2,644 research outputs found
Kinematics of a hot massive accretion disk candidate
Characterizing rotation, infall and accretion disks around high-mass
protostars is an important topic in massive star formation research. With the
Australia Telescope Compact Array and the Very Large Array we studied a massive
disk candidate at high angular resolution in ammonia (NH3(4,4) & (5,5)) tracing
the warm disk but not the envelope. The observations resolved at ~0.4''
resolution (corresponding to ~1400AU) a velocity gradient indicative of
rotation perpendicular to the molecular outflow. Assuming a Keplerian accretion
disk, the estimated protostar-disk mass would be high, similar to the
protostellar mass. Furthermore, the position-velocity diagram exhibits
additional deviation from a Keplerian rotation profile which may be caused by
infalling gas and/or a self-gravitating disk. Moreover, a large fraction of the
rotating gas is at temperatures >100K, markedly different to typical low-mass
accretion disks. In addition, we resolve a central double-lobe cm continuum
structure perpendicular to the rotation. We identify this with an ionized,
optically thick jet.Comment: 5 pages, 3 figures, accepted for Astrophysical Journal Letters, a
high-resolution version of the draft can be found at
http://www.mpia.de/homes/beuther/papers.htm
Optimal experiment design revisited: fair, precise and minimal tomography
Given an experimental set-up and a fixed number of measurements, how should
one take data in order to optimally reconstruct the state of a quantum system?
The problem of optimal experiment design (OED) for quantum state tomography was
first broached by Kosut et al. [arXiv:quant-ph/0411093v1]. Here we provide
efficient numerical algorithms for finding the optimal design, and analytic
results for the case of 'minimal tomography'. We also introduce the average
OED, which is independent of the state to be reconstructed, and the optimal
design for tomography (ODT), which minimizes tomographic bias. We find that
these two designs are generally similar. Monte-Carlo simulations confirm the
utility of our results for qubits. Finally, we adapt our approach to deal with
constrained techniques such as maximum likelihood estimation. We find that
these are less amenable to optimization than cruder reconstruction methods,
such as linear inversion.Comment: 16 pages, 7 figure
Simplified Quantum Process Tomography
We propose and evaluate experimentally an approach to quantum process
tomography that completely removes the scaling problem plaguing the standard
approach. The key to this simplification is the incorporation of prior
knowledge of the class of physical interactions involved in generating the
dynamics, which reduces the problem to one of parameter estimation. This allows
part of the problem to be tackled using efficient convex methods, which, when
coupled with a constraint on some parameters allows globally optimal estimates
for the Kraus operators to be determined from experimental data. Parameterising
the maps provides further advantages: it allows the incorporation of mixed
states of the environment as well as some initial correlation between the
system and environment, both of which are common physical situations following
excitation of the system away from thermal equilibrium. Although the approach
is not universal, in cases where it is valid it returns a complete set of
positive maps for the dynamical evolution of a quantum system at all times.Comment: Added references to interesting related work by Bendersky et a
Benchmarking of Gaussian boson sampling using two-point correlators
Gaussian boson sampling is a promising scheme for demonstrating a quantum
computational advantage using photonic states that are accessible in a
laboratory and, thus, offer scalable sources of quantum light. In this
contribution, we study two-point photon-number correlation functions to gain
insight into the interference of Gaussian states in optical networks. We
investigate the characteristic features of statistical signatures which enable
us to distinguish classical from quantum interference. In contrast to the
typical implementation of boson sampling, we find additional contributions to
the correlators under study which stem from the phase dependence of Gaussian
states and which are not observable when Fock states interfere. Using the first
three moments, we formulate the tools required to experimentally observe
signatures of quantum interference of Gaussian states using two outputs only.
By considering the current architectural limitations in realistic experiments,
we further show that a statistically significant discrimination between quantum
and classical interference is possible even in the presence of loss, noise, and
a finite photon-number resolution. Therefore, we formulate and apply a
theoretical framework to benchmark the quantum features of Gaussian boson
sampling under realistic conditions
Large-Alphabet Time-Frequency Entangled Quantum Key Distribution by means of Time-to-Frequency Conversion
We introduce a novel time-frequency quantum key distribution (TFQKD) scheme
based on photon pairs entangled in these two conjugate degrees of freedom. The
scheme uses spectral detection and phase modulation to enable measurements in
the temporal basis by means of time-to-frequency conversion. This allows
large-alphabet encoding to be implemented with realistic components. A general
security analysis for TFQKD with binned measurements reveals a close connection
with finite-dimensional QKD protocols and enables analysis of the effects of
dark counts on the secure key size.Comment: 14 pages, 3 figures, submitte
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