5,237 research outputs found
Stable retrograde orbits around the triple system 2001 SN263
The NEA 2001 SN263 is the target of the ASTER MISSION - First Brazilian Deep
Space Mission. Araujo et al. (2012), characterized the stable regions around
the components of the triple system for the planar and prograde cases. Knowing
that the retrograde orbits are expected to be more stable, here we present a
complementary study. We now considered particles orbiting the components of the
system, in the internal and external regions, with relative inclinations
between , i.e., particles with retrograde
orbits. Our goal is to characterize the stable regions of the system for
retrograde orbits, and then detach a preferred region to place the space probe.
For a space mission, the most interesting regions would be those that are
unstable for the prograde cases, but stable for the retrograde cases. Such
configuration provide a stable region to place the mission probe with a
relative retrograde orbit, and, at the same time, guarantees a region free of
debris since they are expected to have prograde orbits. We found that in fact
the internal and external stable regions significantly increase when compared
to the prograde case. For particles with and , we found
that nearly the whole region around Alpha and Beta remain stable. We then
identified three internal regions and one external region that are very
interesting to place the space probe. We present the stable regions found for
the retrograde case and a discussion on those preferred regions. We also
discuss the effects of resonances of the particles with Beta and Gamma, and the
role of the Kozai mechanism in this scenario. These results help us understand
and characterize the stability of the triple system 2001 SN263 when retrograde
orbits are considered, and provide important parameters to the design of the
ASTER mission.Comment: 11 pages, 8 figures. Accepted for publication in MNRAS - 2015 March
1
Bilinear and quadratic Hamiltonians in two-mode cavity quantum electrodynamics
In this work we show how to engineer bilinear and quadratic Hamiltonians in
cavity quantum electrodynamics (QED) through the interaction of a single driven
two-level atom with cavity modes. The validity of the engineered Hamiltonians
is numerically analyzed even considering the effects of both dissipative
mechanisms, the cavity field and the atom. The present scheme can be used, in
both optical and microwave regimes, for quantum state preparation, the
implementation of quantum logical operations, and fundamental tests of quantum
theory.Comment: 11 pages, 3 figure
Generation of decoherence-free displaced squeezed states of radiation fields and a squeezed reservoir for atoms in cavity QED
We present a way to engineer an effective anti-Jaynes-Cumming and a
Jaynes-Cumming interaction between an atomic system and a single cavity mode
and show how to employ it in reservoir engineering processes. To construct the
effective Hamiltonian, we analyse considered the interaction of an atomic
system in a \{Lambda} configuration, driven by classical fields, with a single
cavity mode. With this interaction, we firstly show how to generate a
decoherence-free displaced squeezed state for the cavity field. In our scheme,
an atomic beam works as a reservoir for the radiation field trapped inside the
cavity, as employed recently by S. Pielawa et al. [Phys. Rev. Lett. 98, 240401
(2007)] to generate an Einstein-Podolsky-Rosen entangled radiation state in
high-Q resonators. In our scheme, all the atoms have to be prepared in the
ground state and, as in the cited article, neither atomic detection nor precise
interaction times between the atoms and the cavity mode are required. From this
same interaction, we can also generate an ideal squeezed reservoir for atomic
systems. For this purpose we have to assume, besides the engineered atom-field
interaction, a strong decay of the cavity field (i.e., the cavity decay must be
much stronger than the effective atom-field coupling). With this scheme, some
interesting effects in the dynamics of an atom in a squeezed reservoir could be
tested
Nonadiabatic coherent evolution of two-level systems under spontaneous decay
In this paper we extend current perspectives in engineering reservoirs by
producing a time-dependent master equation leading to a nonstationary
superposition equilibrium state that can be nonadiabatically controlled by the
system-reservoir parameters. Working with an ion trapped inside a nonindeal
cavity we first engineer effective Hamiltonians that couple the electronic
states of the ion with the cavity mode. Subsequently, two classes of
decoherence-free evolution of the superposition of the ground and decaying
excited levels are achieved: those with time-dependent azimuthal or polar
angle. As an application, we generalise the purpose of an earlier study [Phys.
Rev. Lett. 96, 150403 (2006)], showing how to observe the geometric phases
acquired by the protected nonstationary states even under a nonadiabatic
evolution.Comment: 5 pages, no figure
Collision and Stable Regions around Bodies with Simple Geometric Shape
We show the expressions of the gravitational potential of homogeneous bodies with well-defined simple geometric shapes to study the phase space of trajectories around these bodies. The potentials of the rectangular and triangular plates are presented. With these expressions we study the phase space of trajectories of a point of mass around the plates, using the Poincaré surface of section technique. We determined the location and the size of the stable and collision regions in the phase space, and the identification of some resonances. This work is the first and an important step for others studies, considering 3D bodies. The study of the behavior of a point of mass orbiting around these plates (2D), near their corners, can be used as a parameter to understand the influence of the gravitational potential when the particle is close to an irregular surface, such as large craters and ridges
Using quantum state protection via dissipation in a quantum-dot molecule to solve the Deutsch problem
The wide set of control parameters and reduced size scale make semiconductor
quantum dots attractive candidates to implement solid-state quantum
computation. Considering an asymmetric double quantum dot coupled by tunneling,
we combine the action of a laser field and the spontaneous emission of the
excitonic state to protect an arbitrary superposition state of the indirect
exciton and ground state. As a by-product we show how to use the protected
state to solve the Deutsch problem.Comment: 8 pages, 1 figure, 2 table
Using low Lift-to-Drag spacecraft to perform upper atmospheric Aero-Gravity Assisted Maneuvers
The Gravity Assisted Maneuver has been applied in lots of space missions, to
change the spacecraft heliocentric velocity vector and the geometry of the
orbit, after the close approach to a celestial body, saving propellant
consumption. It is possible to take advantage of additional forces to improve
the maneuver, like the forces generated by the spacecraft-atmosphere
interaction and/or propulsion systems; reducing the time of flight and the need
for multiple passages around secondary bodies. However, these applications
require improvements in critical subsystems, which are necessary to accomplish
the mission. In this paper, a few combinations of the Gravity-Assist were
classified, including maneuvers with thrust and aerodynamic forces; presenting
the advantages and limitations of these variations. There are analyzed the
effects of implementing low Lift-to-Drag ratios at high altitudes for
Aero-gravity Assist maneuvers, with and without propulsion. The maneuvers were
simulated for Venus and Mars, due to their relevance in interplanetary
missions, the interest in exploration, and the knowledge about their
atmospheres. The Aero-gravity Assist maneuver with low Lift-to-Drag ratios at
high altitudes shows an increase of more than 10{\deg} in the turn angle for
Venus and 2.5{\deg} for Mars. The maneuvers increase the energy gains by more
than 15% when compared to the Gravity-Assist. From the Technology Readiness
Levels, it was observed that the current level of development of the space
technology makes feasible the application of Aero-gravity Assisted Maneuvers at
high altitudes in short term
The use of body weight support on ground level: an alternative strategy for gait training of individuals with stroke
<p>Abstract</p> <p>Background</p> <p>Body weight support (BWS) systems on treadmill have been proposed as a strategy for gait training of subjects with stroke. Considering that ground level is the most common locomotion surface and that there is little information about individuals with stroke walking with BWS on ground level, it is important to investigate the use of BWS on ground level in these individuals as a possible alternative strategy for gait training.</p> <p>Methods</p> <p>Thirteen individuals with chronic stroke (four women and nine men; mean age 54.46 years) were videotaped walking on ground level in three experimental conditions: with no harness, with harness bearing full body weight, and with harness bearing 30% of full body weight. Measurements were recorded for mean walking speed, cadence, stride length, stride speed, durations of initial and terminal double stance, single limb support, swing period, and range of motion of ankle, knee, and hip joints; and foot, shank, thigh, and trunk segments.</p> <p>Results</p> <p>The use of BWS system leads to changes in stride length and speed, but not in stance and swing period duration. Only the hip joint was influenced by the BWS system in the 30% BWS condition. Shank and thigh segments presented less range of motion in the 30% BWS condition than in the other conditions, and the trunk was held straighter in the 30% BWS condition than in the other conditions.</p> <p>Conclusion</p> <p>Individuals with stroke using BWS system on ground level walked slower and with shorter stride length than with no harness. BWS also led to reduction of hip, shank, and thigh range of motion. However, this system did not change walking temporal organization and body side asymmetry of individuals with stroke. On the other hand, the BWS system enabled individuals with chronic stroke to walk safely and without physical assistance. In interventions, the physical therapist can watch and correct gait pattern in patients' performance without the need to provide physical assistance.</p
- …