8,222 research outputs found
Ultrarelativistic electron states in a general background electromagnetic field
The feasibility of obtaining exact analytical results in the realm of QED in
the presence of a background electromagnetic field is almost exclusively
limited to a few tractable cases, where the Dirac equation in the corresponding
background field can be solved analytically. This circumstance has restricted,
in particular, the theoretical analysis of QED processes in intense laser
fields to within the plane-wave approximation even at those high intensities,
achievable experimentally only by tightly focusing the laser energy in space.
Here, within the Wentzel-Kramers-Brillouin (WKB) or eikonal approximation, we
construct analytically single-particle electron states in the presence of a
background electromagnetic field of general space-time structure in the
realistic assumption that the initial energy of the electron is the largest
dynamical energy scale in the problem. The relatively compact expression of
these states opens, in particular, the possibility of investigating
analytically strong-field QED processes in the presence of spatially focused
laser beams, which is of particular relevance in view of the upcoming
experimental campaigns in this field.Comment: 7 pages, 1 figur
Nonlinear Breit-Wheeler pair production in a tightly focused laser beam
The only available analytical framework for investigating QED processes in a
strong laser field systematically relies on approximating the latter as a plane
wave. However, realistic high-intensity laser beams feature much more complex
space-time structures than plane waves. Here, we show the feasibility of an
analytical framework for investigating strong-field QED processes in laser
beams of arbitrary space-time structure by determining the energy spectrum of
positrons produced via nonlinear Breit-Wheeler pair production as a function of
the background field in the realistic assumption that the energy of the
incoming photon is the largest dynamical energy in the problem. A numerical
evaluation of the angular resolved positron spectrum shows significant
quantitative differences with respect to the analogous result in a plane wave,
such that the present results will be also important for the design of upcoming
strong laser facilities aiming at measuring this process.Comment: 6 pages, 1 figur
First-order strong-field QED processes in a tightly focused laser beam
In [Phys. Rev. Lett. \textbf{117}, 213201 (2016)] we have determined the
angular resolved and the total energy spectrum of a positron produced via
nonlinear Breit-Wheeler pair production by a high-energy photon
counterpropagating with respect to a tightly focused laser beam. Here, we first
generalize the results in [Phys. Rev. Lett. \textbf{117}, 213201 (2016)] by
including the possibility that the incoming photon is not exactly
counterpropagating with respect to the laser field. As main focus of the
present paper, we determine the photon angular resolved and total energy
spectrum for the related process of nonlinear Compton scattering by an electron
impinging into a tightly-focused laser beam. Analytical integral expressions
are obtained under the realistic assumption that the energy of the incoming
electron is the largest dynamical energy of the problem and that the electron
is initially almost counterpropagating with respect to the laser field. The
crossing symmetry relation between the two processes in a tightly focused laser
beam is also elucidated.Comment: 24 pages, no figure
Quantum Limitation to the Coherent Emission of Accelerated Charges
Accelerated charges emit electromagnetic radiation. According to classical
electrodynamics if the charges move along sufficiently close trajectories they
emit coherently, i.e., their emitted energy scales quadratically with their
number rather than linearly. By investigating the emission by a two-electron
wave packet in the presence of an electromagnetic plane wave within
strong-field QED, we show that quantum effects deteriorate the coherence
predicted by classical electrodynamics even if the typical quantum nonlinearity
parameter of the system is much smaller than unity. We explain this result by
observing that coherence effects are also controlled by a new quantum parameter
which relates the recoil undergone by the electron with the width of its wave
packet in momentum space.Comment: 6 + 3 SM pages, 3 figure
Ultrarelativistic quasiclassical wave functions in strong laser and atomic fields
The problem of an ultrarelativistic charge in the presence of an atomic and a
plane-wave field is investigated in the quasiclassical regime by including
exactly the effects of both background fields. Starting from the quasiclassical
Green's function obtained in [Phys. Lett. B \textbf{717}, 224 (2012)], the
corresponding in- and out-wave functions are derived in the experimentally
relevant case of the particle initially counterpropagating with respect to the
plane wave. The knowledge of these electron wave functions opens the
possibility of investigating a variety of problems in strong-field QED, where
both the atomic field and the laser field are strong enough to be taken into
account exactly from the beginning in the calculations.Comment: 24 pages, no figure
Nonlinear Compton scattering in ultra-short laser pulses
A detailed analysis of the photon emission spectra of an electron scattered
by a laser pulse containing only very few cycles of the carrying
electromagnetic field is presented. The analysis is performed in the framework
of strong-field quantum electrodynamics, with the laser field taken into
account exactly in the calculations. We consider different emission regimes
depending on the laser intensity, placing special emphasis on the regime of
one-cycle beams and of high laser intensities, where the emission spectra
depend nonperturbatively on the laser intensity. In this regime we in
particular present an accurate stationary phase analysis of the integrals that
are shown to determine the computed emission spectra. The emission spectra show
significant differences with respect to those in a long pulsed or monochromatic
laser field: the emission lines obtained here are much broader and, more
important, no dressing of the electron mass is observed.Comment: 31 pages, 15 figure
Topological soliton-polaritons in 1D systems of light and fermionic matter
Quantum nonlinear optics is a quickly growing field with large technological
promise, at the same time involving complex and novel many-body phenomena. In
the usual scenario, optical nonlinearities originate from the interactions
between polaritons, which are hybrid quasi-particles mixing matter and light
degrees of freedom. Here we introduce a type of polariton which is
intrinsically nonlinear and emerges as the natural quasi-particle in presence
quantum degenerate fermionic matter. It is a composite object made of a fermion
trapped inside an optical soliton forming a topological defect in a
spontaneously formed crystalline structure. Each of these soliton-polaritons
carries a topological quantum number, as they create a domain
wall between two crystalline regions with opposite dimerization so that the
fermion is trapped in an interphase state. These composite objects are formally
equivalent to those appearing in the Su-Schrieffer-Heeger (SSH) model for
electrons coupled to lattice phonons.Comment: Edited version. 6+7 pages, 3 figure
Quantum electron self-interaction in a strong laser field
The quantum state of an electron in a strong laser field is altered if the
interaction of the electron with its own electromagnetic field is taken into
account. Starting from the Schwinger-Dirac equation, we determine the states of
an electron in a plane-wave field with inclusion, at leading order, of its
electromagnetic self-interaction. On the one hand, the electron states show a
pure "quantum" contribution to the electron quasi-momentum, conceptually
different from the conventional "classical" one arising from the quiver motion
of the electron. On the other hand, the electron self-interaction induces a
distinct dynamics of the electron spin, whose effects are shown to be
measurable in principle with available technology.Comment: 5 pages, 2 figure
Stochasticity effects in quantum radiation reaction
When an ultrarelativistic electron beam collides with a sufficiently intense
laser pulse, radiation-reaction effects can strongly alter the beam dynamics.
In the realm of classical electrodynamics, radiation reaction has a beneficial
effect on the electron beam as it tends to reduce its energy spread. Here, we
show that when quantum effects become important, radiation reaction induces the
opposite effect, i.e., the electron beam spreads out after interacting with the
laser pulse. We identify the physical origin of this opposite tendency in the
intrinsic stochasticity of photon emission, which becomes substantial in the
full quantum regime. Our numerical simulations indicated that the predicted
effects of the stochasticity can be measured already with presently available
lasers and electron accelerators.Comment: 5 pages, 2 figure
Macroscopic Superpositions of Phase States with Bose-Einstein Condensates
Quantum superpositions of macroscopically distinguishable states having
distinct phases can be created with a Bose-Einstein condensate trapped in a
periodic potential. The experimental signature is contained in the phase
distribution of the interference patterns obtained after releasing the traps.
Moreover, in the double well case, this distribution exhibits a dramatic
dependence on the parity of the total number of atoms. We finally show that,
for single well occupations up to a few hundred atoms, the macroscopic quantum
superposition can be robust enough against decoherence to be experimentally
revealable within current technology
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