In this article, we present a quantum transistor model based on a network of
coupled quantum oscillators destined to quantum information processing tasks in
linear optics. To this end, we show in an analytical way how a set of $N$
quantum oscillators (data-bus) can be used as an optical quantum switch, in
which the energy gap of the data bus oscillators plays the role of an
adjustable "potential barrier". This enables us to "block or allow" the quantum
information to flow from the source to the drain. In addition, we discuss how

Quantum annealing is an innovative idea and method for avoiding the increase
of the calculation cost of the combinatorial optimization problem. Since the
combinatorial optimization problems are ubiquitous, quantum annealing machine
with high efficiency and scalability will give an immeasurable impact on many
fields. However, the conventional quantum annealing machine may not have a high
success probability for finding the solution because the energy gap closes

We apply variational tensor-network methods for simulating the
Kosterlitz-Thouless phase transition in the classical two-dimensional XY model.
In particular, using uniform matrix product states (MPS) with non-abelian O(2)
symmetry, we compute the universal drop in the spin stiffness at the critical
point. In the critical low-temperature regime, we focus on the MPS entanglement
spectrum to characterize the Luttinger-liquid phase. In the high-temperature
phase, we confirm the exponential divergence of the correlation length and

We approach to the ortho-positronium (o-Ps) as a relativistic two-body
problem in $2+1$ dimensions in which o-Ps is composed of two-oppositely charged
particles interacting via an attractive Coulomb force. In addition to
separation of center of mass and relative coordinates, mapping the background
into the polar space-time gives possibility of construction of possible spin
eigen-states of o-Ps. This approach makes the energy spectrum complex in order
to describe o-Ps that can decay. From the complex energy expression, we find

We study the effect of the shape of different potential barriers on the
transmitted charged current whose fermionic population is not monoenergetic but
is described by means of an energy spectrum. The generalised Kappa Fermi-Dirac
distribution function is able to take into account both equilibrium and
non-equilibrium populations of particles by changing the kappa index. Moreover,
the corresponding current density can be evaluated using such a distribution.
Subsequently, this progressive charged density arrives at the potential

We study a finite spin-1/2 Ising chain with a regularly period-2 alternating
transverse field of period two. By means of a Jordan-Wigner transformation for
even and odd sites, we are able to map it into a one-dimensional model of free
fermions. Next, we determine the ground state energies in the positive and
negative parity subspaces (subspaces with an even or odd total number of down
spins, respectively), and compare them in order to establish the ground state

Advances in nanophotonics, quantum optics, and low-dimensional materials have
enabled precise control of light-matter interactions down to the nanoscale.
Combining concepts from each of these fields, there is now an opportunity to
create and manipulate photonic matter via strong coupling of molecules to the
electromagnetic field. Towards this goal, here we introduce a first principles
framework to calculate polaritonic excited-state potential-energy surfaces for

A nonlinear quantum-optical process is considered: emission of photon pairs
by the reflecting end of a fiber excited by a standing laser wave. Radiation
occurs due to periodic changes in the optical length of the fiber over time.
This radiation can be significantly enhanced in long fibers. Because of this
enhancement, a nonperturbative description of the process is required. Such a
description presented here predicts a strong peak of radiation, if the
amplitude of the oscillations of the optical length coincides with half of the

The paradigm of cavity QED is a two-level emitter interacting with a high
quality factor single mode optical resonator. The hybridization of the emitter
and photon wave functions mandates large vacuum Rabi frequencies and long
coherence times; features that so far have been successfully realized with
trapped cold atoms and ions and localized solid state quantum emitters such as
superconducting circuits, quantum dots, and color centers. Thermal atoms on the

Although regarded today as an important resource in quantum information,
nonlocality has yielded over the years many conceptual conundrums. Among the
latter are nonlocal aspects of single particles which have been of major
interest. In this paper, the nonlocality of single quanta is proved using a
delayed choice modification of quantum measurement outcomes that are
functionally dependent on the complex-valued weak values in a square nested
Mach-Zehnder interferometer with spatially separated measuring devices. We show