9,966 research outputs found
Van der Waals and resonance interactions between accelerated atoms in vacuum and the Unruh effect
We discuss different physical effects related to the uniform acceleration of
atoms in vacuum, in the framework of quantum electrodynamics. We first
investigate the van der Waals/Casimir-Polder dispersion and resonance
interactions between two uniformly accelerated atoms in vacuum. We show that
the atomic acceleration significantly affects the van der Waals force, yielding
a different scaling of the interaction with the interatomic distance and an
explicit time dependence of the interaction energy. We argue how these results
could allow for an indirect detection of the Unruh effect through dispersion
interactions between atoms. We then consider the resonance interaction between
two accelerated atoms, prepared in a correlated Bell-type state, and
interacting with the electromagnetic field in the vacuum state, separating
vacuum fluctuations and radiation reaction contributions, both in the
free-space and in the presence of a perfectly reflecting plate. We show that
nonthermal effects of acceleration manifest in the resonance interaction,
yielding a change of the distance dependence of the resonance interaction
energy. This suggests that the equivalence between temperature and acceleration
does not apply to all radiative properties of accelerated atoms. To further
explore this aspect, we evaluate the resonance interaction between two atoms in
non inertial motion in the coaccelerated (Rindler) frame and show that in this
case the assumption of an Unruh temperature for the field is not required for a
complete equivalence of locally inertial and coaccelerated points of views.Comment: 8 pages, Proceedings of the Eighth International Workshop DICE 2016
Spacetime - Matter - Quantum Mechanic
Mean Escape Time in a System with Stochastic Volatility
We study the mean escape time in a market model with stochastic volatility.
The process followed by the volatility is the Cox Ingersoll and Ross process
which is widely used to model stock price fluctuations. The market model can be
considered as a generalization of the Heston model, where the geometric
Brownian motion is replaced by a random walk in the presence of a cubic
nonlinearity. We investigate the statistical properties of the escape time of
the returns, from a given interval, as a function of the three parameters of
the model. We find that the noise can have a stabilizing effect on the system,
as long as the global noise is not too high with respect to the effective
potential barrier experienced by a fictitious Brownian particle. We compare the
probability density function of the return escape times of the model with those
obtained from real market data. We find that they fit very well.Comment: 9 pages, 9 figures, to be published in Phys. Rev.
Casimir-Polder interatomic potential between two atoms at finite temperature and in the presence of boundary conditions
We evaluate the Casimir-Polder potential between two atoms in the presence of
an infinite perfectly conducting plate and at nonzero temperature. In order to
calculate the potential, we use a method based on equal-time spatial
correlations of the electric field, already used to evaluate the effect of
boundary conditions on interatomic potentials. This method gives also a
transparent physical picture of the role of a finite temperature and boundary
conditions on the Casimir-Polder potential. We obtain an analytical expression
of the potential both in the near and far zones, and consider several limiting
cases of interest, according to the values of the parameters involved, such as
atom-atom distance, atoms-wall distance and temperature.Comment: 11 page
Anomalous lack of decoherence of the Macroscopic Quantum Superpositions based on phase-covariant Quantum Cloning
We show that all Macroscopic Quantum Superpositions (MQS) based on
phase-covariant quantum cloning are characterized by an anomalous high
resilence to the de-coherence processes. The analysis supports the results of
recent MQS experiments and leads to conceive a useful conjecture regarding the
realization of complex decoherence - free structures for quantum information,
such as the quantum computer.Comment: 4 pages, 3 figure
Phase estimation via quantum interferometry for noisy detectors
The sensitivity in optical interferometry is strongly affected by losses
during the signal propagation or at the detection stage. The optimal quantum
states of the probing signals in the presence of loss were recently found.
However, in many cases of practical interest, their associated accuracy is
worse than the one obtainable without employing quantum resources (e.g.
entanglement and squeezing) but neglecting the detector's loss. Here we detail
an experiment that can reach the latter even in the presence of imperfect
detectors: it employs a phase-sensitive amplification of the signals after the
phase sensing, before the detection. We experimentally demonstrated the
feasibility of a phase estimation experiment able to reach its optimal working
regime. Since our method uses coherent states as input signals, it is a
practical technique that can be used for high-sensitivity interferometry and,
in contrast to the optimal strategies, does not require one to have an exact
characterization of the loss beforehand.Comment: 4 pages + supplementary information (10 pages), 3 + 4 figure
Noise driven translocation of short polymers in crowded solutions
In this work we study the noise induced effects on the dynamics of short
polymers crossing a potential barrier, in the presence of a metastable state.
An improved version of the Rouse model for a flexible polymer has been adopted
to mimic the molecular dynamics by taking into account both the interactions
between adjacent monomers and introducing a Lennard-Jones potential between all
beads. A bending recoil torque has also been included in our model. The polymer
dynamics is simulated in a two-dimensional domain by numerically solving the
Langevin equations of motion with a Gaussian uncorrelated noise. We find a
nonmonotonic behaviour of the mean first passage time and the most probable
translocation time, of the polymer centre of inertia, as a function of the
polymer length at low noise intensity. We show how thermal fluctuations
influence the motion of short polymers, by inducing two different regimes of
translocation in the molecule transport dynamics. In this context, the role
played by the length of the molecule in the translocation time is investigated.Comment: 11 pages, 3 figures, to appear in J. Stat. Mechanics: Theory and
Experiment, 200
Thermal noise suppression: how much does it cost?
In order to stabilize the behavior of noisy systems, confining it around a
desirable state, an effort is required to suppress the intrinsic noise. This
noise suppression task entails a cost. For the important case of thermal noise
in an overdamped system, we show that the minimum cost is achieved when the
system control parameters are held constant: any additional deterministic or
random modulation produces an increase of the cost. We discuss the implications
of this phenomenon for those overdamped systems whose control parameters are
intrinsically noisy, presenting a case study based on the example of a Brownian
particle optically trapped in an oscillating potential.Comment: 6 page
The problem of analytical calculation of barrier crossing characteristics for Levy flights
By using the backward fractional Fokker-Planck equation we investigate the
barrier crossing event in the presence of Levy noise. After shortly review
recent results obtained with different approaches on the time characteristics
of the barrier crossing, we derive a general differential equation useful to
calculate the nonlinear relaxation time. We obtain analytically the nonlinear
relaxation time for free Levy flights and a closed expression in quadrature of
the same characteristics for cubic potential.Comment: 12 pages, 2 figures, presented at 5th International Conference on
Unsolved Problems on Noise, Lyon, France, 2008, to appear in J. Stat. Mech.:
Theory and Experimen
Van der Waals Interactions in a Magneto-Dielectric Medium
The van der Waals interaction between two ground-state atoms is calculated
for two electrically or magnetically polarizable particles embedded in a
dispersive magneto-dielectric medium. Unlike previous calculations which infer
the atom-atom interaction from the dilute-medium limit of the macroscopic,
many-body van der Waals interaction, the interaction is calculated directly for
the system of two atoms in a magneto-dielectric medium. Two approaches are
presented, the first based on the quantized electromagnetic field in a
dispersive medium without absorption and the second on Green functions that
allow for absorption. We show that the correct van der Waals interactions are
obtained regardless of whether absorption in the host medium is taken into
account.Comment: 9 pages, 1 figur
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Cyber attacks, spillovers and contagion in the cryptocurrency markets
Copyright © 2021 The Author(s). This paper examines mean and volatility spillovers between three major cryptocurrencies (Bitcoin, Litecoin and Ethereum) and the role played by cyber-attacks. Specifically, trivariate GARCH-BEKK models are estimated which include suitably defined dummies corresponding to different types, targets and number per day of cyber-attacks. Significant dynamic linkages (interdependence) between the three cryptocurrencies under investigation are found in most cases when cyber-attacks are taken into account, Bitcoin appearing to be the dominant cryptocurrency. Further, Wald tests for parameter shifts during episodes of turbulence resulting from cyber-attacks provide evidence that the latter affect the transmission mechanism between cryptocurrency returns and volatilities (contagion). More precisely, cyber-attacks appear to strengthen cross-market linkages, thereby reducing portfolio diversification opportunities for cryptocurrency investors. Finally, the conditional correlation analysis confirms the previous findings
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