6,178 research outputs found
Nonlocality threshold for entanglement under general dephasing evolutions: A case study
Determining relationships between different types of quantum correlations in
open composite quantum systems is important since it enables the exploitation
of a type by knowing the amount of another type. We here review, by giving a
formal demonstration, a closed formula of the Bell function, witnessing
nonlocality, as a function of the concurrence, quantifying entanglement, valid
for a system of two noninteracting qubits initially prepared in extended
Werner-like states undergoing any local pure-dephasing evolution. This formula
allows for finding nonlocality thresholds for the concurrence depending only on
the purity of the initial state. We then utilize these thresholds in a
paradigmatic system where the two qubits are locally affected by a quantum
environment with an Ohmic class spectrum. We show that steady entanglement can
be achieved and provide the lower bound of initial state purity such that this
stationary entanglement is above the nonlocality threshold thus guaranteeing
the maintenance of nonlocal correlations.Comment: 7 pages, 4 figures. Revised versio
Quantum entanglement of identical particles by standard information-theoretic notions
Quantum entanglement of identical particles is essential in quantum
information theory. Yet, its correct determination remains an open issue
hindering the general understanding and exploitation of many-particle systems.
Operator-based methods have been developed that attempt to overcome the issue.
We introduce a state-based method which, as second quantization, does not label
identical particles and presents conceptual and technical advances compared to
the previous ones. It establishes the quantitative role played by arbitrary
wave function overlaps, local measurements and particle nature (bosons or
fermions) in assessing entanglement by notions commonly used in quantum
information theory for distinguishable particles, like partial trace. Our
approach furthermore shows that bringing identical particles into the same
spatial location functions as an entangling gate, providing fundamental
theoretical support to recent experimental observations with ultracold atoms.
These results pave the way to set and interpret experiments for utilizing
quantum correlations in realistic scenarios where overlap of particles can
count, as in Bose-Einstein condensates, quantum dots and biological molecular
aggregates.Comment: 6+3 pages, 3 Figures. Stories on: Physics World
(http://physicsworld.com/cws/article/news/2016/feb/12/theorists-disentangle-particle-identity);
Phys.org
(http://phys.org/news/2016-02-entanglement-identical-particles-doesnt-textbook.html).
Invited article on 2Physics.com, presenting key developments in physics
(http://www.2physics.com/2016/03/a-new-approach-to-quantum-entanglement.html
Validating and controlling quantum enhancement against noise by the motion of a qubit
Experimental validation and control of quantum traits for an open quantum system are important for any quantum information purpose. We consider a traveling atom qubit as a quantum memory with adjustable velocity inside a leaky cavity, adopting a quantum witness as a figure of merit for quantumness assessment. We show that this model constitutes an inherent physical instance where the quantum witness does not work properly if not suitably optimized. We then supply the optimal intermediate blind measurements which make the quantum witness a faithful tester of quantum coherence. We thus find that larger velocities protect quantumness against noise, leading to a lifetime extension of hybrid qubit-photon entanglement and to higher phase estimation precision. Control of qubit motion thus reveals itself as a quantum enhancer
Simple Non-Markovian Microscopic Models for the Depolarizing Channel of a Single Qubit
The archetypal one-qubit noisy channels ---depolarizing, phase-damping and
amplitude-damping channels--- describe both Markovian and non-Markovian
evolution. Simple microscopic models for the depolarizing channel, both
classical and quantum, are considered. Microscopic models which describe phase
damping and amplitude damping channels are briefly reviewed.Comment: 13 pages, 2 figures. Title corrected. Paper rewritten. Added
references. Some typos and errors corrected. Author adde
Coherence and entanglement dynamics of vibrating qubits
We investigate the dynamics of coherence and entanglement of vibrating
qubits. Firstly, we consider a single trapped ion qubit inside a perfect cavity
and successively we use it to construct a bipartite system made of two of such
subsystems, taken identical and noninteracting. As a general result, we find
that qubit vibration can lead to prolonging initial coherence in both
single-qubit and two-qubit system. However, despite of this coherence
preservation, we show that the decay of the entanglement between the two qubits
is sped up by the vibrational motion of the qubits. Furthermore, we highlight
how the dynamics of photon-phonon correlations between cavity mode and
vibrational mode, which may serve as a further useful resource stored in the
single-qubit system, is strongly affected by the initial state of the qubit.
These results provide new insights about the ability of systems made of moving
qubits in maintaining quantum resources compared to systems of stationary
qubits.Comment: 7 pages, 5 figures. Prepared for the Virtual Special Issue (VSI) on
Quantum Correlations, in the journal Optics Communications
Comparison of non-Markovianity criteria in a qubit system under random external fields
We give the map representing the evolution of a qubit under the action of
non-dissipative random external fields. From this map we construct the
corresponding master equation that in turn allows us to phenomenologically
introduce population damping of the qubit system. We then compare, in this
system, the time-regions when non-Markovianity is present on the basis of
different criteria both for the non-dissipative and dissipative case. We show
that the adopted criteria agree both in the non-dissipative case and in the
presence of population damping.Comment: 8 pages, 1 figure. Some changes made. In press on Physica Scripta T
(special issue
Protecting entanglement by adjusting the velocities of moving qubits inside non-Markovian environments
Efficient entanglement preservation in open quantum systems is a crucial
scope towards a reliable exploitation of quantum resources. We address this
issue by studying how two-qubit entanglement dynamically behaves when two atom
qubits move inside two separated identical cavities. The moving qubits
independently interact with their respective cavity. As a main general result,
we find that under resonant qubit-cavity interaction the initial entanglement
between two moving qubits remains closer to its initial value as time passes
compared to the case of stationary qubits. In particular, we show that the
initial entanglement can be strongly protected from decay by suitably adjusting
the velocities of the qubits according to the non-Markovian features of the
cavities. Our results supply a further way of preserving quantum correlations
against noise with a natural implementation in cavity-QED scenarios and are
straightforwardly extendable to many qubits for scalability.Comment: To be published in Laser Physics Letter
Wealth distribution across communities of adaptive financial agents
This paper studies the trading volumes and wealth distribution of a novel
agent-based model of an artificial financial market. In this model,
heterogeneous agents, behaving according to the Von Neumann and Morgenstern
utility theory, may mutually interact. A Tobin-like tax (TT) on successful
investments and a flat tax are compared to assess the effects on the agents'
wealth distribution. We carry out extensive numerical simulations in two
alternative scenarios: i) a reference scenario, where the agents keep their
utility function fixed, and ii) a focal scenario, where the agents are adaptive
and self-organize in communities, emulating their neighbours by updating their
own utility function. Specifically, the interactions among the agents are
modelled through a directed scale-free network to account for the presence of
community leaders, and the herding-like effect is tested against the reference
scenario. We observe that our model is capable of replicating the benefits and
drawbacks of the two taxation systems and that the interactions among the
agents strongly affect the wealth distribution across the communities.
Remarkably, the communities benefit from the presence of leaders with
successful trading strategies, and are more likely to increase their average
wealth. Moreover, this emulation mechanism mitigates the decrease in trading
volumes, which is a typical drawback of TTs.Comment: 18 pages, 7 figures, published in New Journal of Physic
Cavity-based architecture to preserve quantum coherence and entanglement
Quantum technology relies on the utilization of resources, like quantum
coherence and entanglement, which allow quantum information and computation
processing. This achievement is however jeopardized by the detrimental effects
of the environment surrounding any quantum system, so that finding strategies
to protect quantum resources is essential. Non-Markovian and structured
environments are useful tools to this aim. Here we show how a simple
environmental architecture made of two coupled lossy cavities enables a switch
between Markovian and non-Markovian regimes for the dynamics of a qubit
embedded in one of the cavity. Furthermore, qubit coherence can be indefinitely
preserved if the cavity without qubit is perfect. We then focus on entanglement
control of two independent qubits locally subject to such an engineered
environment and discuss its feasibility in the framework of circuit quantum
electrodynamics. With up-to-date experimental parameters, we show that our
architecture allows entanglement lifetimes orders of magnitude longer than the
spontaneous lifetime without local cavity couplings. This cavity-based
architecture is straightforwardly extendable to many qubits for scalability.Comment: 12 pages, 9 figures, 1 table. To appear on Nature Scientific Report
Partial containment control over signed graphs
In this paper, we deal with the containment control problem in presence of
antagonistic interactions. In particular, we focus on the cases in which it is
not possible to contain the entire network due to a constrained number of
control signals. In this scenario, we study the problem of selecting the nodes
where control signals have to be injected to maximize the number of contained
nodes. Leveraging graph condensations, we find a suboptimal and computationally
efficient solution to this problem, which can be implemented by solving an
integer linear problem. The effectiveness of the selection strategy is
illustrated through representative simulations.Comment: 6 pages, 3 figures, accepted for presentation at the 2019 European
Control Conference (ECC19), Naples, Ital
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