We revisit the issue of defining the entropy of a spatial region in a broad
class of quantum theories. In theories with explicit regularizations, working
within an elementary but general algebraic framework applicable to matter and
gauge theories alike, we give precise path integral expressions for three known
types of entanglement entropy that we call full, distillable, and
gauge-invariant. For a class of gauge theories that do not necessarily have a
regularization in our framework, including Chern-Simons theory, we describe a

For the characterization of dynamics in quantum many-body systems the
question how information spreads and becomes distributed over the constituent
degrees of freedom is of fundamental interest. The delocalization of
information under many-body dynamics has been dubbed "scrambling" and
out-of-time-order correlators were proposed to probe this behavior. In this
work we investigate the time-evolution of tripartite information as a natural
operator-independent measure of scrambling, which quantifies to which extent

We study scattering of propagating microwave fields by a DC-voltage biased
Josephson junction. At sub-gap voltages, a small Josephson junction works
merely as a non-linear boundary that can absorb, amplify, and diversely convert
propagating microwaves. In the leading-order perturbation theory of the
Josephson coupling energy, the spectral density and quadrature fluctuations of
scattered thermal and coherent radiation can be described in terms of the
well-known $P(E)$ function. Applying this, we study how thermal and coherent

The Noisy Intermediate-Scale Quantum (NISQ) technology is currently
investigated by major players in the field to build the first practically
useful quantum computer. IBM QX architectures are the first ones which are
already publicly available today. However, in order to use them, the respective
quantum circuits have to be compiled for the respectively used target
architecture. While first approaches have been proposed for this purpose, they
are infeasible for a certain set of SU(4) quantum circuits which recently have

The resonance fluorescence of a four-level atom in J = 1/2 to J = 1/2
transition driven by two coherent fields is studied. We find that the
incoherent fluorescence spectrum shows a direct indication of vacuum-induced
coherence in the atomic system. We show that such coherence manifests itself
via an enhancement or suppression of the spectral peaks in the $\pi$-polarized
fluorescence. The effect of the relative phase of the driving fields on the
spectral features is also investigated. We show that phase-dependent

Optimization problems in disciplines such as machine learning are commonly
solved with iterative methods. Gradient descent algorithms find local minima by
moving along the direction of steepest descent while Newton's method takes into
account curvature information and thereby often improves convergence. Here, we
develop quantum versions of these iterative optimization algorithms and apply
them to polynomial optimization with a unit norm constraint. In each step,

We introduce a protocol capable of generating a general measurement operator
for a mechanical resonator. The technique requires a qubit-resonator
interaction and uses a coherent pulse to drive qubit transitions. This is
followed by projective measurement of the qubit's energy, constraining the
resonator in a state that depends on the pulse shape. The freedom to choose a
pulse shape for the coherent drive enables an arbitrary position-basis
measurement operator. Using this measurement operator, we outline a two pulse

Quantum computers, which take advantage of the superposition and entanglement
of physical states, could outperform their classical counterparts in solving
problems with technological impact, such as factoring large numbers and
searching databases. A quantum processor executes algorithms by applying a
programmable sequence of gates to an initialized state of qubits, which
coherently evolves into a final state containing the result of the computation.
Although quantum processors with a few qubits have been demonstrated on

We propose the concept of machine learning configuration interaction (MLCI)
whereby an artificial neural network is trained on-the-fly to predict important
new configurations in an iterative selected configuration interaction
procedure. We demonstrate that the neural network can discriminate between
important and unimportant configurations, that it has not been trained on, much
better than by chance. MLCI is then used to find compact wavefunctions for
carbon monoxide at both stretched and equilibrium geometries. We also consider

Valid transformations between quantum states are necessarily described by
completely positive maps, instead of just positive maps. Positive but not
completely positive maps such as the transposition map cannot be implemented
due to the existence of entanglement in composite quantum systems, but there
are classes of states for which the positivity is guaranteed, e.g., states not
correlated to other systems. In this paper, we introduce the concept of N-copy