273 research outputs found
Realization of quantum walks with negligible decoherence in waveguide lattices
Quantum random walks are the quantum counterpart of classical random walks, and were recently studied in the context of quantum computation. Physical implementations of quantum walks have only been made in very small scale systems severely limited by decoherence. Here we show that the propagation of photons in waveguide lattices, which have been studied extensively in recent years, are essentially an implementation of quantum walks. Since waveguide lattices are easily constructed at large scales and display negligible decoherence, they can serve as an ideal and versatile experimental playground for the study of quantum walks and quantum algorithms. We experimentally observe quantum walks in large systems (similar to 100 sites) and confirm quantum walks effects which were studied theoretically, including ballistic propagation, disorder, and boundary related effects
Hyper-runaway and hypervelocity white dwarf candidates in Gaia Data Release 3: possible remnants from Ia/Iax supernova explosions or dynamical encounters
Type Ia and other peculiar supernovae (SNe) are thought to originate from the thermonuclear explosions of white dwarfs (WDs). Some of the proposed channels involve the ejection of a partly exploded WD (e.g. Iax SN remnant) or the companion of an exploding WD at extremely high velocities (>400 km s−1). Characterisation of such hyper-runaway/hypervelocity (HVS) WDs might therefore shed light on the physics and origins of SNe. Here we analyse the Gaia DR3 data to search for HVS WDs candidates, and peculiar sub-main-sequence (sub-MS) objects. We retrieve the previously identified HVSs, and find 46 new HVS candidates. Among these we identify two new unbound WDs and two new unbound sub-MS candidates. The remaining stars are hyper-runaway WDs and hyper-runaway sub-MS stars. The numbers and properties of the HVS WD and sub-MS candidates suggest that extreme velocity ejections (>1000 km s−1) can accompany at most a small fraction of type Ia SNe, disfavouring a significant contribution of the D6-scenario to the origin of Ia SNe. The rate of HVS ejections following the hybrid WD reverse-detonation channel could be consistent with the identified HVSs. The numbers of lower-velocity HVS WDs could be consistent with type Iax SNe origin and/or contribution from dynamical encounters. We also searched for HVS WDs related to known SN remnants, but identified only one such candidate
On the origin of planets at very wide orbits from the re-capture of free floating planets
In recent years several planets have been discovered at wide orbits (>100 AU)
around their host stars. Theoretical studies encounter difficulties in
explaining their formation and origin. Here we propose a novel scenario for the
production of planetary systems at such orbits, through the dynamical recapture
of free floating planets (FFPs) in dispersing stellar clusters. This process is
a natural extension of the recently suggested scenario for the formation of
wide stellar binaries. We use N-body simulations of dispersing clusters with
10-1000 stars and comparable numbers of FFPs to study this process. We find
that planets are captured into wide orbits in the typical range ~100-10^6 AU,
and have a wide range of eccentricities (thermal distribution). Typically, 3-6
x (f_FFP/1) % of all stars capture a planetary companion with such properties
(where f_FFP is the number of FFP per star). The planetary capture efficiency
is comparable to that of capture-formed stellar-binaries, and shows a similar
dependence on the cluster size and structure. It is almost independent of the
specific planetary mass; planets as well as sub-stellar companions of any mass
can be captured. The capture efficiency decreases with increasing cluster size,
and for a given cluster size the it increases with the host/primary mass. More
than one planet can be captured around the same host and planets can be
captured into binary systems. Planets can also be captured into pre-existing
planetary and into orbits around black holes and massive white dwarfs, if these
formed early enough before the cluster dispersal. In particular, stellar black
holes have a high capture efficiency (>50 % and 5-10 x (f_FFP/1) % for capture
of stars and planetary companions, respectively) due to their large mass.
Finally, although rare, two FFPs or brown dwarfs can become bound and form a
FFP-binary system with no stellar host.Comment: ApJ, in press. Added two figure
The S-Star Cluster at the Center of the Milky Way: On the nature of diffuse NIR emission in the inner tenth of a parsec
Sagittarius A*, the super-massive black hole at the center of the Milky Way,
is surrounded by a small cluster of high velocity stars, known as the S-stars.
We aim to constrain the amount and nature of stellar and dark mass associated
with the cluster in the immediate vicinity of Sagittarius A*. We use
near-infrared imaging to determine the -band luminosity function
of the S-star cluster members, and the distribution of the diffuse background
emission and the stellar number density counts around the central black hole.
This allows us to determine the stellar light and mass contribution expected
from the faint members of the cluster. We then use post-Newtonian N-body
techniques to investigate the effect of stellar perturbations on the motion of
S2, as a means of detecting the number and masses of the perturbers. We find
that the stellar mass derived from the -band luminosity
extrapolation is much smaller than the amount of mass that might be present
considering the uncertainties in the orbital motion of the star S2. Also the
amount of light from the fainter S-cluster members is below the amount of
residual light at the position of the S-star cluster after removing the bright
cluster members. If the distribution of stars and stellar remnants is strongly
enough peaked near Sagittarius A*, observed changes in the orbital elements of
S2 can be used to constrain both their masses and numbers. Based on simulations
of the cluster of high velocity stars we find that at a wavelength of 2.2
m close to the confusion level for 8 m class telescopes blend stars will
occur (preferentially near the position of Sagittarius A*) that last for
typically 3 years before they dissolve due to proper motions.Comment: 14 pages, 11 figures, minor changes to match the published version in
Astronomy & Astrophysic
Quantum walks of correlated particles
Quantum walks of correlated particles offer the possibility to study
large-scale quantum interference, simulate biological, chemical and physical
systems, and a route to universal quantum computation. Here we demonstrate
quantum walks of two identical photons in an array of 21 continuously
evanescently-coupled waveguides in a SiOxNy chip. We observe quantum
correlations, violating a classical limit by 76 standard deviations, and find
that they depend critically on the input state of the quantum walk. These
results open the way to a powerful approach to quantum walks using correlated
particles to encode information in an exponentially larger state space
Observational Evidence Linking Interstellar UV Absorption to PAH Molecules
Interstellar matter and star formatio
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