3,639 research outputs found
Critical decay index at the onset of solar eruptions
Magnetic flux ropes are topological structures consisting of twisted magnetic
field lines that globally wrap around an axis. The torus instability model
predicts that a magnetic flux rope of major radius undergoes an eruption
when its axis reaches a location where the decay index of the ambient magnetic field is larger than a critical value. In
the current-wire model, the critical value depends on the thickness and
time-evolution of the current channel. We use magneto-hydrodynamic (MHD)
simulations to investigate if the critical value of the decay index at the
onset of the eruption is affected by the magnetic flux rope's internal current
profile and/or by the particular pre-eruptive photospheric dynamics. The
evolution of an asymmetric, bipolar active region is driven by applying
different classes of photospheric motions. We find that the critical value of
the decay index at the onset of the eruption is not significantly affected by
either the pre-eruptive photospheric evolution of the active region or by the
resulting different magnetic flux ropes. As in the case of the current-wire
model, we find that there is a `critical range' , rather than a
`critical value' for the onset of the torus instability. This range is in good
agreement with the predictions of the current-wire model, despite the inclusion
of line-tying effects and the occurrence of tether-cutting magnetic
reconnection.Comment: 15 pages, 9 figures. To appear in The Astrophysical Journa
Influence of anneal atmosphere on ZnO-nanorod photoluminescent and morphological properties with self-powered photodetector performance
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Quantum computation with optical coherent states
We show that quantum computation circuits using coherent states as the
logical qubits can be constructed from simple linear networks, conditional
photon measurements and "small" coherent superposition resource states
Improving the entanglement transfer from continuous variable systems to localized qubits using non Gaussian states
We investigate the entanglement transfer from a bipartite continuous-variable
(CV) system to a pair of localized qubits assuming that each CV mode couples to
one qubit via the off-resonance Jaynes-Cummings interaction with different
interaction times for the two subsystems. First, we consider the case of the CV
system prepared in a Bell-like superposition and investigate the conditions for
maximum entanglement transfer. Then we analyze the general case of two-mode CV
states that can be represented by a Schmidt decomposition in the Fock number
basis. This class includes both Gaussian and non Gaussian CV states, as for
example twin-beam (TWB) and pair-coherent (TMC, also known as two-mode-coher
ent) states respectively. Under resonance conditions, equal interaction times
for both qubits and different initial preparations, we find that the
entanglement transfer is more efficient for TMC than for TWB states. In the
perspective of applications such as in cavity QED or with superconducting
qubits, we analyze in details the effects of off-resonance interactions
(detuning) and different interaction times for the two qubits, and discuss
conditions to preserve the entanglement transfer.Comment: revised version, 11 pages, 7 figures (few of them low-res
Distance measures to compare real and ideal quantum processes
With growing success in experimental implementations it is critical to
identify a "gold standard" for quantum information processing, a single measure
of distance that can be used to compare and contrast different experiments. We
enumerate a set of criteria such a distance measure must satisfy to be both
experimentally and theoretically meaningful. We then assess a wide range of
possible measures against these criteria, before making a recommendation as to
the best measures to use in characterizing quantum information processing.Comment: 15 pages; this version in line with published versio
A Check on the Validity of Magnetic Field Reconstructions
We investigate a method to test whether a numerically computed model coronal magnetic field B departs from the divergence-free condition (also known as the solenoidality condition). The test requires a potential field B0 to be calculated, subject to Neumann boundary conditions, given by the normal components of the model field B at the boundaries. The free energy of the model field may be calculated using the volume integral of (B-B0)^2, where the integral is over the computational volume of the model field. A second estimate of the free energy is provided by calculating the difference between the volume integral of B^2 and the volume integral of B0^2. If B is divergence-free, the two estimates of the free energy should be the same. A difference between the two estimates indicates a departure from div B = 0 in the volume. The test is an implementation of a procedure proposed by Moraitis et al. (Sol. Phys. 289, 4453, 2014) and is a simpler version of the Helmholtz decomposition procedure presented by Valori et al. (Astron. Astrophys. 553, A38, 2013). We demonstrate the test in application to previously published nonlinear force-free model fields, and also investigate the influence on the results of the test of a departure from flux balance over the boundaries of the model field. Our results underline the fact that, to make meaningful statements about magnetic free energy in the corona, it is necessary to have model magnetic fields which satisfy the divergence-free condition to a good approximation.Australian Research Counci
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