105,877 research outputs found
Hypercyclic operators on countably dimensional spaces
According to Grivaux, the group of invertible linear operators on a
separable infinite dimensional Banach space acts transitively on the set
of countable dense linearly independent subsets of . As a
consequence, each is an orbit of a hypercyclic operator on
. Furthermore, every countably dimensional normed space supports a
hypercyclic operator.
We show that for a separable infinite dimensional Fr\'echet space ,
acts transitively on if and only if possesses a
continuous norm. We also prove that every countably dimensional metrizable
locally convex space supports a hypercyclic operator
Dynamics of Nanometer-Scale Foil Targets Irradiated with Relativistically Intense Laser Pulses
In this letter we report on an experimental study of high harmonic radiation
generated in nanometer-scale foil targets irradiated under normal incidence.
The experiments constitute the first unambiguous observation of odd-numbered
relativistic harmonics generated by the component of the
Lorentz force verifying a long predicted property of solid target harmonics.
Simultaneously the observed harmonic spectra allow in-situ extraction of the
target density in an experimental scenario which is of utmost interest for
applications such as ion acceleration by the radiation pressure of an
ultraintense laser.Comment: 5 pages, 4 figure
Supernovae and radio transients in M 82
We present optical and near-infrared (IR) photometry and near-IR spectroscopy
of SN 2004am, the only optically detected supernova (SN) in M 82. These
demonstrate that SN 2004am was a highly reddened type II-P SN similar to the
low luminosity type II-P events such as SNe 1997D and 2005cs. We show that SN
2004am was located coincident with the obscured super star cluster M 82-L, and
from the cluster age infer a progenitor mass of 12 +7/-3 Msun. In addition to
this, we present a high spatial resolution Gemini-N K-band adaptive optics
image of the site of SN 2008iz and a second transient of uncertain nature, both
detected so far only at radio wavelengths. Using image subtraction techniques
together with archival data from the Hubble Space Telescope, we are able to
recover a near-IR transient source co-incident with both objects. We find the
likely extinction towards SN 2008iz to be not more than Av ~ 10. The nature of
the second transient remains elusive and we regard an extremely bright
microquasar in M 82 as the most plausible scenario.Comment: 14 pages, 8 figures, accepted for publication in MNRA
An ultraviolet-optical flare from the tidal disruption of a helium-rich stellar core
The flare of radiation from the tidal disruption and accretion of a star can
be used as a marker for supermassive black holes that otherwise lie dormant and
undetected in the centres of distant galaxies. Previous candidate flares have
had declining light curves in good agreement with expectations, but with poor
constraints on the time of disruption and the type of star disrupted, because
the rising emission was not observed. Recently, two `relativistic' candidate
tidal disruption events were discovered, each of whose extreme X-ray luminosity
and synchrotron radio emission were interpreted as the onset of emission from a
relativistic jet. Here we report the discovery of a luminous
ultraviolet-optical flare from the nuclear region of an inactive galaxy at a
redshift of 0.1696. The observed continuum is cooler than expected for a simple
accreting debris disk, but the well-sampled rise and decline of its light curve
follows the predicted mass accretion rate, and can be modelled to determine the
time of disruption to an accuracy of two days. The black hole has a mass of
about 2 million solar masses, modulo a factor dependent on the mass and radius
of the star disrupted. On the basis of the spectroscopic signature of ionized
helium from the unbound debris, we determine that the disrupted star was a
helium-rich stellar core.Comment: To appear in Nature on May 10, 201
Probing magnetic order in ultracold lattice gases
A forthcoming challenge in ultracold lattice gases is the simulation of
quantum magnetism. That involves both the preparation of the lattice atomic gas
in the desired spin state and the probing of the state. Here we demonstrate how
a probing scheme based on atom-light interfaces gives access to the order
parameters of nontrivial quantum magnetic phases, allowing us to characterize
univocally strongly correlated magnetic systems produced in ultracold gases.
This method, which is also nondemolishing, yields spatially resolved spin
correlations and can be applied to bosons or fermions. As a proof of principle,
we apply this method to detect the complete phase diagram displayed by a chain
of (rotationally invariant) spin-1 bosons.Comment: published versio
D-outcome measurement for a nonlocality test
For the purpose of the nonlocality test, we propose a general correlation
observable of two parties by utilizing local -outcome measurements with
SU() transformations and classical communications. Generic symmetries of the
SU() transformations and correlation observables are found for the test of
nonlocality. It is shown that these symmetries dramatically reduce the number
of numerical variables, which is important for numerical analysis of
nonlocality. A linear combination of the correlation observables, which is
reduced to the Clauser-Horne-Shimony-Holt (CHSH) Bell's inequality for two
outcome measurements, is led to the Collins-Gisin-Linden-Massar-Popescu (CGLMP)
nonlocality test for -outcome measurement. As a system to be tested for its
nonlocality, we investigate a continuous-variable (CV) entangled state with
measurement outcomes. It allows the comparison of nonlocality based on
different numbers of measurement outcomes on one physical system. In our
example of the CV state, we find that a pure entangled state of any degree
violates Bell's inequality for measurement outcomes when the
observables are of SU() transformations.Comment: 16 pages, 2 figure
Enhanced dynamical entanglement transfer with multiple qubits
We present two strategies to enhance the dynamical entanglement transfer from
continuous variable (CV) to finite dimensional systems by employing multiple
qubits. First, we consider the entanglement transfer to a composite finite
dimensional system of many qubits simultaneously interacting with a bipartite
CV field. We show that, considering realistic conditions in the generation of
CV entanglement, a small number of qubits resonantly coupled to the CV system
is sufficient for an almost complete dynamical transfer of the entanglement.
Our analysis also sheds further light on the transition between microscopic and
macroscopic behaviours of composite finite dimensional systems coupled to
bosonic fields (like atomic clouds interacting with light). Furthermore, we
present a protocol based on sequential interactions of the CV system with some
ancillary qubit systems and on subsequent measurements, allowing to
probabilistically convert CV entanglement into `almost perfect' Bell pairs of
two qubits. Our proposals are suited for realizations in various experimental
settings, ranging from cavity-QED to cavity-integrated superconducting devices.Comment: 10 pages, 8 figures, RevTeX4; terminology revised; accepted for
publicatio
Tuning non-Markovianity by spin-dynamics control
We study the interplay between forgetful and memory-keeping evolution
enforced on a two-level system by a multi-spin environment whose elements are
coupled to local bosonic baths. Contrarily to the expectation that any
non-Markovian effect would be buried by the forgetful mechanism induced by the
spin-bath coupling, one can actually induce a full Markovian-to-non-Markovian
transition of the two-level system's dynamics, controllable by parameters such
as the mismatch between the energy of the two-level system and of the spin
environment. For a symmetric coupling, the amount of non-Markovianity
surprisingly grows with the number of decoherence channels.Comment: 7 pages, 6 figures, PRA versio
A frequency measure robust to linear filtering
A definition of frequency (cycles per unit-time) based on an approximate
reconstruction of the phase-space trajectory of an oscillator from a signal is
introduced. It is shown to be invariant under linear filtering, and therefore
inaccessible by spectral methods. The effect of filtering on frequency in cases
where this definition does not perfectly apply is quantified.Comment: 10 pages, 2 figure
Free energy and molecular dynamics calculations for the cubic-tetragonal phase transition in zirconia
The high-temperature cubic-tetragonal phase transition of pure stoichiometric
zirconia is studied by molecular dynamics (MD) simulations and within the
framework of the Landau theory of phase transformations. The interatomic forces
are calculated using an empirical, self-consistent, orthogonal tight-binding
(SC-TB) model, which includes atomic polarizabilities up to the quadrupolar
level. A first set of standard MD calculations shows that, on increasing
temperature, one particular vibrational frequency softens. The temperature
evolution of the free energy surfaces around the phase transition is then
studied with a second set of calculations. These combine the thermodynamic
integration technique with constrained MD simulations. The results seem to
support the thesis of a second-order phase transition but with unusual, very
anharmonic behaviour above the transition temperature
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