Recent years have seen an increasing interest in quantum chaos and related
aspects of spatially extended systems, such as spin chains. However, the
results are seemingly contradictory as generic approaches suggest the presence
of many-body localization while analytical calculations for certain system
classes, here referred to as the "self-dual case", prove adherence to universal
(chaotic) spectral behavior. We address these issues studying the level
statistics in the vicinity of the latter case, thereby revealing transitions to

The equations of electrodynamics are altered in the presence of a classical
coherent axion dark matter background field, changing the dispersion relation
for electromagnetic waves. Careful measurements of the frequency stability in
sensitive atomic clocks could in principle provide evidence for such a
background for $f_a \ge 10^7$ GeV. Turning on a background magnetic field might
enhance these effects in a controllable way, and interferometric measurements

We study the maximal amount of energy that can be extracted from a finite
quantum system by means of projective measurements. For this quantity we coin
the expression "metrotropy" $\mathcal{M}$, in analogy with "ergotropy"
$\mathcal{W}$, which is the maximal amount of energy that can be extracted by
means of unitary operations. The study is restricted to the case when the
system is initially in a stationary state, and therefore the ergotropy is
achieved by means of a permutation of the energy eigenstates. We show that i)

Quantum process tomography has become increasingly critical as the need grows
for robust verification and validation of candidate quantum processors. Here,
we present an approach for efficient quantum process tomography that uses a
physically motivated ansatz for an unknown quantum process. Our ansatz
bootstraps to an effective description for an unknown process on a multi-qubit
processor from pairwise two-qubit tomographic data. Further, our approach can
inherit insensitivity to system preparation and measurement error from the

We show unusual cooperative two-photon resonance between two-modes of field
inside a photonic crystal cavity. The two-photon resonance occurs when two off
resonant quantum dots emit one photon in each cavity mode and de-excite
simultaneously. In the presence of phonon coupling the conditions for
two-photon resonance change significantly. Using such two-photon two-mode
interaction we propose to generate entangled state of two qutrits. The basis of
a qutrit are formed by the state of the cavity mode containing $0$, $1$ and $2$

We describe and experimentally demonstrate a three-party quantum secret
sharing protocol using polarization-entangled photon pairs. The source itself
serves as an active participant and can switch between the required photon
states by modulating the pump beam only, thereby making the protocol less
susceptible to loss and amenable to fast switching. Compared to three-photon
protocols, the practical efficiency is dramatically improved as there is no
need to generate, transmit, or detect a third photon.

We study systematically the quantum corrections to a weakly interacting
Bose-Einstein condensate with spin-orbit coupling. We show that quantum
fluctuations, enhanced by the spin-orbit coupling, modify quantitatively the
mean-field properties such as the superfluid density, spin polarizability, and
sound velocity. We find that the phase boundary between the plane wave and zero
momentum phases is shifted to a smaller transverse field. We also calculate the

Some effects of vacuum polarization in QED due to the presence of field
sources are investigated. We focus on effects with no counter-part in Maxwell
electrodynamics. The the Uehling interaction energy between two stationary
point-like charges is calculated exactly in terms of Meijer-G functions.
Effects induced on a hydrogen atom by the vacuum polarization in the vicinity
of a Dirac string are considered. We also calculate the interaction between two
parallel Dirac strings and corrections to the energy levels of a quantum

We investigate the quantum dynamics of two bosons, trapped in a
two-dimensional harmonic trap, upon quenching arbitrarily their interaction
strength thereby covering the entire energy spectrum. Utilizing the exact
analytical solution of the stationary system we derive a closed analytical form
of the expansion coefficients of the time-evolved two-body wavefunction, whose
dynamics is determined by an expansion over the postquench eigenstates. The
emergent dynamical response of the system is analyzed in detail by inspecting

Recent experiments with strongly interacting, driven Rydberg ensembles have
introduced a promising setup for the study of self-organized criticality (SOC)
in cold atom systems. Based on this setup, we theoretically propose a control
mechanism for the paradigmatic avalanche dynamics of SOC in terms of a
time-dependent drive amplitude. This gives access to a variety of avalanche
dominated, self-organization scenarios, prominently including self-organized
criticality, as well as sub- and supercritical dynamics. We analyze the