5,966 research outputs found
Noisy metrology beyond the standard quantum limit
Parameter estimation is of fundamental importance in areas from atomic
spectroscopy and atomic clocks to gravitational wave detection. Entangled
probes provide a significant precision gain over classical strategies in the
absence of noise. However, recent results seem to indicate that any small
amount of realistic noise restricts the advantage of quantum strategies to an
improvement by at most a multiplicative constant. Here, we identify a relevant
scenario in which one can overcome this restriction and attain superclassical
precision scaling even in the presence of uncorrelated noise. We show that
precision can be significantly enhanced when the noise is concentrated along
some spatial direction, while the Hamiltonian governing the evolution which
depends on the parameter to be estimated can be engineered to point along a
different direction. In the case of perpendicular orientation, we find
superclassical scaling and identify a state which achieves the optimum.Comment: Erroneous expressions with inconsistent units have been corrected. 5
pages, 3 figures + Appendi
Electrostatics of electron-hole interactions in van der Waals heterostructures
The role of dielectric screening of electron-hole interaction in van der
Waals heterostructures is theoretically investigated. A comparison between
models available in the literature for describing these interactions is made
and the limitations of these approaches are discussed. A simple numerical
solution of Poissons equation for a stack of dielectric slabs based on a
transfer matrix method is developed, enabling the calculation of the
electron-hole interaction potential at very low computational cost and with
reasonable accuracy. Using different potential models, direct and indirect
exciton binding energies in these systems are calculated within Wannier-Mott
theory, and a comparison of theoretical results with recent experiments on
excitons in two-dimensional materials is discussed.Comment: 10 pages, 8 figure
Exploring the Local Orthogonality Principle
Nonlocality is arguably one of the most fundamental and counterintuitive
aspects of quantum theory. Nonlocal correlations could, however, be even more
nonlocal than quantum theory allows, while still complying with basic physical
principles such as no-signaling. So why is quantum mechanics not as nonlocal as
it could be? Are there other physical or information-theoretic principles which
prohibit this? So far, the proposed answers to this question have been only
partially successful, partly because they are lacking genuinely multipartite
formulations. In Nat. Comm. 4, 2263 (2013) we introduced the principle of Local
Orthogonality (LO), an intrinsically multipartite principle which is satisfied
by quantum mechanics but is violated by non-physical correlations.
Here we further explore the LO principle, presenting new results and
explaining some of its subtleties. In particular, we show that the set of
no-signaling boxes satisfying LO is closed under wirings, present a
classification of all LO inequalities in certain scenarios, show that all
extremal tripartite boxes with two binary measurements per party violate LO,
and explain the connection between LO inequalities and unextendible product
bases.Comment: Typos corrected; data files uploade
Detection of very-high-energy gamma-ray emission from the vicinity of PSR B1706-44 with H.E.S.S
The energetic pulsar PSR B1706-44 and the adjacent supernova remnant (SNR)
candidate G 343.1-2.3 were observed by H.E.S.S. during a dedicated
observational campaign in 2007. A new source of very-high-energy (VHE; E > 100
GeV) gamma-ray emission, HESS J1708-443, was discovered with its centroid at
RA(J2000) = 17h08m10s and Dec(J2000) = -44d21', with a statistical error of 3
arcmin on each axis. The VHE gamma-ray source is significantly more extended
than the H.E.S.S. point-spread function, with an intrinsic Gaussian width of
0.29 +/- 0.04 deg. Its energy spectrum can be described by a power law with a
photon index Gamma = 2.0 +/- 0.1 (stat) +/- 0.2 (sys). The integral flux
measured between 1-10 TeV is ~17% of the Crab Nebula flux in the same energy
range. The possible associations with PSR B1706-44 and SNR G343.1-2.3 are
discussed.Comment: 4+ pages, 2 figures; v1 submitted to ICRC Proceedings on 15 May 2009;
v2 has additional references and minor change
Índices espectrais, diagnóstico da vegetação e da degradação da caatinga da bacia do Rio Taperoá-PB.
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