We show that a strong laser pulse combined with a strong x-ray pulse can be employed in a detection scheme
for characterizing high-energy γ-ray pulses down to the zeptosecond timescale. The scheme employs streak
imaging technique built upon the high-energy process of electron-positron pair production in vacuum through
the collision of a test pulse with intense laser pulses. The role of quantum radiation reaction in multiphoton
Compton scattering process and limitations imposed by it on the detection scheme are examined.
The validity of the few-level approximation is investigated in a system of two dipole-dipole interacting four-level
atoms. Each atom is modelled by two complete sets of angular momentum multiplets. We provide two
independent arguments demonstrating that the few-level approximation in general leads to incorrect predictions
if it is applied to the Zeeman sublevels of the atomic level scheme. First, we show that the artificial omission
of sublevels generally leads to incorrect eigenenergies of the system. The second counterexample involves an
external laser field and illustrates that the relevant states in each atom are not only determined by the laser
field polarization, but also by the orientation of the atomic separation vector. As the physical origin of this
outcome, we identify the dipole-dipole interaction between orthogonal dipole transitions of different atoms. Our
interpretation enables us to identify conditions on the atomic level structure as well as special geometries in
which (partial) few-level approximations are valid.
We describe a scheme capable of localizing an ensemble of interacting two-level atoms. The atoms are assumed to
be bunched together in a volume much smaller than an emission wavelength, and they interact with a standing
wave laser field. Due to the laser-pumping of the atomic sample, it collectively emits fluorescence light with
properties depending on the ensemble position in the standing wave. This relation can be described by a
fluorescence intensity profile, which depends on the standing wave field parameters, the ensemble properties, and
which is modified due to collective effects in the ensemble of nearby particles. We demonstrate that the intensity
profile can be tailored to suit different localization setups.
Light scattered by a regular structure of atoms exhibits spatial interference signatures, similar to Young's classical
double-slit experiment. The first-order interferences, however, are known to vanish for strong light intensities,
where the incoherently fluctuating part of the emitted light dominates. Here, we show how to overcome these
limitations to quantum interference in stronger laser fields, and how to recover the first-order interference in strong
fields by a tailored electromagnetic vacuum with a suitable frequency dependence. We also discuss higher-order
correlation functions of the scattered light, with applications, e.g., to lithography. In the second part, we study
light propagation of a probe field pulse in closed-loop atomic systems. The closed interaction loop induces a
sensitivity to the relative driving field phase, but in general prohibits a stationary steady state. In particular,
the finite frequency width of the short probe pulse requires a time-dependent analysis beyond the so-called
multiphoton resonance assumption. Using a Floquet decomposition, we identify the different contributions to
the medium response, and demonstrate sub- and superluminal light propagation with small absorption or even
gain, where a coupling field Rabi frequency allows to switch between sub- and superluminal light propagation.
Different physical processes using strong laser fields with intensities likely available in the near future are studied.
We focus on the possibility of probing experimentally the nonlinear properties of the quantum vacuum that arise
due to the existence of the so-called "quantum vacuum fluctuations", as predicted by quantum electrodynamics
(QED). In particular, we consider the laser-assisted photon-photon scattering process and the diffractive effects
arising during the interaction between an x-ray probe and a strong, focused optical standing wave. Also, the
enhancement of vacuum polarization effects due to the presence of a cold relativistic plasma is pointed out.
Finally, direct nuclear excitation by an intense, high-frequency laser field is studied.
We discuss various aspects of the incoherent spontaneous emission in atomic few-level systems arising from the coupling of the atom to the surrounding vacuum. First, we consider systems where the decoherence due to spontaneous emission acts as a limiting factor. Here, we combine collective effects in larger samples of atoms with control mechanisms known from single-atom schemes, or modify the system dynamics by externally inducing multiphoton quantum interference effects. In the second part, we discuss ground-state laser cooling of trapped atoms and ions. Here, the momentum transfer in the spontaneous emission events is required to cool the particles, but needs to be controlled in order to achieve a low cooling limit. In our scheme, we make use of double electromagnetically induced transparency in order to design the absorption spectrum of the trapped particle. In the final part, we show that the incoherent part of the resonance fluorescence spectrum of a two-level system may serve as an interesting candidate for high-precision spectroscopy. For this, we discuss relativistic and radiative corrections to the resonance fluorescence spectra of laser-driven few-level systems.