5,167 research outputs found
Realistic Anisotropic Neutron Stars: Pressure Effects
In this paper, we study the impact of anisotropy on neutron stars with
different equations of state, which have been modeled by a piecewise polytropic
function with continuous sound speed. Anisotropic pressure in neutron stars is
often attributed to interior magnetic fields, rotation, and the presence of
exotic matter or condensates. We quantify the presence of anisotropy within the
star by assuming a quasi-local relationship. We find that the radial and
tangential sound velocities constrain the range of anisotropy allowed within
the star. As expected, the anisotropy affects the macroscopic properties of
stars, and it can be introduced to reconcile them with astrophysical
observations. For instance, the maximum mass of anisotropic neutron stars can
be increased by up to 15\% compared to the maximum mass of the corresponding
isotropic configuration. This allows neutron stars to reach masses greater than
, which may explain the secondary compact object of the GW190814
event. Additionally, we propose a universal relation for the binding energy of
an anisotropic neutron star as a function of the star's compactness and the
degree of anisotropy.Comment: 12 pages, 7 figure
Adaptive Phase Estimation with Squeezed Vacuum Approaching the Quantum Limit
Phase estimation plays a central role in communications, sensing, and
information processing. Quantum correlated states, such as squeezed states,
enable phase estimation beyond the shot-noise limit, and in principle approach
the ultimate quantum limit in precision, when paired with optimal quantum
measurements. However, physical realizations of optimal quantum measurements
for optical phase estimation with quantum correlated states are still unknown.
Here we address this problem by introducing an adaptive Gaussian measurement
strategy for optical phase estimation with squeezed vacuum states that, by
construction, approaches the quantum limit in precision. This strategy builds
from a comprehensive set of locally optimal POVMs through rotations and
homodyne measurements and uses the Adaptive Quantum State Estimation framework
for optimizing the adaptive measurement process, which, under certain
regularity conditions, guarantees asymptotic optimality for this quantum
parameter estimation problem. As a result, the adaptive phase estimation
strategy based on locally-optimal homodyne measurements achieves the quantum
limit within the phase interval of . Furthermore, we generalize
this strategy by including heterodyne measurements, enabling phase estimation
across the full range of phases from , where squeezed vacuum allows
for unambiguous phase encoding. Remarkably, for this phase interval, which is
the maximum range of phases that can be encoded in squeezed vacuum, this
estimation strategy maintains an asymptotic quantum-optimal performance,
representing a significant advancement in quantum metrology.Comment: 14 pages, 9 figure
Correlated photon pairs generated from a warm atomic ensemble
We present measurements of the cross-correlation function of photon pairs at
780 nm and 1367 nm, generated in a hot rubidium vapor cell. The temporal
character of the biphoton is determined by the dispersive properties of the
medium where the pair generation takes place. We show that short correlation
times occur for optically thick samples, which can be understood in terms of
off-resonant pair generation. By modifying the linear response of the sample,
we produce near-resonant photon pairs, which could in principle be used for
entanglement distribution
The -metric naked singularity: A viable explanation for the nature of the central object in the Milky Way
In this work, we investigate whether the compact object at the center of the
Milky Way is a naked singularity described by the -metric
spacetime. Our fitting of the astrometric and spectroscopic data for the S2
star implies that similarly to the Schwarzschild black hole, the
-metric naked singularity offers a satisfactory fit to the observed
measurements. Additionally, it is shown that the shadow produced by the naked
singularity is consistent with the shadow observed by the Event Horizon
Telescope collaboration for Sgr-A*. It is worth mentioning that the spatial
distribution of the S-stars favors the notion that the compact object at the
center of our Galaxy can be described by an almost static spacetime. Based on
these findings, the -metric naked singularity turns up as a
compelling candidate for further investigation.Comment: Accepted for publication in Classical and Quantum Gravit
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