135 research outputs found

### Interferometric modulation of quantum cascade interactions

We consider many-body quantum systems dissipatively coupled by a cascade
network, i.e. a setup in which interactions are mediated by unidirectional
environmental modes propagating through a linear optical interferometer. In
particular we are interested in the possibility of inducing different effective
interactions by properly engineering an external dissipative network of
beam-splitters and phase-shifters. In this work we first derive the general
structure of the master equation for a symmetric class of translation-invariant
cascade networks. Then we show how, by tuning the parameters of the
interferometer, one can exploit interference effects to tailor a large variety
of many-body interactions.Comment: 12 pages, 10 figure

### Interferometric Quantum Cascade Systems

In this work we consider quantum cascade networks in which quantum systems
are connected through unidirectional channels that can mutually interact giving
rise to interference effects. In particular we show how to compute master
equations for cascade systems in an arbitrary interferometric configuration by
means of a collisional model. We apply our general theory to two specific
examples: the first consists in two systems arranged in a Mach-Zender-like
configuration; the second is a three system network where it is possible to
tune the effective chiral interactions between the nodes exploiting
interference effects.Comment: 15 pages, 5 figure

### Quantum Engineering in Open Quantum Systems

The huge technological advancement achieved in the last years has allowed for the emergence
of a new field of physics dubbed \u201cquantum engineering\u201d: with this term people
refer to a wide range of topics, from planning and building physical systems for specific
tasks to developing algorithms to control those systems, from ways to create specific
quantum states to new theoretical tools to describe and plan new physical systems.
As the field of quantum engineering covers many topics in physics, this is reflected
in the community interested in it, ranging from quantum optics theorists to solid state
experimentalists. This also includes the possibility, and sometimes the necessity, for a
scientist willing to enter the field to study very different problems, as it happened for
the material in this thesis, where at least two main topics are covered.
One of them is the study of open quantum systems, more specifically in the context
of collisional model and cascade networks. The latter are networks of quantum systems
interacting through the interaction with a common environment with unidirectional,
i.e. chiral, propagation of the signal. Thanks to the chirality of the environment it is
possible to obtain non symmetrical couplings between the quantum systems composing
the network, opening the way to engineer the steady state of the system.
The tool used to derive master equation describing dynamics and properties of such
systems is the one of collisional models: these models are nowadays extensively used in
a wide range of topics concerning open quantum systems, from the description of both
Markovian and non Markovian dynamics, to quantum optics and quantum thermodynamics.
In collisional models the environment is depicted as a collection of smaller systems, dubbed ancillas, which interact in a collisional fashion with the quantum system under examination. This way of describing open systems dynamics leads to a discrete master
equation on which it is then possible to enforce a continuous time limit. Among the
advantages provided by such an approach there is the simplicity with which is possible
to switch from a Markovian to a non-Markovian dynamics and the possibility of keeping
track of the environmental degrees of freedom.
The last feature cited is the one exploited in this thesis when studying a quantum
system thermalizing through the interaction with a thermal bath: having at disposal
the environmental state at each discrete step of the thermalization process, it is possible
to compute the thermodynamic functionals relative to the environment. Specifically, by
computing the quantum mutual information between the system and the environment, it
is possible to show that the final joint state reached by the system and the environment
is a factorized state.
The other part of this thesis focuses instead on quantum state engineering by potential
engineering. By appropriately engineering a potential profile, it is possible to obtain
a class of quantum states, dubbed stretchable, which have the property of having a
flat wave function in some regions, somehow analogously to what happens in photonic
metamaterials: in this materials, where either the permittivity or the permeability is
zero, the temporal and spatial variation of the electric field are decoupled, leading to the
possibility of having a stretched wave with both large frequency and large wavelength.
Finally, in this thesis it is shown how, by properly engineering a spatially varying
potential landscape, it is possible to attach a geometric phase to the quantum state
of a traveling wave. More specifically, as the confining potential of a traveling wave
varies along a closed loop in parameters space, it is possible to implement an operation,
usually called holonomy, which attaches a geometric phase to the state, analogously to
what happens in the Berry phase phenomenon for a time dependent Hamiltonian

### Contractors as a Second Best Option: The Italian Hybrid Approach to Maritime Security

Initially based mainly on the use of Navy Vessel Protection Detachments (VPDs) paid by shipowners, Italian maritime security legislation also allows for the use of Privately Contracted Armed Security Personnel (PCASP) when VPDs are not available. Hence, Italy has adopted a hybrid antipiracy approach that entails two different forms of private sector involvement: the financing and partial control of public military forces by the maritime industry and the provision of armed security by PCASP, an option that includes Private Military and Security Companies (PMSCs). This opening to the commercial sector is significant as one of the first of its kind in a state that has adopted a tight monopoly over the provision of armed services and can be explained as the interplay between the willingness to respond to the needs of the maritime industry and a long-standing resistance against loosening state control over the use of force.Political Culture and National Identit

### The Political Cost-Effectiveness of Private Vessel Protection: The Italian Case

Italy has traditionally been wary of private providers of security. Still, private military and security companies (PMSCs) have recently started to play an important role in protecting Italian merchant vessels, eventually replacing the military vessel protection detachment units (VPDs) provided by the Italian Navy. Drawing on neoclassical realism, the increasing involvement of PMSCs in protecting Italian merchant ships is presented as an attempt to reduce the political costs associated with the use of military personnel abroad, epitomised by the arrest of two Italian Navy fusiliers by Indian authorities in February 2012History and International Relation

### Structured quantum collision models: generating coherence with thermal resources

Quantum collision models normally consist of a system interacting with a set
of ancillary units representing the environment. While these ancillary systems
are usually assumed to be either two level systems (TLS) or harmonic
oscillators, in this work we move further and represent each ancillary system
as a structured system, i.e., a system made out of two or more subsystems. We
show how this scenario modifies the kind of master equation that one can obtain
for the evolution of the open systems. Moreover, we are able to consider a
situation where the ancilla state is thermal yet has some coherence. This
allows the generation of coherence in the steady state of the open system and,
thanks to the simplicity of the collision model, this allows us to better
understand the thermodynamic cost of creating coherence in a system.
Specifically, we show that letting the system interact with the coherent
degrees of freedom requires a work cost, leading to the natural fulfillment of
the first and second law of thermodynamics without the necessity of {\it ad
hoc} formulations.Comment: 16 pages, 8 figure

### Entropy production and asymptotic factorization via thermalization: a collisional model approach

The Markovian evolution of an open quantum system is characterized by a
positive entropy production, while the global entropy gets redistributed
between the system and the environment degrees of freedom. Starting from these
premises, we analyze the entropy variation of an open quantum system in terms
of two distinct relations: the Clausius inequality, that provides an intrinsic
bound for the entropy variation in terms of the heat absorbed by the system,
and an extrinsic inequality, which instead relates the former to the
corresponding entropy increment of the environment. By modeling the
thermalization process with a Markovian collisional model, we compare and
discuss the two bounds, showing that the latter is asymptotically saturated in
the limit of large interaction time. In this regime not only the reduced
density matrix of the system reaches an equilibrium configuration, but it also
factorizes from the environment degrees of freedom. This last result is proven
analytically when the system-bath coupling is sufficiently strong and through
numerical analysis in the weak-coupling regime.Comment: 10 pages, 2 figure

### Thermodynamics and correlations in quantum cascade systems

Interest in quantum cascaded systems first arose in the 80â€™, when new exotic
forms of light, like squeezed light, were discovered. This led to a strong demand
for a new quantum formalism able to describe the evolution of a total system in
which one subsystem is driven with the light from another quantum subsystem
(e.g. two atoms). Gardiner and Collet [15] and Charmichael [6] first developed
the so-called input-output formalism which allowed to describe the evolution
of the systemâ€™s operator through Langevinâ€™s equations, and moreover led to
a standard method to derive from these equations a master equation for the
density matrix. This formalism has been subsequently developed for various
case of interests [14, 16], up to become a well-based theory presented in books
[17].
In recent years the interest towards cascaded system has undergone a revival
due to important application in quantum information theory and many-body
physics. In general quantum cascaded systems are studied in the wider context
of quantum open systems: while in the past the noise from an external environ-
ment was seen only as a detrimental feature causing decoherence [37], nowadays
it is considered as a tool to control systemâ€™s evolution [29, 34] in order to obtain
states of interest, like entangled states [24, 33] or particular many-body states
[32, 35].
In this thesis a quantum cascaded system composed of bosonic subsystems
will be studied, mainly focusing on his thermodynamics and on the correlations
arising during the evolution.
Thermodynamics has been since its dawn in the 19th a quite difficult sub-
ject: it started as a purely phenomenological science, until the atomic theory
became popular. It was then that Boltzmann made a first attempt to derive
thermodynamics entirely from classical mechanics. Even if his theory clarified
some points of thermodynamics, nonetheless it was still unsatisfactory from a
foundational point of view, because it relied on unproven assumptions like the
ergodicity postulate or the a priori probabilities hypothesis. Many other sci-
entists tried to solve this problem, but none of them gave a fully satisfactory
answer.
With the emergence of quantum theory the old image of a gas as a set of
balls in a box started to seem just a sketch of reality, so that many efforts were
given to establish the theory of quantum thermodynamics [18]. On one side it
is necessary to reconcile quantum mechanics and classical thermodynamics [36,
38], while on the other side the main thermodynamical quantities like work,
heat and entropy have to be redefined in the quantum framework [2â€“4, 22, 25].
Concerning cascaded systems the interest in thermodynamics is due to the
peculiar features that heat flux showed in the case of a quantum system com-
posed by two subsystem, as analyzed in [26], so that it is interesting to know
what happens if there are more subsystems, if the features remain unchanged
or they are sensible to the number of subsystems. Moreover one asks if it would
be possible to engineer these systems in order to create heat cells able to release
energy slower, faster or in another desired way. Moreover it is interesting to see
if it is possible to create a heat interferometer with this kind of systems [19, 27].
The interest in correlations is mainly due the importance that they have
in several information theory protocols: it is well known that entanglement is
considered the main source for many quantum computation tasks [9, 10, 30],
and one of the most striking feature of quantum mechanics [37]. Many efforts
have been given all over the years to find methods for generating [24, 33, 35]
and distributing [5, 23] entangled states.
Moreover in recent years correlations have been analyzed in the new frame-
work of quantum discord [21, 31], a quantity that allows to measure all quantum
correlations of a state beyond entanglement. One of the most striking feature
of quantum discord is that while every entangled state has a non-null discord,
there exist non-entangled states also with non-zero discord [11, 13]. In recent
years several studies have been done to understand quantum discord [8, 12], to
define it operationally [7], to quantify it [28] and at least in some cases to find
closed formulas for its evaluation [1, 11, 20].
The first chapters of this thesis are focused on the basic arguments necessary
to understand the following chapters, such as open systems dynamics, cascaded
systems definition and Gaussian states. The last chapters are instead focused
on deriving and explaining all the results obtained about thermodynamics and
correlations. Here it is a detailed outline of the thesis.
In the second chapter a brief introduction about quantum mechanics and
open system dynamics is given. Dynamical semigroups and the concept of mas-
ter equation are introduced, together with the assumptions usually made in the
analysis of such systems.
The third chapter is focused on cascade systems, how they are defined and
which are the physical assumption made. The collisonal model used to find the
master equation is explained, and then the master equation is derived under
different physical assumptions.
Chapter four deals with Gaussian states and the formalism used to describe
them. The covariance matrix formalism is introduced and the dynamics of the
covariance matrix is derived through the master equation.
In the fifth chapter the thermodynamics of the system is analyzed, focusing
on the behaviour of the heat flux. First all the quantities used, such as heat
flux and transferred heat, are defined. Then, using the master equation, it is
studied how heat flux is affected by the systemâ€™s characteristics, like the number
of subsystems or their temperature.
Fynally in the sixth chapter correlations in the system are studied, observing
how they can arise during the transient dynamics. All the necessary formulas
to compute the amount of correlations in the system are explained and then
correlations dynamics is examined.
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### Geometric phase through spatial potential engineering

We propose a spatial analog of the Berry's phase mechanism for the coherent
manipulation of states of non-relativistic massive particles moving in a
two-dimensional landscape. In our construction the temporal modulation of the
system Hamiltonian is replaced by a modulation of the confining potential along
the transverse direction of the particle propagation. By properly tuning the
model parameters the resulting scattering input-output relations exhibit a
Wilczek-Zee non-abelian phase shift contribution that is intrinsically
geometrical, hence insensitive to the specific details of the potential
landscape. A theoretical derivation of the effect is provided together with
practical examples.Comment: 10 pages, 5 figure

### First report of Melittobia australica Girault in Europe and new record of M. acasta (Walker) for Italy

Melittobia acasta and M. australica are newly recorded from Sicily, Italy, and the second species is reported in Europe for the first time. A short historical background about Melittobia parasitoid wasps, their hosts, and distribution, with emphasis in those two species is presented together with illustrations to facilitate their identification. Brief discussion about the presence and possible distribution of the species in Sicily is also included

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