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

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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