26,299 research outputs found
A wall interference assessment/correction system
The Hackett method (a Wall Pressure Signature Method) was selected to be adapted for the 12 ft Wind Tunnel WIAC system. This method uses limited measurements of the static pressure at the wall, in conjunction with the solid wall boundary condition, to determine the strength and distribution of singularities representing the test article. The singularities are used in term for estimating wall interference at the model location. Hackett's method will have to be formulated for application to the unique geometry of the 12 ft tunnel. The WIAC code will be validated by conducting numerically simulated experiments rather than actual wind tunnel experiments. The simulations will be used to generate both free air and confined wind tunnel flow fields for each of the test articles over a range of test configurations. Specifically the pressure signature at the test section wall will be computed for the confined case to provide the simulated 'measured' data. These data will serve as the input for the WIAC method. The performance of the WIAC method then may be evaluated by comparing the corrected parameters with those for the free air simulation
Field-induced structure transformation in electrorheological solids
We have computed the local electric field in a body-centered tetragonal (BCT)
lattice of point dipoles via the Ewald-Kornfeld formulation, in an attempt to
examine the effects of a structure transformation on the local field strength.
For the ground state of an electrorheological solid of hard spheres, we
identified a novel structure transformation from the BCT to the face-centered
cubic (FCC) lattices by changing the uniaxial lattice constant c under the hard
sphere constraint. In contrast to the previous results, the local field
exhibits a non-monotonic transition from BCT to FCC. As c increases from the
BCT ground state, the local field initially decreases rapidly towards the
isotropic value at the body-centered cubic lattice, decreases further, reaching
a minimum value and increases, passing through the isotropic value again at an
intermediate lattice, reaches a maximum value and finally decreases to the FCC
value. An experimental realization of the structure transformation is
suggested. Moreover, the change in the local field can lead to a generalized
Clausius-Mossotti equation for the BCT lattices.Comment: Submitted to Phys. Rev.
Heisenberg-picture approach to the exact quantum motion of a time-dependent forced harmonic oscillator
In the Heisenberg picture, the generalized invariant and exact quantum
motions are found for a time-dependent forced harmonic oscillator. We find the
eigenstate and the coherent state of the invariant and show that the
dispersions of these quantum states do not depend on the external force. Our
formalism is applied to several interesting cases.Comment: 15 pages, two eps files, to appear in Phys. Rev. A 53 (6) (1996
Charge dynamics and spin blockade in a hybrid double quantum dot in silicon
Electron spin qubits in silicon, whether in quantum dots or in donor atoms,
have long been considered attractive qubits for the implementation of a quantum
computer due to the semiconductor vacuum character of silicon and its
compatibility with the microelectronics industry. While donor electron spins in
silicon provide extremely long coherence times and access to the nuclear spin
via the hyperfine interaction, quantum dots have the complementary advantages
of fast electrical operations, tunability and scalability. Here we present an
approach to a novel hybrid double quantum dot by coupling a donor to a
lithographically patterned artificial atom. Using gate-based rf reflectometry,
we probe the charge stability of this double quantum dot system and the
variation of quantum capacitance at the interdot charge transition. Using
microwave spectroscopy, we find a tunnel coupling of 2.7 GHz and characterise
the charge dynamics, which reveals a charge T2* of 200 ps and a relaxation time
T1 of 100 ns. Additionally, we demonstrate spin blockade at the inderdot
transition, opening up the possibility to operate this coupled system as a
singlet-triplet qubit or to transfer a coherent spin state between the quantum
dot and the donor electron and nucleus.Comment: 6 pages, 4 figures, supplementary information (3 pages, 4 figures
The Megamaser Cosmology Project. X. High Resolution Maps and Mass Constraint for SMBHs
We present high resolution (sub-mas) VLBI maps of nuclear H2O megamasers for
seven galaxies. In UGC6093, the well-aligned systemic masers and high-velocity
masers originate in an edge-on, flat disk and we determine the mass of the
central SMBH to be M_SMBH = 2.58*10^7Msun(+-7%). For J1346+5228, the
distribution of masers is consistent with a disk, but the faint high-velocity
masers are only marginally detected, and we constrain the mass of the SMBH to
be in the range 1.5-2.0*10^7Msun. The origin of the masers in Mrk1210 is less
clear, as the systemic and high-velocity masers are misaligned and show a
disorganized velocity structure. We present one possible model in which the
masers originate in a tilted, warped disk, but we do not rule out the
possibility of other explanations including outflow masers. In NGC6926, we
detect a set of redshifted masers, clustered within a pc of each other, and a
single blueshifted maser about 4.4pc away, an offset that would be unusually
large for a maser disk system. Nevertheless, if it is a disk system, we
estimate the enclosed mass to be M_SMBH<4.8*10^7 Msun . For NGC5793, we detect
redshifted masers spaced about 1.4pc from a clustered set of blueshifted
features. The orientation of the structure supports a disk scenario as
suggested by Hagiwara et al.(2001). We estimate the enclosed mass to be M
SMBH<1.3*10^7 Msun. For NGC2824 and J0350-0127, the masers may be associated
with pc or sub-pc scale jets or outflows.Comment: Accepted by Ap
Security Analysis of an Untrusted Source for Quantum Key Distribution: Passive Approach
We present a passive approach to the security analysis of quantum key
distribution (QKD) with an untrusted source. A complete proof of its
unconditional security is also presented. This scheme has significant
advantages in real-life implementations as it does not require fast optical
switching or a quantum random number generator. The essential idea is to use a
beam splitter to split each input pulse. We show that we can characterize the
source using a cross-estimate technique without active routing of each pulse.
We have derived analytical expressions for the passive estimation scheme.
Moreover, using simulations, we have considered four real-life imperfections:
Additional loss introduced by the "plug & play" structure, inefficiency of the
intensity monitor, noise of the intensity monitor, and statistical fluctuation
introduced by finite data size. Our simulation results show that the passive
estimate of an untrusted source remains useful in practice, despite these four
imperfections. Also, we have performed preliminary experiments, confirming the
utility of our proposal in real-life applications. Our proposal makes it
possible to implement the "plug & play" QKD with the security guaranteed, while
keeping the implementation practical.Comment: 35 pages, 19 figures. Published Versio
The HARPS search for southern extra-solar planets. XXVII. Up to seven planets orbiting HD 10180: probing the architecture of low-mass planetary systems
Context. Low-mass extrasolar planets are presently being discovered at an
increased pace by radial velocity and transit surveys, opening a new window on
planetary systems. Aims. We are conducting a high-precision radial velocity
survey with the HARPS spectrograph which aims at characterizing the population
of ice giants and super-Earths around nearby solar-type stars. This will lead
to a better understanding of their formation and evolution, and yield a global
picture of planetary systems from gas giants down to telluric planets. Methods.
Progress has been possible in this field thanks in particular to the sub-m/s
radial velocity precision achieved by HARPS. We present here new high-quality
measurements from this instrument. Results. We report the discovery of a
planetary system comprising at least five Neptune-like planets with minimum
masses ranging from 12 to 25 M_Earth, orbiting the solar-type star HD 10180 at
separations between 0.06 and 1.4 AU. A sixth radial velocity signal is present
at a longer period, probably due to a 65-M_Earth object. Moreover, another body
with a minimum mass as low as 1.4 M_Earth may be present at 0.02 AU from the
star. This is the most populated exoplanetary system known to date. The planets
are in a dense but still well-separated configuration, with significant secular
interactions. Some of the orbital period ratios are fairly close to integer or
half-integer values, but the system does not exhibit any mean-motion
resonances. General relativity effects and tidal dissipation play an important
role to stabilize the innermost planet and the system as a whole. Numerical
integrations show long-term dynamical stability provided true masses are within
a factor ~3 from minimum masses. We further note that several low-mass
planetary systems exhibit a rather "packed" orbital architecture with little or
no space left for additional planets. (Abridged)Comment: 20 pages, 15 figures, accepted for publication in A&
Quantum models related to fouled Hamiltonians of the harmonic oscillator
We study a pair of canonoid (fouled) Hamiltonians of the harmonic oscillator
which provide, at the classical level, the same equation of motion as the
conventional Hamiltonian. These Hamiltonians, say and , result
to be explicitly time-dependent and can be expressed as a formal rotation of
two cubic polynomial functions, and , of the canonical variables
(q,p).
We investigate the role of these fouled Hamiltonians at the quantum level.
Adopting a canonical quantization procedure, we construct some quantum models
and analyze the related eigenvalue equations. One of these models is described
by a Hamiltonian admitting infinite self-adjoint extensions, each of them has a
discrete spectrum on the real line. A self-adjoint extension is fixed by
choosing the spectral parameter of the associated eigenvalue
equation equal to zero. The spectral problem is discussed in the context of
three different representations. For , the eigenvalue equation is
exactly solved in all these representations, in which square-integrable
solutions are explicity found. A set of constants of motion corresponding to
these quantum models is also obtained. Furthermore, the algebraic structure
underlying the quantum models is explored. This turns out to be a nonlinear
(quadratic) algebra, which could be applied for the determination of
approximate solutions to the eigenvalue equations.Comment: 24 pages, no figures, accepted for publication on JM
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