5,042 research outputs found
Discovery of molecular gas around HD 131835 in an APEX molecular line survey of bright debris disks
Debris disks are considered to be gas-poor, but recent observations revealed
molecular or atomic gas in several 10-40 Myr old systems. We used the APEX and
IRAM 30m radiotelescopes to search for CO gas in 20 bright debris disks. In one
case, around the 16 Myr old A-type star HD 131835, we discovered a new
gas-bearing debris disk, where the CO 3-2 transition was successfully detected.
No other individual system exhibited a measurable CO signal. Our Herschel Space
Observatory far-infrared images of HD 131835 marginally resolved the disk both
at 70 and 100m, with a characteristic radius of ~170 au. While in stellar
properties HD 131835 resembles Pic, its dust disk properties are
similar to those of the most massive young debris disks. With the detection of
gas in HD 131835 the number of known debris disks with CO content has increased
to four, all of them encircling young (40 Myr) A-type stars. Based on
statistics within 125 pc, we suggest that the presence of detectable amount of
gas in the most massive debris disks around young A-type stars is a common
phenomenon. Our current data cannot conclude on the origin of gas in HD 131835.
If the gas is secondary, arising from the disruption of planetesimals, then HD
131835 is a comparably young and in terms of its disk more massive analogue of
the Pic system. However, it is also possible that this system similarly
to HD 21997 possesses a hybrid disk, where the gas material is predominantly
primordial, while the dust grains are mostly derived from planetesimals.Comment: Accepted for publication in ApJ, 18 pages, 9 figures, 5 table
High-Dimensional Quantum Key Distribution based on Multicore Fiber using Silicon Photonic Integrated Circuits
Quantum Key Distribution (QKD) provides an efficient means to exchange
information in an unconditionally secure way. Historically, QKD protocols have
been based on binary signal formats, such as two polarisation states, and the
transmitted information efficiency of the quantum key is intrinsically limited
to 1 bit/photon. Here we propose and experimentally demonstrate, for the first
time, a high-dimensional QKD protocol based on space division multiplexing in
multicore fiber using silicon photonic integrated lightwave circuits. We
successfully realized three mutually unbiased bases in a four-dimensional
Hilbert space, and achieved low and stable quantum bit error rate well below
both coherent attack and individual attack limits. Compared to previous
demonstrations, the use of a multicore fiber in our protocol provides a much
more efficient way to create high-dimensional quantum states, and enables
breaking the information efficiency limit of traditional QKD protocols. In
addition, the silicon photonic circuits used in our work integrate variable
optical attenuators, highly efficient multicore fiber couplers, and
Mach-Zehnder interferometers, enabling manipulating high-dimensional quantum
states in a compact and stable means. Our demonstration pave the way to utilize
state-of-the-art multicore fibers for long distance high-dimensional QKD, and
boost silicon photonics for high information efficiency quantum communications.Comment: Please see the complementary work arXiv:1610.01682 (2016
STIM map: detection map for exoplanets imaging beyond asymptotic Gaussian residual speckle noise
Direct imaging of exoplanets is a challenging task as it requires to reach a
high contrast at very close separation to the star. Today, the main limitation
in the high-contrast images is the quasi-static speckles that are created by
residual instrumental aberrations. They have the same angular size as planetary
companions and are often brighter, hence hindering our capability to detect
exoplanets. Dedicated observation strategies and signal processing techniques
are necessary to disentangle these speckles from planetary signals. The output
of these methods is a detection map in which the value of each pixel is related
to a probability of presence of a planetary signal. The detection map found in
the literature relies on the assumption that the residual noise is Gaussian.
However, this is known to lead to higher false positive rates, especially close
to the star. In this paper, we re-visit the notion of detection map by
analyzing the speckle noise distribution, namely the Modified Rician
distribution. We use non-asymptotic analysis of the sum of random variables to
show that the tail of the distribution of the residual noise decays as an
exponential distribution, hence explaining the high false detection rate
obtained with the Gaussian assumption. From this analysis, we introduce a novel
time domain detection map and we demonstrate its capabilities and the relevance
of our approach through experiments on real data. We also provide an empirical
rule to determine detection threshold providing a good trade off between true
positive and false positive rates for exoplanet detection
Incremental Relaying for the Gaussian Interference Channel with a Degraded Broadcasting Relay
This paper studies incremental relay strategies for a two-user Gaussian
relay-interference channel with an in-band-reception and
out-of-band-transmission relay, where the link between the relay and the two
receivers is modelled as a degraded broadcast channel. It is shown that
generalized hash-and-forward (GHF) can achieve the capacity region of this
channel to within a constant number of bits in a certain weak relay regime,
where the transmitter-to-relay link gains are not unboundedly stronger than the
interference links between the transmitters and the receivers. The GHF relaying
strategy is ideally suited for the broadcasting relay because it can be
implemented in an incremental fashion, i.e., the relay message to one receiver
is a degraded version of the message to the other receiver. A
generalized-degree-of-freedom (GDoF) analysis in the high signal-to-noise ratio
(SNR) regime reveals that in the symmetric channel setting, each common relay
bit can improve the sum rate roughly by either one bit or two bits
asymptotically depending on the operating regime, and the rate gain can be
interpreted as coming solely from the improvement of the common message rates,
or alternatively in the very weak interference regime as solely coming from the
rate improvement of the private messages. Further, this paper studies an
asymmetric case in which the relay has only a single single link to one of the
destinations. It is shown that with only one relay-destination link, the
approximate capacity region can be established for a larger regime of channel
parameters. Further, from a GDoF point of view, the sum-capacity gain due to
the relay can now be thought as coming from either signal relaying only, or
interference forwarding only.Comment: To appear in IEEE Trans. on Inf. Theor
ISR spectra simulations with electron-ion Coulomb collisions
Incoherent scatter radars (ISR) rely on Thomson scattering of very high frequency or ultrahigh frequency radio waves off electrons in the ionosphere and measure the backscattered power spectra in order to estimate altitude profiles of plasma density, electron temperature, ion temperature, and ion drift speed. These spectra result from the collective behavior of coupled ion and electron dynamics, and, for most cases, existing theories predict these well. However, when the radar points nearly perpendicular to the Earth's magnetic field, the motion of the plasma across the field lines becomes complex and Coulomb collisions between electrons and ions become important in interpreting ISR measurements. This paper presents the first fully kinetic, self‐consistent, particle‐in‐cell simulations of ISR spectra with electron‐ion Coulomb collisions. We implement a grid‐based Coulomb collision algorithm in the Electrostatic Parallel Particle‐in‐Cell simulator and obtain ISR spectra from simulations both with and without collisions. For radar directions greater than 5° away from perpendicular to the magnetic field, both sets of simulations match collisionless ISR theory well. For angles between 3° and 5°, the collisional simulation is well described by a simplified Brownian motion collision process. At angles less than 3° away from perpendicular the Brownian motion model fails, and the collisional simulation qualitatively agrees with previous single particle simulations. For radar directions exactly perpendicular to the magnetic field the simulated collisional spectra match those from the Brownian motion collision theory, in agreement with previous single particle simulations.This work was supported by NASA grants NNX14AI13G and NNX16AB80G and NSF grant PHY-1500439. This work used the XSEDE and TACC computational facilities, supported by NSF grant ACI-1053575. The work by Alex Fletcher was supported by NSF-AGS Postdoctoral Research Fellowship award 1433536 while at the Center for Space Physics, Boston University. Simulation produced data are archived at TACC and are available upon request. We thank John Swoboda of MIT Haystack Observatory for his suggestions on processing the simulated ISR spectra. (NNX14AI13G - NASA; NNX16AB80G - NASA; PHY-1500439 - NSF; ACI-1053575 - NSF; 1433536 - NSF-AGS Postdoctoral Research Fellowship at the Center for Space Physics, Boston University)First author draf
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