16,743 research outputs found
On the heating of source of the Orion KL hot core
We present images of the J=10-9 rotational lines of HC3N in the vibrationally
excited levels 1v7, 1v6 and 1v5 of the hot core (HC) in Orion KL. The images
show that the spatial distribution and the size emission from the 1v7 and 1v5
levels are different. While the J=10-9 1v7 line has a size of 4''x 6'' and
peaks 1.1'' NE of the 3 mm continuum peak, the J=10--9 1v5 line emission is
unresolved (<3'') and peaks 1.3'' south of the 3 mm peak. This is a clear
indication that the HC is composed of condensations with very different
temperatures (170 K for the 1v7 peak and K for the 1v5 peak). The
temperature derived from the 1v7 and 1v5 lines increases with the projected
distance to the suspected main heating source I. Projection effects along the
line of sight could explain the temperature gradient as produced by source I.
However, the large luminosity required for source I, >5 10^5 Lsolar, to explain
the 1v5 line suggests that external heating by this source may not dominate the
heating of the HC. Simple model calculations of the vibrationally excited
emission indicate that the HC can be internally heated by a source with a
luminosity of 10^5 Lsolar, located 1.2'' SW of the 1v5 line peak (1.8'' south
of source I). We also report the first detection of high-velocity gas from
vibrationally excited HC3N emission. Based on excitation arguments we conclude
that the main heating source is also driving the molecular outflow. We
speculate that all the data presented in this letter and the IR images are
consistent with a young massive protostar embedded in an edge-on disk.Comment: 13 pages, 3 figures, To be published in Ap.J. Letter
Speed limits for quantum gates in multi-qubit systems
We use analytical and numerical calculations to obtain speed limits for
various unitary quantum operations in multiqubit systems under typical
experimental conditions. The operations that we consider include single-, two-,
and three-qubit gates, as well as quantum-state transfer in a chain of qubits.
We find in particular that simple methods for implementing two-qubit gates
generally provide the fastest possible implementations of these gates. We also
find that the three-qubit Toffoli gate time varies greatly depending on the
type of interactions and the system's geometry, taking only slightly longer
than a two-qubit controlled-NOT (CNOT) gate for a triangle geometry. The speed
limit for quantum-state transfer across a qubit chain is set by the maximum
spin-wave speed in the chain.Comment: 7 pages (two-column), 2 figures, 2 table
A simple preconditioner for a discontinuous Galerkin method for the Stokes problem
In this paper we construct Discontinuous Galerkin approximations of the
Stokes problem where the velocity field is H(div)-conforming. This implies that
the velocity solution is divergence-free in the whole domain. This property can
be exploited to design a simple and effective preconditioner for the final
linear system.Comment: 27 pages, 4 figure
Majorana fermions in pinned vortices
Exploiting the peculiar properties of proximity-induced superconductivity on
the surface of a topological insulator, we propose a device which allows the
creation of a Majorana fermion inside the core of a pinned Abrikosov vortex.
The relevant Bogolyubov-de Gennes equations are studied analytically. We
demonstrate that in this system the zero-energy Majorana fermion state is
separated by a large energy gap, of the order of the zero-temperature
superconducting gap , from a band of single-particle non-topological
excitations. In other words, the Majorana fermion remains robust against
thermal fluctuations, as long as the temperature remains substantially lower
than the critical superconducting temperature. Experimentally, the Majorana
state may be detected by measuring the tunneling differential conductance at
the center of the Abrikosov vortex. In such an experiment, the Majorana state
manifests itself as a zero-bias anomaly separated by a gap, of the order of
, from the contributions of the nontopological excitations.Comment: 9 pages, 2 eps figures, new references are added, several typos are
correcte
Hindcast and forecast of the Parsifal storm
On 2 November 1995 a Mistral storm in the Gulf of Lions sank the 16 metre yacht Parsifal claiming six lives out of the nine member crew. We analyse the storm with different meteorological and wave models, verifying the results against the available buoy and satellite measurements. Then we consider the accuracy of the storm forecasts and the information available the days before the accident. The limitations related to the resolution of the meteorological models are explored by hindcasting the storm also with the winds produced by some limited area models. Finally, we discuss the present situation of wind and wave hindcast and forecast in theMediterranean Sea, and the distribution of these results to the public
Hindcast and forecast of the Parsifal storm
On 2 November 1995 a Mistral storm in the Gulf of Lions sank the 16 metre yacht Parsifal claiming six lives out of the nine member crew. We analyse the storm with different meteorological and wave models, verifying the results against the available buoy and satellite measurements. Then we consider the accuracy of the storm forecasts and the information available the days before the accident. The limitations related to the resolution of the meteorological models are explored by hindcasting the storm also with the winds produced by some limited area models. Finally, we discuss the present situation of wind and wave hindcast and forecast in theMediterranean Sea, and the distribution of these results to the public
Effects of periodic potentials on the critical velocity of superfluid Fermi gases in the BCS-BEC crossover
We study the effects of an external periodic potential on the critical
velocity of a superfluid Fermi gas in the crossover between the
Bardeen-Cooper-Schrieffer (BCS) phase and Bose-Einstein condensation (BEC). We
numerically solve the Bogoliubov-de Gennes equations to model a
three-dimensional (3D) gas of ultracold atoms in the superfluid phase flowing
through a 1D optical lattice. We find that when the recoil energy is comparable
to the Fermi energy, the presence of the periodic potential reduces the effect
of pair-breaking excitations. This behavior is a consequence of the peculiar
band structure of the quasiparticle energy spectrum in the lattice. When the
lattice height is much larger than the Fermi energy, the periodic potential
makes pairs of atoms to be strongly bound even in the BCS regime and
pair-breaking excitations are further suppressed. We have also found that when
the recoil energy is comparable to or larger than the Fermi energy, the
critical velocity due to long-wavelength phonon excitations shows a
non-monotonic behavior along the BCS-BEC crossover.Comment: 9 pages, 7 figures, added an appendix on the dynamical instabilit
Selective darkening of degenerate transitions for implementing quantum controlled-NOT gates
We present a theoretical analysis of the selective darkening method for
implementing quantum controlled-NOT (CNOT) gates. This method, which we
recently proposed and demonstrated, consists of driving two
transversely-coupled quantum bits (qubits) with a driving field that is
resonant with one of the two qubits. For specific relative amplitudes and
phases of the driving field felt by the two qubits, one of the two transitions
in the degenerate pair is darkened, or in other words, becomes forbidden by
effective selection rules. At these driving conditions, the evolution of the
two-qubit state realizes a CNOT gate. The gate speed is found to be limited
only by the coupling energy J, which is the fundamental speed limit for any
entangling gate. Numerical simulations show that at gate speeds corresponding
to 0.48J and 0.07J, the gate fidelity is 99% and 99.99%, respectively, and
increases further for lower gate speeds. In addition, the effect of
higher-lying energy levels and weak anharmonicity is studied, as well as the
scalability of the method to systems of multiple qubits. We conclude that in
all these respects this method is competitive with existing schemes for
creating entanglement, with the added advantages of being applicable for qubits
operating at fixed frequencies (either by design or for exploitation of
coherence sweet-spots) and having the simplicity of microwave-only operation.Comment: 25 pages, 5 figure
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