29 research outputs found
Enhancing Quantum Effects via Periodic Modulations in Optomechanical Systems
Parametrically modulated optomechanical systems have been recently proposed
as a simple and efficient setting for the quantum control of a micromechanical
oscillator: relevant possibilities include the generation of squeezing in the
oscillator position (or momentum) and the enhancement of entanglement between
mechanical and radiation modes. In this paper we further investigate this new
modulation regime, considering an optomechanical system with one or more
parameters being modulated over time. We first apply a sinusoidal modulation of
the mechanical frequency and characterize the optimal regime in which the
visibility of purely quantum effects is maximal. We then introduce a second
modulation on the input laser intensity and analyze the interplay between the
two. We find that an interference pattern shows up, so that different choices
of the relative phase between the two modulations can either enhance or cancel
the desired quantum effects.Comment: 10 pages, 4 figure
Non classical states of an oscillator in optomechanical systems
Riassunto Analitico
Questa tesi teorica si sviluppa nel campo dell'optomeccanica, ovvero studia l'interazione di luce con oscillatori (nano-) micromeccanici e in questo modo la possibilità di controllare quantisticamente sistemi meccanici grandi. A un livello fondamentale, l'obiettivo è studiare la meccanica quantistica applicata ad oggetti macroscopici e investigare il sottile confine tra regime classico e regime quantistico. Tradotto in concreto, questo lavoro si concentra sulla ricerca di parametri ottimali (possibilmente vicini ai parametri utilizzati in recenti esperimenti) per cui la generazione di stati non classici in un sistema meccanico sia ben visibile e facilmente realizzabile. Particolare interesse è dato ad effetti di ``squeezing'' e ``entanglement''.
Viene presentata una trattazione dettagliata dell’interazione tramite pressione di radiazione, spiegando gli aspetti principali che rendono l'accoppiamento optomeccanico uno strumento fondamentale per il controllo di oscillatori meccanici. Un modello per gli effetti di rumore e dissipazione dovuti all’accoppiamento con l’ambiente esterno completa la cornice teorica e rende possibile analizzare la dinamica quantistica di sistemi optomeccanici senza perdere contatto con la realtà . In questo modo i risultati trovati possono essere sufficientemente precisi e direttamente confrontabili con i futuri risultati sperimentali.
Seguendo alcuni recenti lavori apparsi in letteratura viene costruito un metodo di analisi, usato come esempio per discutere la generazione, la quantificazione e la misura di ``entanglement'' tra modi di oscillazione meccanici e modi di radiazione in una cavità Fabry-Perot. Cuore di questa tesi è poi la generazione di ``squeezing`` e ``entanglement`` in sistemi optomeccanici modulati periodicamente. Quando un sistema viene modulato periodicamente, entrano infatti in gioco effetti addizionali (come ad esempio la risonanza parametrica), che possono risultare molto utili se sfruttati adeguatamente. In questa prospettiva possono essere trovati schemi efficaci per amplificare la visibilità e migliorare il controllo degli effetti quantistici desiderati. Giocando sui parametri della modulazione viene infine caratterizzato il regime ideale che massimizza la presenza di effetti quantistici ed è allo stesso tempo
compatibile con le piĂą recenti implementazioni sperimentali
Quantum optomechanical piston engines powered by heat
We study two different models of optomechanical systems where a temperature
gradient between two radiation baths is exploited for inducing self-sustained
coherent oscillations of a mechanical resonator. Viewed from a thermodynamic
perspective, such systems represent quantum instances of self-contained thermal
machines converting heat into a periodic mechanical motion and thus they can be
interpreted as nano-scale analogues of macroscopic piston engines. Our models
are potentially suitable for testing fundamental aspects of quantum
thermodynamics in the laboratory and for applications in energy efficient
nanotechnology.Comment: 10 pages, 6 figure
Steady-state entanglement activation in optomechanical cavities
Quantum discord, and a number of related indicators, are currently raising a
relentless interest as a novel paradigm of non-classical correlations beyond
entanglement. Beside merely fundamental aspects, various works have shown that
discord is a valuable -- so far largely unexplored -- resource in quantum
information processing. Along this line, quite a striking scheme is
{entanglement activation}. An initial amount of discord between two
disentangled parties of a multipartite system affects the dynamics so as to
establish entanglement across a bipartition, which would not arise otherwise.
To date, such a process was proven to be achievable only dynamically, i.e.,
with no guarantee of a stationary entanglement throughput in the presence of
noise. Here, we discover a {\it discord-activated mechanism yielding
steady-state entanglement} production in a realistic continuous-variable setup.
This comprises two coupled optomechanical cavities, where the optical modes
(OMs) communicate through a fiber. We first use a simplified model to highlight
the creation of steady-state discord between the OMs. We show next that such
discord improves the level of stationary optomechanical entanglement attainable
in the system, making it more robust against temperature and thermal noise.Comment: 5+4 pages, 5+1 figures (main text + supplementary materials
Building versatile bipartite probes for quantum metrology
We consider bipartite systems as versatile probes for the estimation of
transformations acting locally on one of the subsystems. We investigate what
resources are required for the probes to offer a guaranteed level of
metrological performance, when the latter is averaged over specific sets of
local transformations. We quantify such a performance via the average skew
information, a convex quantity which we compute in closed form for bipartite
states of arbitrary dimensions, and which is shown to be strongly dependent on
the degree of local purity of the probes. Our analysis contrasts and
complements the recent series of studies focused on the minimum, rather than
the average, performance of bipartite probes in local estimation tasks, which
was instead determined by quantum correlations other than entanglement. We
provide explicit prescriptions to characterize the most reliable states
maximizing the average skew information, and elucidate the role of state
purity, separability and correlations in the classification of optimal probes.
Our results can help in the identification of useful resources for sensing,
estimation and discrimination applications when complete knowledge of the
interaction mechanism realizing the local transformation is unavailable, and
access to pure entangled probes is technologically limited.Comment: 13+5 pages, 2 figures (added new section
Gaussian Discriminating Strength
We present a quantifier of non-classical correlations for bipartite,
multi-mode Gaussian states. It is derived from the Discriminating Strength
measure, introduced for finite dimensional systems in A. Farace et al., New. J.
Phys. 16, 073010 (2014). As the latter the new measure exploits the Quantum
Chernoff Bound to gauge the susceptibility of the composite system with respect
to local perturbations induced by unitary gates extracted from a suitable set
of allowed transformations (the latter being identified by posing some general
requirements). Closed expressions are provided for the case of two-mode
Gaussian states obtained by squeezing or by linearly mixing via a beam-splitter
a factorized two-mode thermal state. For these density matrices, we study how
non-classical correlations are related with the entanglement present in the
system and with its total photon number.Comment: 11+6 pages, 4 figure
Digital lattice gauge theories
We propose a general scheme for a digital construction of lattice gauge
theories with dynamical fermions. In this method, the four-body interactions
arising in models with dimensions and higher, are obtained
stroboscopically, through a sequence of two-body interactions with ancillary
degrees of freedom. This yields stronger interactions than the ones obtained
through pertubative methods, as typically done in previous proposals, and
removes an important bottleneck in the road towards experimental realizations.
The scheme applies to generic gauge theories with Lie or finite symmetry
groups, both Abelian and non-Abelian. As a concrete example, we present the
construction of a digital quantum simulator for a lattice
gauge theory with dynamical fermionic matter in dimensions, using
ultracold atoms in optical lattices, involving three atomic species,
representing the matter, gauge and auxiliary degrees of freedom, that are
separated in three different layers. By moving the ancilla atoms with a proper
sequence of steps, we show how we can obtain the desired evolution in a clean,
controlled way
Digital quantum simulation of lattice gauge theories in three spatial dimensions
In the present work, we propose a scheme for digital formulation of lattice
gauge theories with dynamical fermions in 3+1 dimensions. All interactions are
obtained as a stroboscopic sequence of two-body interactions with an auxiliary
system. This enables quantum simulations of lattice gauge theories where the
magnetic four-body interactions arising in two and more spatial dimensions are
obtained without the use of perturbation theory, thus resulting in stronger
interactions compared with analogue approaches. The simulation scheme is
applicable to lattice gauge theories with either compact or finite gauge
groups. The required bounds on the digitization errors in lattice gauge
theories, due to the sequential nature of the stroboscopic time evolution, are
provided. Furthermore, an implementation of a lattice gauge theory with a
non-abelian gauge group, the dihedral group , is proposed employing the
aforementioned simulation scheme using ultracold atoms in optical lattices.Comment: 38 pages, 5 figure
Heat flux dynamics in dissipative cascaded systems
We study the dynamics of heat flux in the thermalization process of a pair of
identical quantum system that interact dissipatively with a reservoir in a {\it
cascaded} fashion. Despite the open dynamics of the bipartite system S is
globally Lindbladian, one of the subsystems "sees" the reservoir in a state
modified by the interaction with the other subsystem and hence it undergoes a
non-Markovian dynamics. As a consequence, the heat flow exhibits a
non-exponential time behaviour which can greatly deviate from the case where
each party is independently coupled to the reservoir. We investigate both
thermal and correlated initial states of and show that the presence of
correlations at the beginning can considerably affect the heat flux rate. We
carry out our study in two paradigmatic cases -- a pair of harmonic oscillators
with a reservoir of bosonic modes and two qubits with a reservoir of fermionic
modes -- and compare the corresponding behaviours. In the case of qubits and
for initial thermal states, we find that the trace distance discord is at any
time interpretable as the correlated contribution to the total heat flux.Comment: Final accepted versio
Versatile Gaussian probes for squeezing estimation
We consider an instance of “black-box” quantum metrology in the Gaussian framework, where we aim to estimate the amount of squeezing applied on an input probe, without previous knowledge on the phase of the applied squeezing. By taking the quantum Fisher information (QFI) as the figure of merit, we evaluate its average and variance with respect to this phase in order to identify probe states that yield good precision for many different squeezing directions. We first consider the case of single-mode Gaussian probes with the same energy, and find that pure squeezed states maximize the average quantum Fisher information (AvQFI) at the cost of a performance that oscillates strongly as the squeezing direction is changed. Although the variance can be brought to zero by correlating the probing system with a reference mode, the maximum AvQFI cannot be increased in the same way. A different scenario opens if one takes into account the effects of photon losses: coherent states represent the optimal single-mode choice when losses exceed a certain threshold and, moreover, correlated probes can now yield larger AvQFI values than all single-mode states, on top of having zero variance