photons. We have shown that the diffraction pattern becomes visible in a few operating hours, if the strong fields have an intensity exceeding 10²⁴W/cm².     

Consistency restrictions on maximal electric-field strength in quantum field theory

Quantum field theory with an external background can be considered as a consistent model only if backreaction is relatively small with respect to the background. To find the corresponding consistency restrictions on an external electric field and its duration in QED and QCD, we analyze the mean-energy density of quantized fields for an arbitrary constant electric field E, acting during a large but finite time T. Using the corresponding asymptotics with respect to the dimensionless parameter eET2, one can see that the leading contributions to the energy are due to the creation of particles by the electric field. Assuming that these contributions are small in comparison with the energy density of the electric background, we establish the above-mentioned restrictions, which determine, in fact, the time scales from above of depletion of an electric field due to the backreaction.     

Phase space evolution of pairs created in strong electric fields

We study the process of energy conversion from overcritical electric field into electron–positron–photon plasma. We solve numerically Vlasov–Boltzmann equations for pairs and photons assuming the system to be homogeneous and anisotropic. All the 2-particle QED interactions between pairs and photons are described by collision terms. We evidence several epochs of this energy conversion, each of them associated to a specific physical process. Firstly pair creation occurs, secondly back reaction results in plasma oscillations. Thirdly photons are produced by electron–positron annihilation. Finally particle interactions lead to completely equilibrated thermal electron–positron–photon plasma.     

超强激光驱动的辐射反作用力效应与极化粒子加速

光强超过10~(22) W/cm~2的极端超强激光将光与物质的相互作用推进到辐射主导区域,激发高能伽马光子辐射,产生明显的辐射反作用力效应.辐射反作用力可以显著影响强场中带电粒子的动力学行为,并从根本上改变了极端强场区域的激光等离子体相互作用规律.如何理解和验证辐射反作用力效应是强场物理研究的核心内容之一.本文结合该方向的国内外研究进展,论述了辐射反作用力的经典形式与强场量子电动力学的理论计算与模拟方法,详细讨论了单粒子在强场中的反射、量子随机辐射、自旋-辐射耦合等效应,介绍了激光等离子体相互作用中的电子冷却、辐射俘获、高效伽马辐射等机制,并给出了目前辐射反作用力效应的实验验证方法与进展.针对自旋在强场量子电动力学方面的效应,介绍了激光加速产生极化粒子源的方法.     

Signatures of the Unruh effect via high-power, short-pulse lasers

The ultra-high fields of high-power short-pulse lasers are expected to contribute to understanding fundamental properties of the quantum vacuum and quantum theory in very strong fields. For example, the neutral QED vacuum breaks down at the Schwinger field strength of 1.3×10¹⁸ V/m, where a virtual e⁺e^- pair gains its rest mass energy over a Compton wavelength and materializes as a real pair. At such an ultra-high field strength, an electron experiences an acceleration of a_S=2×10²⁸g and hence fundamental phenomena such as the long predicted Unruh effect start to play a role. The Unruh effect implies that the accelerated electron experiences the vacuum as a thermal bath with the Unruh temperature. In its accelerated frame the electron scatters photons off the thermal bath, corresponding to the emission of an entangled pair of photons in the laboratory frame. While it remains an experimental challenge to reach the critical Schwinger field strength within the immediate future even in view of the enormous thrust in high-power laser developments in recent years, the near-future laser technology may allow to probe the signatures of the Unruh effect mentioned above. Using a laser-accelerated electron beam (γ～300) and a counter-propagating laser beam acting as optical undulator should allow to create entangled Unruh photon pairs (i.e., signatures of the Unruh effect) with energies of the order of several hundred keV. An even substantially improved experimental scenario can be realized by using a brilliant 20 keV photon beam as X-ray undulator together with a low-energy (γ≈2) electron beam. In this case the separation of the Unruh photon pairs from background originating from linearly accelerated electrons (classical Larmor radiation) is significantly improved. Detection of the Unruh photons may be envisaged by Compton polarimetry in a 2D-segmented position-sensitive germanium detector.     

Kinetic theory of radiation in non-equilibrium relativistic plasmas

Many-particle QED is applied to kinetic theory of radiative processes in many-component plasmas with relativistic electrons and non-relativistic heavy particles. Within the framework of non-equilibrium Green’s function technique, transport and mass-shell equations for fluctuations of the electromagnetic field are obtained. We show that the transverse field correlation functions can be decomposed into sharply peaked (non-Lorentzian) parts that describe resonant (propagating) photons and off-shell parts corresponding to virtual photons in plasmas. Analogous decompositions are found for the longitudinal field correlation functions and the correlation functions of relativistic electrons. As a novel result a kinetic equation for the resonant photons with a finite spectral width is derived. The off-shell parts of the particle and field correlation functions are shown to be essential to calculate the local radiating power in relativistic plasmas and recover the results of vacuum QED. The influence of plasma effects and collisional broadening of the relativistic quasiparticle spectral function on radiative processes is discussed.     

量子电动力学—粒子模拟极端等离子体动力学研究

新一代拍瓦激光装置有望将激光强度提升至10²³~10²⁴ W·cm^-2,在此极端强场条件下非线性量子电动力学效应对等离子体动力学过程产生重要影响.相对论电子在强电磁场作用下会同步辐射大量伽马光子,当后者穿过超强电磁场时会级联产生正负电子对.与此同时,这些量子电动力学效应也会反作用于激光等离子体相互作用过程,如辐射阻尼严重影响电子运动过程.为了研究这样极端的等离子体动力学,我们介绍最近几年发展的量子电动力学数值模拟模块,并将其耦合到传统的粒子模拟程序中,即量子电动力学-粒子模拟程序.由于大量新辐射的光子和产生的正负电子对会造成模拟粒子数目的不断增加,我们发展了粒子融合技术来减小模拟规模.利用此量子电动力学-粒子模拟程序,我们对极端强场激光物质相互作用以及极端天体物理现象开展了数值模拟研究.     

飞秒超强激光驱动太赫兹辐射特性的实验研究

强太赫兹源是太赫兹科学技术发展的关键,其中大能量强场太赫兹脉冲源在超快物态调控、新型电子加速器等领域具有重要的应用前景.超快超强激光与等离子体相互作用是近年来发展起来的一种新型的强场太赫兹辐射产生途径.本文报道了利用超强飞秒激光脉冲与金属薄膜靶作用产生太赫兹辐射的实验结果,研究了激光能量和离焦量对靶后太赫兹辐射能量的影响,并通过监测激光背向散射光谱,定性揭示了其变化规律与不同光强下的电子加热机制的相关性.实验表征了太赫兹辐射的频谱、偏振及聚焦光斑情况.测量结果表明,实验产生了脉冲能量达458μJ、聚焦场强高达GV/m量级的超宽带太赫兹辐射,为开展极端太赫兹脉冲与物质相互作用研究提供了一种新的强场太赫兹光源.     

New method for calculating helicity amplitudes of jet-like QED processes for high-energy colliders

Inelastic QED processes, the cross sections of which do not drop with increasing energy, play an important role at high-energy colliders. Such reactions have the form of two-jet processes with the exchange of a virtual photon in the t-channel. We consider them in the region of small scattering angles , which yields the dominant contribution to their total cross sections. A new effective method is presented and applied to QED processes with emission of real photons to calculate the helicity amplitudes of these processes. Its basic idea is similar to the well-known equivalent-lepton method. Compact analytical expressions for those amplitudes up to are derived omitting only terms of the order of and higher order. The helicity amplitudes are presented in a compact form in which large compensating terms are already cancelled. Some common properties for all jet-like processes are found and we discuss their origin. Received: 19 December 2001 / Published online: 5 April 2002     

Quantum Statistical Behaviors of Interaction of an Atomic Bose-Einstein Condensate with Laser

We have investigated quantum statistical behaviors of photons and atoms in interaction of an atomic Bose Einstein condensate with quantized laser field. When the quantized laser field is initially prepared in a superposition state which exhibits holes in its photon-number distribution, while the atomic field is initially in a Fock state, it is found that there is energy exchange between photons and atoms. For the input and output states, the photons and atoms may exhibit the sub-Poissonian distribution. The input and output laser fields may exhibit quadrature squeezing, but for the atomic field, only the output state exhibits quadrature squeezing. It is shown that there exists the violation of the Cauchy-Schwartz inequality, which means that the correlation between photons and atoms is nonclassical.``     

Extreme quantum field theory and particle physics with IZEST

The prospect of next-generation ultra-high-intensity laser sources has prompted recent renewed study of nonlinear QED processes, such as the Schwinger effect, in which the instability of the QED vacuum is probed by external fields. Experimental observation of these nonlinear QED effects would provide unprecedented controlled access to non-perturbative processes in quantum field theory under extreme conditions, which is of direct interest in particle physics and astrophysical applications. I summarize important theoretical issues, both conceptual and computational, related to these nonlinear QED effects.