384 research outputs found
The antenna DSA 3 and its potential use for Radio Astronomy
The European Space Agency (ESA) will inaugurate its third Deep Space Antenna
(DSA 3) by the end of 2012. DSA 3 will be located in Argentina near the city of
Malarg"ue in the Mendoza province. While the instrument will be primarily
dedicated to communications with interplanetary missions, the characteristics
of its antenna and receivers will also enable standalone leading scientific
contributions, with a high scientific-technological return. We outline here
scientific proposals for a radio astronomical use of DSA 3.Comment: 4 pages, submitted as Proceedings for the BAA
Microcavity phonoritons -- a coherent optical-to-microwave interface
Optomechanical systems provide a pathway for the bidirectional
optical-to-microwave interconversion in (quantum) networks. We demonstrate the
implementation of this functionality and non-adiabatic optomechanical control
in a single, m-sized potential trap for phonons and exciton-polariton
condensates in a structured semiconductor microcavity. The exciton-enhanced
optomechanical coupling leads to self-oscillations (phonon lasing) -- thus
proving reversible photon-to-phonon conversion. We show that these oscillations
are a signature of the optomechanical strong coupling signalizing the emergence
of elusive phonon-exciton-photon quasiparticles -- the phonoritons. We then
demonstrate full control of the phonoriton spectrum as well as coherent
microwave-to-photon interconversion using electrically generated GHz-vibrations
and a resonant optical laser beam. These findings establish the
zero-dimensional polariton condensates as a scalable coherent interface between
microwave and optical domains with enhanced microwave-to-mechanical and
mechanical-to-optical coupling rates
Cosmic Ray Acceleration at the Forward Shock in Tycho's Supernova Remnant: Evidence from Chandra X-ray Observations
We present evidence for cosmic ray acceleration at the forward shock in
Tycho's supernova remnant (SNR) from three X-ray observables: (1) the proximity
of the contact discontinuity to the forward shock, or blast wave, (2) the
morphology of the emission from the rim of Tycho, and (3) the spectral nature
of the rim emission. We determine the locations of the blast wave (BW), contact
discontinuity (CD), and reverse shock (RS) around the rim of Tycho's supernova
remnant using a principal component analysis and other methods applied to new
Chandra data. The azimuthal-angle-averaged radius of the BW is 251". For the CD
and RS we find average radii of 241" and 183", respectively. Taking account of
projection effects, we find ratios of 1:0.93:0.70 (BW:CD:RS). We show these
values to be inconsistent with adiabatic hydrodynamical models of SNR
evolution. The CD:BW ratio can be explained if cosmic ray acceleration of ions
is occurring at the forward shock. The RS:BW ratio, as well as the strong Fe Ka
emission from the Tycho ejecta, imply that the RS is not accelerating cosmic
rays. We also extract radial profiles from ~34% of the rim of Tycho and compare
them to models of surface brightness profiles behind the BW for a purely
thermal plasma with an adiabatic shock. The observed morphology of the rim is
much more strongly peaked than predicted by the model, indicating that such
thermal emission is implausible here. Spectral analysis also implies that the
rim emission is non-thermal in nature, lending further support to the idea that
Tycho's forward shock is accelerating cosmic rays.Comment: 39 pages, 10 figures, accepted by Ap
Deep optical observations of the central X-ray source in the Puppis A supernova remnant
X-ray observations reveiled a group of radio-silent isolated neutron stars
(INSs) at the centre of young supernova remnants (SNRs), dubbed central compact
objects or CCOs, with properties different from those of classical
rotation-powered pulsars. In at least three cases, evidence points towards CCOs
being low-magnetized INSs, born with slow rotation periods, and possibly
accreting from a debris disc of material formed out of the supernova event.
Understanding the origin of the diversity of the CCOs can shed light on
supernova explosion and neutron star formation models. Optical/infrared (IR)
observations are crucial to test different CCO interpretations. The aim of our
work is to perform a deep optical investigation of the CCO RX J0822.0-4300 in
the Puppis A SNR, one of the most poorly understood in the CCO family. By using
as a reference the Chandra X-ray coordinates of RX J0822.0-4300, we performed
deep optical observations in the B, V and I bands with the Very Large Telescope
(VLT). We found no candidate optical counterpart within 3 sigma of the computed
Chandra X-ray position down to 5 sigma limits of B~27.2, V~26.9, and I~25.6,
the deepest obtained in the optical band for this source. These limits confirm
the non-detection of a companion brighter than an M5 dwarf. At the same time,
they do not constrain optical emission from the neutron star surface, while
emission from the magnetosphere would require a spectral break in the
optical/IR.Comment: 4 pages, 2 figures, Astronomy and Astrophysics, accepte
Valley-spin blockade and spin resonance in carbon nanotubes
Manipulation and readout of spin qubits in quantum dots made in III-V
materials successfully rely on Pauli blockade that forbids transitions between
spin-triplet and spin-singlet states. Quantum dots in group IV materials have
the advantage of avoiding decoherence from the hyperfine interaction by
purifying them with only zero-spin nuclei. Complications of group IV materials
arise from the valley degeneracies in the electronic bandstructure. These lead
to complicated multiplet states even for two-electron quantum dots thereby
significantly weakening the selection rules for Pauli blockade. Only recently
have spin qubits been realized in silicon devices where the valley degeneracy
is lifted by strain and spatial confinement. In carbon nanotubes Pauli blockade
can be observed by lifting valley degeneracy through disorder. In clean
nanotubes, quantum dots have to be made ultra-small to obtain a large energy
difference between the relevant multiplet states. Here we report on
low-disorder nanotubes and demonstrate Pauli blockade based on both valley and
spin selection rules. We exploit the bandgap of the nanotube to obtain a large
level spacing and thereby a robust blockade. Single-electron spin resonance is
detected using the blockade.Comment: 31 pages including supplementary informatio
Neutrino tomography - Learning about the Earth's interior using the propagation of neutrinos
Because the propagation of neutrinos is affected by the presence of Earth
matter, it opens new possibilities to probe the Earth's interior. Different
approaches range from techniques based upon the interaction of high energy
(above TeV) neutrinos with Earth matter, to methods using the MSW effect on the
neutrino oscillations of low energy (MeV to GeV) neutrinos. In principle,
neutrinos from many different sources (sun, atmosphere, supernovae, beams etc.)
can be used. In this talk, we summarize and compare different approaches with
an emphasis on more recent developments. In addition, we point out other
geophysical aspects relevant for neutrino oscillations.Comment: 22 pages, 9 figures. Proceedings of ``Neutrino sciences 2005:
Neutrino geophysics'', December 14-16, 2005, Honolulu, USA. Minor changes,
some references added. Final version to appear in Earth, Moon, and Planet
A valley-spin qubit in a carbon nanotube
Although electron spins in III-V semiconductor quantum dots have shown great
promise as qubits, a major challenge is the unavoidable hyperfine decoherence
in these materials. In group IV semiconductors, the dominant nuclear species
are spinless, allowing for qubit coherence times that have been extended up to
seconds in diamond and silicon. Carbon nanotubes are a particularly attractive
host material, because the spin-orbit interaction with the valley degree of
freedom allows for electrical manipulation of the qubit. In this work, we
realise such a qubit in a nanotube double quantum dot. The qubit is encoded in
two valley-spin states, with coherent manipulation via electrically driven spin
resonance (EDSR) mediated by a bend in the nanotube. Readout is performed by
measuring the current in Pauli blockade. Arbitrary qubit rotations are
demonstrated, and the coherence time is measured via Hahn echo. Although the
measured decoherence time is only 65 ns in our current device, this work offers
the possibility of creating a qubit for which hyperfine interaction can be
virtually eliminated
VLT observations of the Central Compact Object in the Vela Jr. supernova remnant
X-ray observations have unveiled the existence of enigmatic point-like
sources at the center of young (a few kyrs) supernova remnants. These sources,
known as Central Compact Objects (CCOs), are thought to be neutron stars
produced by the supernova explosion, although their X-ray phenomenology makes
them markedly different from all the other young neutron stars discovered so
far.The aim of this work is to search for the optical/IR counterpart of the
Vela Junior CCO and to understand the nature of the associated Halpha nebula
discovered by Pellizzoni et al. (2002).}{We have used deep optical (R band) and
IR (J,H,Ks bands) observations recently performed by our group with the ESO VLT
to obtain the first deep, high resolution images of the field with the goal of
resolving the nebula structure and pinpointing a point-like source possibly
associated with the neutron star.Our R-band image shows that both the nebula's
flux and its structure are very similar to the Halpha ones, suggesting that the
nebula spectrum is dominated by pure Halpha line emission. However, the nebula
is not detected in our IR observations, whick makes it impossible to to
constrain its spectrum. A faint point-like object (J>22.6, H~21.6, Ks ~ 21.4)
compatible with the neutron star's Chandra X-ray position is detected in our IR
images (H and Ks) but not in the optical one (R > 25.6), where it is buried by
the nebula background. The nebula is most likely a bow-shock produced by the
neutron star motion through the ISM or, alternatively, a photo-ionization
nebula powered by UV radiation from a hot neutron star.Comment: 8 pages, 4 figures, A&Aaccepte
Optomechanical parametric oscillation of a quantum light-fluid lattice
Two-photon coherent states are one of the main building pillars of nonlinear and quantum optics. They are the basis for the generation of minimum-uncertainty quantum states and entangled photon pairs, applications not obtainable from standard coherent states or one-photon lasers. Here, we describe a fully resonant optomechanical parametric amplifier involving a polariton condensate in a trap lattice quadratically coupled to mechanical modes. The quadratic coupling derives from nonresonant virtual transitions to extended discrete excited states induced by the optomechanical coupling. Nonresonant continuous-wave laser excitation leads to striking experimental consequences, including the emergence of optomechanically induced intersite parametric oscillations and intersite tunneling of polaritons at discrete intertrap detunings corresponding to sums of energies of the two involved mechanical oscillations (20- and 60-GHz confined vibrations). We show that the coherent mechanical oscillations correspond to parametric resonances with a threshold condition different from that of standard linear optomechanical self-oscillation. The associated Arnold tongues display a complex scenario of states within the instability region. The observed phenomena can have applications for the generation of entangled phonon pairs and squeezed mechanical states relevant in sensing and quantum computation and for the bidirectional frequency conversion of signals in a technologically relevant range.Fil: Reynoso, Andres Alejandro. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche.; ArgentinaFil: Usaj, Gonzalo. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche.; ArgentinaFil: Chafatinos, Dimitri Lisandro. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche.; ArgentinaFil: Mangussi, Franco. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche.; ArgentinaFil: Bruchhausen, Axel Emerico. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche.; ArgentinaFil: Kuznetsov, A. S.. Paul-drude-institut Für Festkörperelektronik; AlemaniaFil: Biermann, K.. Paul-drude-institut Für Festkörperelektronik; AlemaniaFil: Santos, P. V.. Paul-drude-institut Für Festkörperelektronik; AlemaniaFil: Fainstein, Alejandro. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche.; Argentin
Hubble Space Telescope WFPC-2 Imaging of Cassiopeia A
The young SNR Cassiopeia A was imaged with WFPC-2 through four filters
selected to capture the complete velocity range of the remnant's main shell in
several important emission lines. Primary lines detected were [O III]
4959,5007, [N II] 6583, [S II] 6716,6731 + [O II] 7319,7330 + [O I] 6300,6364,
and [S III] 9069,9532. About 3/4th of the remnant's main shell was imaged in
all four filters. Considerable detail is observed in the reverse-shocked ejecta
with typical knot scale lengths of 0.2"-0.4" (1 - 2 x 10^16 cm). Both bright
and faint emission features appear highly clumped. Large differences in [S III]
and [O III] line intensities indicating chemical abundance differences are also
seen, particularly in knots located along the bright northern limb and near the
base of the northeast jet. A line of curved overlapping filament in the
remnant's northwestern rim appears to mark the location of the remnant's
reverse shock front in this region. Finger-like ejecta structures elsewhere
suggest cases where the reverse shock front is encountering the remnant's
clumped ejecta. Narrow-band [N II] images of the remnant's circumstellar knots
("QSFs") reveal them to be 0.1"-0.6" thick knots and filaments, often with
diffuse edges facing away from the center of expansion. Three color composite
images of the whole remnant and certain sections along with individual filter
enlargements of selected regions of the bright optical shell are presented and
discussed.Comment: 26 pages, 12 figures Accepted to the Astronomical Journa
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