380 research outputs found
On the spatial Markov property of soups of unoriented and oriented loops
We describe simple properties of some soups of unoriented Markov loops and of
some soups of oriented Markov loops that can be interpreted as a spatial Markov
property of these loop-soups. This property of the latter soup is related to
well-known features of the uniform spanning trees (such as Wilson's algorithm)
while the Markov property of the former soup is related to the Gaussian Free
Field and to identities used in the foundational papers of Symanzik, Nelson,
and of Brydges, Fr\"ohlich and Spencer or Dynkin, or more recently by Le Jan
Decentralized Formation Pose Estimation for Spacecraft Swarms
For spacecraft swarms, the multi-agent localization algorithm must scale well with the number of spacecraft and adapt to time-varying communication and relative sensing networks. In this paper, we present a decentralized, scalable algorithm for swarm localization, called the Decentralized Pose Estimation (DPE) algorithm. The DPE considers both communication and relative sensing graphs and defines an observable local formation. Each spacecraft jointly localizes its local subset of spacecraft using direct and communicated measurements. Since the algorithm is local, the algorithm complexity does not grow with the number of spacecraft in the swarm. As part of the DPE, we present the Swarm Reference Frame Estimation (SRFE) algorithm, a distributed consensus algorithm to co-estimate a common Local-Vertical, Local-Horizontal (LVLH) frame. The DPE combined with the SRFE provides a scalable, fully-decentralized navigation solution that can be used for swarm control and motion planning. Numerical simulations and experiments using Caltech’s robotic spacecraft simulators are presented to validate the effectiveness and scalability of the DPE algorithm
Decentralized formation pose estimation for spacecraft swarms
For spacecraft swarms, the multi-agent localization algorithm must scale well with the number of spacecraft and adapt to time-varying communication and relative sensing networks. In this paper, we present a decentralized, scalable algorithm for swarm localization, called the Decentralized Pose Estimation (DPE) algorithm. The DPE considers both communication and relative sensing graphs and defines an observable local formation. Each spacecraft jointly localizes its local subset of spacecraft using direct and communicated measurements. Since the algorithm is local, the algorithm complexity does not grow with the number of spacecraft in the swarm. As part of the DPE, we present the Swarm Reference Frame Estimation (SRFE) algorithm, a distributed consensus algorithm to co-estimate a common Local-Vertical, Local-Horizontal (LVLH) frame. The DPE combined with the SRFE provides a scalable, fully-decentralized navigation solution that can be used for swarm control and motion planning. Numerical simulations and experiments using Caltech’s robotic spacecraft simulators are presented to validate the effectiveness and scalability of the DPE algorithm
Negative quasiprobabilities enhance phase estimation in quantum-optics experiment
Operator noncommutation, a hallmark of quantum theory, limits measurement
precision, according to uncertainty principles. Wielded correctly, though,
noncommutation can boost precision. A recent foundational result relates a
metrological advantage with negative quasiprobabilities -- quantum extensions
of probabilities -- engendered by noncommuting operators. We crystallize the
relationship in an equation that we prove theoretically and observe
experimentally. Our proof-of-principle optical experiment features a filtering
technique that we term partially postselected amplification (PPA). Using PPA,
we measure a waveplate's birefringent phase. PPA amplifies, by over two orders
of magnitude, the information obtained about the phase per detected photon. In
principle, PPA can boost the information obtained from the average filtered
photon by an arbitrarily large factor. The filter's amplification of systematic
errors, we find, bounds the theoretically unlimited advantage in practice. PPA
can facilitate any phase measurement and mitigates challenges that scale with
trial number, such as proportional noise and detector saturation. By
quantifying PPA's metrological advantage with quasiprobabilities, we reveal
deep connections between quantum foundations and precision measurement.Comment: 5 pages, 4 figures in main text; 8 pages, 1 figure in appendice
Generating a 4-photon Tetrahedron State: Towards Simultaneous Super-sensitivity to Non-commuting Rotations
It is often thought that the super-sensitivity of a quantum state to an
observable comes at the cost of a decreased sensitivity to other non-commuting
observables. For example, a squeezed state squeezed in position quadrature is
super-sensitive to position displacements, but very insensitive to momentum
displacements. This misconception was cleared with the introduction of the
compass state, a quantum state equally super-sensitive to displacements in
position and momentum. When looking at quantum states used to measure spin
rotations, N00N states are known to be more advantageous than classical methods
as long as they are aligned to the rotation axis. When considering the
estimation of a rotation with unknown direction and amplitude, a certain class
of states stands out with interesting properties. These states are equally
sensitive to rotations around any axis, are second-order unpolarized, and can
possess the rotational properties of platonic solids in particular dimensions.
Importantly, these states are optimal for simultaneously estimating the three
parameters describing a rotation. In the asymptotic limit, estimating all d
parameters describing a transformation simultaneously rather than sequentially
can lead to a reduction of the appropriately-weighted sum of the measured
parameters' variances by a factor of d. We report the experimental creation and
characterization of the lowest-dimensional such state, which we call the
"tetrahedron state" due to its tetrahedral symmetry. This tetrahedron state is
created in the symmetric subspace of four optical photons' polarization in a
single spatial and temporal mode, which behaves as a spin-2 particle. While
imperfections due to the hardware limit the performance of our method, we argue
that better technology can improve our method to the point of outperforming any
other existing strategy in per-photon comparisons.Comment: 13 pages, 6 figure
MANIFESTĂRILE TUBULUI DIGESTIV SUPERIOR LA PACIENTUL CU IRC DIALIZAT
Introducere: bolnavii cu IRC sunt susceptibili la o gamă largă de complicaţii: refl ux gastro-esofagian, esofagită,
gastropatie, gastrită şi duodenită, ulcer gastric şi duodenal, angiodisplazie.
Obiective: studiul a constat în evidenţierea simptomelor gastro-duodenale, a frecvenţei modifi cărilor patologice
ale mucoasei eso-gastro-duodenale şi a prezenţei H.pylori la copiii cu IRC afl aţi în program de dializă.
Material şi metodă: lotul de studiu a fost format din 55 de copii internaţi în Clinica a IV-a Nefrologie, afl aţi în
program de substituţie renală cronică, la care s-a aplicat un protocol ce a sistematizat: simptomele obiective şi
subiective şi rezultatele endoscopiei digestive superioare. Lotul cuprinde bolnavii cu modifi cări ale mucoasei
depistate endoscopic.
Concluzii: modifi cările patologice ale mucoasei eso-gastro-duodenale la copilul dializat cronic se refl ectă clinic inconstant, ceea ce sugerează necesitatea endoscopiei pentru fi ecare caz. Prezenţa H.pylori la 24/55 (43,63%) cazuri biopsiate evidenţiază asocierea relativ frecventă între leziunile gastro-duodenale şi H.pylori la pacienţii
dializaţi cronic
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